Post Harvest Tech for Leafy Veg

Postharvest Technology for Leafy Vegetables

AVRDC – The World Vegetable Center Shanhua, Taiwan

Postharvest Technology for Leafy Vegetables

AVRDC-ADB Postharvest Projects RETA 6208/6376

Antonio L. Acedo, Jr.

AVRDC - The World Vegetable Center The leading international nonprofit organization committed to alleviating poverty and malnutrition in the developing world through the increased production and consumption of safe vegetables.

AVRDC - The World Vegetable Center P.O. Box 42 Shanhua, Tainan 74199 TAIWAN Tel: Fax:

+886 6 583 7801 +886 6 583 0009

Email: [email protected] Web: www.avrdc.org AVRDC Publication No: 10-733 Editor: Maureen Mecozzi AVRDC Publication Team: Kathy Chen, Vanna Liu, Chen Ming-che, Shiu-luan Lu © 2010 AVRDC - The World Vegetable Center

Project partner

Citation Acedo AL Jr. 2010. Postharvest technology for leafy vegetables. AVRDC-ADB Postharvest Projects RETA 6208/6376. AVRDC Publication No. 10-733. AVRDC - The World Vegetable Center, Taiwan. 67 p.

Contents Contents ..................................................................................................................................... i Tables .........................................................................................................................................ii Figures........................................................................................................................................iii Foreword.....................................................................................................................................iv 1 INTRODUCTION ...................................................................................................................1 2 PRODUCT QUALITY AND POSTHARVEST LOSS .............................................................2 Quality Factors .......................................................................................................................2 Quality Deterioration ..............................................................................................................2 Physiological deterioration.................................................................................................2 Pathological decay.............................................................................................................5 Mechanical injury ...............................................................................................................5 Postharvest Loss....................................................................................................................5 3 POSTHARVEST TECHNOLOGIES FOR FRESH LEAFY VEGETABLES............................6 Improved Crop Variety ...........................................................................................................6 Production Factors .................................................................................................................6 Harvesting ..............................................................................................................................7 Harvest maturity.................................................................................................................7 Time of harvesting .............................................................................................................7 Harvesting method.............................................................................................................8 Field Handling ........................................................................................................................8 Packinghouse Operations ......................................................................................................9 Trimming ............................................................................................................................9 Sorting/Grading................................................................................................................10 Washing and sanitizing....................................................................................................10 Other commodity treatments ...........................................................................................11 Packaging.............................................................................................................................12 Produce packages ...........................................................................................................12 Reinforcing and handling packages.................................................................................12 Modified atmosphere packaging (MAP)...........................................................................14 Cooling and Storage ............................................................................................................15 Precooling ........................................................................................................................17 Optimum storage conditions ............................................................................................17 Cooling methods in developing countries ........................................................................18 Transport Techniques .......................................................................................................... 21 Market Handling ................................................................................................................... 22 Other PHT Developments .................................................................................................... 22 Ethylene removal from postharvest chain ....................................................................... 22 Cold chain system............................................................................................................ 23 Supply chain management .............................................................................................. 24 Economic Analysis of Postharvest Technologies ................................................................ 26 Example 1: postharvest technical advice ........................................................................ 27 Example 2: postharvest technique from exploratory investigation .................................. 27 Example 3: introduction of better postharvest material ................................................... 27 4 PROCESSING TECHNOLOGIES FOR LEAFY VEGETABLES......................................... 30 Importance of Processing .................................................................................................... 30 Commodity Considerations .................................................................................................. 30 Preprocessing Operations.................................................................................................... 30 Washing ........................................................................................................................... 30 Cutting.............................................................................................................................. 31 Blanching ......................................................................................................................... 31 Salting Technology............................................................................................................... 31 Fermentation Technologies.................................................................................................. 32

i

Basic information ............................................................................................................. 32 Sauerkraut processing..................................................................................................... 33 Kimchi processing............................................................................................................ 35 Fermented mustards and cabbage.................................................................................. 35 Pickling Technologies .......................................................................................................... 46 Drying/Dehydration Technologies........................................................................................ 46 Basic principles................................................................................................................ 46 Pre-drying operations ...................................................................................................... 47 Simple dryers................................................................................................................... 47 Producing dehydrated cabbage....................................................................................... 48 Producing dehydrated Kangkong .................................................................................... 48 Dehydrated leafy vegetables in China............................................................................. 50 Packaging dried vegetables ............................................................................................ 50 Canning Technologies ......................................................................................................... 53 Cabbage canning............................................................................................................. 53 Canning techniques for other leafy vegetables ............................................................... 54 5

SUMMARY ......................................................................................................................... 55

References ............................................................................................................................... 56

ii

Tables Table 1. Chinese cabbage grade standards in Yunnan, China................................................ 18 Table 2. Postharvest cooling methods and suitable commodities. .......................................... 16 Table 3. Cost benefit analysis of keeping 2-3 outer leaves of Chinese cabbage during transport compared to removing all outer leaves. .................................................................... 28 Table 4. Potential cost and benefit of using guava leaf extract and alum for bacterial soft rot control in common cabbage. .................................................................................................... 28 Table 5. Cost-benefit analysis of using poly sacks and plastic crates for transporting vegetables from the collecting center, Keppetipola to the central (Manning) market in Colombo, Sri Lanka................................................................................................................................... 29 Table 7. Sample recipe for Kimchi production.......................................................................... 36 Table 8. Desired fresh and dry weight ratio after drying for some vegetables........................ 59

Figures Figure 1. Ethylene-induced cabbage yellowing and culinary herbs epinasty, abscission and yellowing. .................................................................................................................................. 11 Figure 2. Chilling injury development in sweet basil leaves. ...................................................... 4 Figure 3. Trimming, cleaning, and bundling leafy vegetables. ................................................... 9 Figure 4. Packaging containers for leafy vegetables. .............................................................. 13 Figure 5. Protective packaging practices. ................................................................................ 14 Figure 6. MAP practices for leafy vegetables........................................................................... 16 Figure 7. Ice packing and use of ice bottles for cooling leafy vegetables. ............................... 19 Figure 8. Simple refrigeration equipment for the storage of leafy vegetables.......................... 19 Figure 9. Low-cost evaporative cooler developed in Sri Lanka................................................ 28 Figure 10. Application of evaporative cooling and/or modified atmosphere in packing and transport of leafy vegetables. ................................................................................................... 29 Figure 11. Traditional and innovative stacking procedures for containers of leafy vegetables 23 Figure 12. Traditional supply chain system for leafy vegetables.............................................. 25 Figure 13. Corporate supply chain system for leafy vegetables. ............................................. 26 Figure 14. Cooperative system of supply chain management. ................................................ 26 Figure 15. Myanmar government-initiated supply chain for export vegetables produced by contract farmers and brought to the packinghouse for sorting, hydrocooling, packing in carton box, cold storage, and transport in refrigerated trucks for cargo flight..................................... 27 Figure 16. Process flow for sauerkraut production................................................................... 42 Figure 17. Process flow for Kimchi production. ........................................................................ 36 Figure 18. Flow chart of fermented leaf mustard processing. .................................................. 45 Figure 19. Process flow for producing fermented mustard leaves. .......................................... 39 Figure 20. Lao process for producing fermented Chinese mustard......................................... 39 Figure 21. Fermentation pond/plastic barrel and earthen jar for fermented product................ 48 Figure 22. General flow chart of fermented leaf mustard processing. ..................................... 48 Figure 23. Company processing operation for fermented mustards in Thailand. .................... 49 Figure 24. Process flow for Chinese mustard fermentation technique ................................... 42 Figure 25. Process flow for Chinese mustard and cabbage fermentation . ............................ 43 Figure 26. Process flow for fermented cabbage and mustard canning................................... 45 Figure 27. Process flow for producing pickled cabbage........................................................... 46 Figure 28. Different commercial solar dryers in Thailand......................................................... 49 Figure 29. Gas- and electric-powered cabinet dryers. ............................................................. 49 Figure 30. Washing, cutting, blanching and removal of excess water in leafy vegetable for processing to dehydrated product. ........................................................................................... 51 Figure 31. Drying leafy vegetable with or without pre-drying dextrose treatment. .................. 52 Figure 32. Sorting, packing and storage of dehydrated leafy vegetable................................. 53

iii

Foreword Postharvest losses of vegetables contribute to food insecurity, poverty, and economic hopelessness in developing countries. Two overriding gaps are the inadequacies in fresh produce handling and processing, and lack of awareness and adoption of available technologies. Bridging these gaps, which the AVRDC-ADB Postharvest Projects (RETA 6208/6376) strive to achieve, would advance the postharvest sector as a major source of employment for millions of rural people, especially women. These are prerequisites for sustained economic progress. Vegetables are among the most nutritious agricultural products and are widely and preferably consumed fresh. However, fresh vegetables are subject to rapid quality deterioration after harvest due to their high water content, active metabolism, spoilage pathogens, and insect pests. Processing can minimize the problem and is ideal if done without sacrificing nutritional quality, market availability, and price of fresh produce. Many advantages of processed products have been reported, particularly the extended availability of seasonal, perishable products like vegetables beyond the growing season, thereby stabilizing supplies and increasing food security. Processed products are also more stable, have improved digestibility, and permit great diet diversity, giving consumers access to a wider choice of products and better range of vitamins and minerals. There are various techniques for handling fresh vegetables and producing processed products. Knowing these technologies widens alternatives for adaptation and opens more opportunities for innovation. This document summarizes the results of a literature search on low-cost postharvest handling and processing technologies for tomato and chilli, the targeted vegetable crops of the RETA 6208 project, and for selected leafy vegetables, the target crops of RETA 6376. It focuses on technologies considered for adaptation in the RETA 6208/6376 countries (Cambodia, Lao PDR, and Vietnam). Furthermore, this report could serve as a reference for research and development workers and institutions seeking to advance the vegetable industry in developing countries. Dr. Antonio L. Acedo Jr. Dr. Katinka Weinberger AVRDC - The World Vegetable Center

iv

1 INTRODUCTION Leafy vegetables are important crops in Greater Mekong Subregion (GMS) countries, providing income to many small farmers, processors, and other entrepreneurs, and serving as health foods for the rural and urban populace. They are rich sources of vitamins, minerals, and dietary fiber. Leafy vegetables also play a vital role in the global drive to end “hidden hunger”—micronutrient deficiency (Buyckx, undated). Deficiency in vitamin A is a major problem in 37 countries, mostly in Southeast Asia and Africa, with 250,000-500,000 people, mostly children, becoming partially or totally blind yearly. More than 2 billion people worldwide are anemic or have insufficient iron intake. Green leafy vegetables are rich in both vitamin A and iron. Sustainable and reliable food supply systems are needed to ensure global food availability, access to food at the household level, and more opportunities for development of people’s well-being. A vital strategy is to reduce postharvest losses, especially highly perishable leafy vegetables. Reducing postharvest losses through appropriate postharvest technologies for fresh and processed produce would not only increase food availability to the growing world population but also decrease the area needed for production and conserve natural resources (Kader, 2006). While there are many postharvest technologies that extend the market availability of vegetables, the appropriateness of these technologies has to be ascertained through sitespecific and commodity-specific studies. A technology proven effective and commercially used elsewhere is not necessarily the best for use under conditions of another country. Technologies developed in a developing country may better suit the need of another developing country, as they typically are much simpler and less costly than technologies created in developed countries. In recent years, reducing postharvest losses has become only part of efforts to improve food availability; assuring food quality and safety are increasingly important as well. Market demand is responding to consumers' rising nutritional expectations and awareness of food safety. Maintaining product quality and safety can greatly determine marketing success (Sullivan et al., 1991). Countries can increase their competitiveness and world market shares by providing higher quality, safe products and promoting lower prices through technological innovation. Leafy vegetables given emphasis in this report include the leafy brassicas, such as common cabbage, which is also known as round cabbage or head cabbage (Brassica oleracea var capitata); Chinese cabbage (Brassica rapa. var. pekinensis); bok choy or pak choi (Brassica rapa. var. chinensis), a non-heading form of Chinese cabbage; Chinese mustard (Brassica juncea var. rugosa); and other priority vegetables of the partners in Cambodia, Laos, and Vietnam (e.g. amaranth, kangkong). This report consolidates the literature search results and relevant information from papers presented during the RETA 6376 Workshop on “Best Practices in Postharvest Management of Leafy Vegetables in GMS Countries” held on 25-27 October 2007 in Hanoi, Vietnam. Information and actual observations during country visits and study missions in some GMS countries are also included. This report has three main sections. The first section provides some basic information on leafy vegetable quality and postharvest loss, for better understanding of the underlying reasons for the technological recommendations in fresh produce handling and processing described in the second and third sections. The focus throughout is on simple, low-cost methods. Other techniques and information that can be considered in future initiatives are incorporated. In addition, sample cost-benefit analyses of selected techniques is introduced in preparation for training on this topic, and for similar analyses to be done for the techniques developed in the AVRDC-ADB postharvest projects.

Postharvest Technology of Leafy Vegetables

1

2 PRODUCT QUALITY AND POSTHARVEST LOSS Quality Factors Leafy vegetable quality is mainly based on appearance (e.g. fresh-looking, well-formed or well-shaped, right size, right maturity, right color, turgid or not wilted, free of defects such as rot, physical damage, yellowing, or wilting) and to a certain extent, other attributes that cannot be seen but can be discerned by the other human senses, such as firmness, tenderness, and taste. For common cabbage and Chinese cabbage, the heads should be light green, compact but not overmature (no seed stalk), right size, and free of defects. Freshness of cabbages can be tested by rubbing two heads together; if they are fresh, they will make a squeaking sound. For other leafy vegetables, consumers seek similar visual quality attributes (freshness, uniformity of size, shape and typical color, and free of defects). Chinese mustard should have the typical flavor or aroma. Concerns about food safety and nutritional value have made quality definition more complex. Safety factors include pesticide residues (the most important safety issue among consumers), microbial contamination (the number one safety concern among health authorities and scientists), natural toxicants (antinutrients), natural contaminants (mycotoxins, bacterial toxins, and heavy metals such as cadmium, lead, mercury), and environmental pollutants (Kader and Rolle, 2004). These safety concerns, particularly microbial contamination, are the basis for the worldwide promotion of safety standards in production (Good Agricultural Practices or GAP), postharvest handling (Good Hygienic Practices or GHP) and processing (Good Manufacturing Practices or GMP), which incorporate principles and recommendations from Hazard Analysis & Critical Control Points (HACCP) and Codex Alimentarius. Quality standards compliance would greatly raise market competitiveness and consumer confidence in the produce and its source.

Quality Deterioration Wilting due to water loss, senescence-associated discoloration (yellowing or browning), mechanical injury, high respiration rate, and decay or rotting are the main causes of quality deterioration and postharvest loss of leafy vegetables. These causes of quality loss are physiological, pathological, and mechanical in nature.

Physiological deterioration Water loss and wilting. Leafy vegetables are mostly water (>90%) and have the propensity to lose water through transpiration (evaporation of water from plant tissues). Water loss is the main cause of weight loss (loss in saleable weight) and wilting (Fig. 1). A loss of 5-10% of fresh weight would make leafy vegetables to appear wilted and become unusable (Kanlayanarat, 2007). Water loss also induces degradation of nutritional components (e.g. vitamin C loss) and imposes stress (i.e. water stress) that increases respiration and ethylene production. In pak choi, wilting is primarily due to water loss through the stomata (O’Hare et al., 2001). Water loss was measured at 2.8% per hour at 35°C. Complete closure of all stomata occurs between 10-15% moisture loss. Wilting occurred more rapidly in leaves with lower initial water potential. Water potential in pak choi was highest when harvested at 0400 and 2200h. Respiration and ethylene production. Leafy vegetables are non-climacteric, that is, they do not exhibit a final surge in respiration and ethylene production during senescence (Jobling, undated). Cabbages generally have lower respiration and ethylene production rates, partly due to their morphology, in which the young inner leaves are fully covered by the more mature outer leaves compared with Chinese kale or green mustards. However, cabbage is sensitive to ethylene (senescence hormone), which causes yellowing, epinasty (leaf curving), and abscission (Fig. 1) (Cantwell and Suslow, 2006; Cantwell and Reid, 2006; Jobling, undated). In lettuce, ethylene induces russet spotting manifested as dark brown spotting of the midribs.

2

AVRDC-ADB Postharvest Projects RETA 6208/6376

A. Ethylene-induced disorders

B. Bacterial soft rot, wilting and yellowing in cabbages

Figure 1. Ethylene-induced cabbage yellowing and culinary herbs epinasty, abscission and yellowing (Cantwell & Suslow, 2006; Cantwell & Reid, 2006); bacterial soft rot in common cabbage and Chinese cabbage; and out leaf wilting and yellowing in Chinese cabbage (Acedo et al., 1999; Acedo et al., 2003).

Postharvest Technology of Leafy Vegetables

3

Chilling injury score

Days of storage Figure 2. Chilling injury development in sweet basil leaves (Cantwell & Reid, 2006). (Score: 0 = no injury, 8 = severe. Score of 3: limit of commercial acceptability.) Prolonged exposure to ethylene, as low as 0.01 ppm, could cause significant losses of fresh produce. Ethylene easily accumulates in packages, packinghouses, storage areas, and even markets. All plant tissues produce ethylene, although at varying levels. In markets (wholesale, retail, distribution centers), the main sources of ethylene, in addition to the fresh produce, include ripening fruit, decaying produce, and exhaust gases of vehicles; concentration could reach 0.02-0.06 ppm, which can cause a 10-30% loss in product shelf life (Wills et al., 2000). The effect of ethylene is cumulative, so continuous exposure to a low concentration throughout marketing can cause significant harm. The loss of shelf life will be most frustrating for the final consumer, as the loss of quality will not be obvious during marketing and retail. Aside from accelerating aging, ethylene increases product susceptibility to decay. In pak choi, leaf yellowing was found to be controlled by the sugar level (the main energy substrate) rather than ethylene, which explains the poor performance of anti-ethylene agents (e.g. 1-methylcyclopropene) in extending shelf life (O’Hare et al., 2001). Understanding this mechanism also avoided the potentially expensive error of designing genetically modified pak choi through manipulation of ethylene metabolism. Sugars tend to be highest in younger leaves and lowest in leaves toward the base of stem even though the leaves may look similar in size and appearance. As a result, shelf life was longer in younger leaves than older leaves. Leaf yellowing may also be related to genetics (i.e. cultivars), exposure to temperature abuse (i.e. high temperatures), and the level of stress tolerance inherent in the leaf tissue (Kanlayanarat, 2007). Physiological disorders. Chilling injury is induced by storage below the recommended low temperature requirement but above the freezing point of tissues, usually between -2°C to 0°C. In Chinese cabbage, chilling sensitivity varies with cultivar and the injury symptom is mainly midrib discoloration, especially on outer leaves (Cantwell and Suslow, 2006). Chinese cabbage developed patchy papery necrosis more severely at 0°C and 2°C, while none was noted at 20°C. For tropical leafy vegetables such as kangkong and some mustard greens, chilling injury is induced at 10°C and lower. This was shown in sweet basil leaves, which developed chilling injury symptoms (browning of leaves and growing tip, bronzing of leaf veins, and loss of glossy appearance of leaves) more rapidly at lower chilling temperatures (Fig. 2) (Cantwell and Reid, 2006). For common cabbage, physiological deterioration during storage is associated with stem or seed stalk growth (bolting), root growth, internal breakdown, leaf abscission, discoloration, and black speck (Cantwell and Suslow, 2006).

4

AVRDC-ADB Postharvest Projects RETA 6208/6376

Pathological decay Leafy vegetables are susceptible to postharvest diseases that render the produce unfit to sell. Postharvest diseases can be spread through field boxes contaminated by soil or decaying produce or both, contaminated water used to wash produce before packing, decaying rejected produce left lying around the packinghouse, and contaminated healthy produce in packages. Microbial infection can occur both before and after harvest. The infection after harvest can be found at any time between the field and final consumer (Kanlayanarat, 2007). Among postharvest diseases, bacterial soft rot is the most important problem, particularly in brassicas such as cabbages and Chinese kale (Fig. 1). It is caused by various bacterial species including Erwinia, Pseudomonas, and Xanthomonas and is characterized by slimy breakdown of infected tissue with offensive foul odor. The disease usually develops at the cut portion (butt end) and injured leaves of cabbages. Other common decays in cabbage are watery soft rot (Sclerotinia), gray mold rot (Botrytis cinerea), and alternaria leaf spot (Alternaria spp.) (Cantwell and Suslow, 2006).

Mechanical injury Leafy vegetables are very susceptible to mechanical injury (physical damage). Leaf tearing and crushing, midrib breakage, and head cracking or bursting are common forms of damage. Physical injuries increase physiological deterioration through browning as a result of oxidation of phenolics substances, and susceptibility to decay. Postharvest rots have been found to be more prevalent in bruised or damaged produce (Bachmann and Earles, 2000). Mechanical damage also increases moisture loss by as much as 3-4 times more than that of undamaged produce.

Postharvest Loss Quality deterioration results in partial or total loss of fresh produce. It is predisposed by a number of interacting factors, which may be preharvest, harvest and/or postharvest in origin, such as poor crop variety, unfavorable climate, inadequate cultural practices, lack of harvesting techniques, improper handling, and poor storage conditions. Non-technological factors also contribute to postharvest loss, such as lack of capable human resources, lack of knowledge about technical and scientific technologies, inefficient commercialization and marketing systems, lack of logistical support, and lack of enabling policy for the use and administration of human, economic, technical, and scientific resources. Postharvest losses of leafy vegetables vary with commodity, location, growing season, and other factors such as standards of quality and consumer preferences and purchasing power, which differ greatly among countries and across cultures (Kader and Rolle, 2004). Postharvest loss estimates in developing countries are alarming (e.g. 20-50% of production) but efforts are lacking to establish the seriousness of the problem and the interventions needed. In the AVRDC-ADB postharvest projects, postharvest losses were determined at specific stages in the supply chain in Cambodia, Laos, and Vietnam and outright volume loss of specific vegetables including the leafy types (e.g. Chinese cabbage) was estimated at an average of 17% (Weinberger et al., 2007). The loss situation maybe more serious if qualitative losses (e.g. loss in price due to reduced quality, loss in nutritional quality, edibility or caloric value) were factored in. Contributing factors to these losses were identified and prioritized for R&D interventions. In the RETA 6376 initiative, more specific assessment of postharvest loss is being pursued, covering selected leafy vegetables in two upland areas of each country at the farmer and processor levels.

Postharvest Technology of Leafy Vegetables

5

3 POSTHARVEST TECHNOLOGIES FOR FRESH LEAFY VEGETABLES The farm-to-table approach (other terms: system approach, whole chain approach, supply chain approach) is increasingly used to grow and market fresh produce. With this approach, factors before and after harvest are taken into account to obtain the desired product quantity and quality, reduce postharvest loss, and ensure delivery of good quality, safe produce to consumers. Any intervention that manipulates postharvest quality and shelf life could be considered under postharvest technology (PHT). This section introduces some production factors that influence product quality and shelf life, and discusses PHT innovations and recommendations, from harvesting to market handling.

Improved Crop Variety Leafy vegetables have limited storage life even under optimum storage conditions. The potential shelf life is partly under genetic control and can be manipulated by breeding. Breeding leafy vegetables for long shelf life and desired shipping quality is a particular need in developing countries with hot and humid climates where refrigerated facilities are lacking due to the high cost. Some specific shelf-life attributes that can be manipulated through breeding include green color retention and resistance to postharvest stress such as high temperature and microbial infection. More effort is now being exerted to develop vegetable varieties with desired shelf life, shipping and processing attributes, and high levels of nutrients. Breeding leafy vegetables with high carotenoids content has been reported and results in lettuce were promising due to observed genetic variations in B-carotene and lutein contents (Fonseca, 2004). B-carotene and lutein were observed to be higher in leaves with higher chlorophyll content. Other nutrients in vegetables include those with therapeutic or pharmaceutical uses, hence the term nutraceuticals (e.g. glucosinolates). A growing concern is placed on the possibility of accidentally lowering beneficial non-target components while enhancing targeted ones with new technologies.

Production Factors Environmental conditions and cultural practices during production have tremendous effects on produce quality, safety, and shelf life. For example, lettuce harvested during a period of rain does not ship well and product losses are increased. Produce stressed by too much or too little water (by irrigation or rainfall), high rates of nitrogen fertilization, or mechanical injury (scrapes, bruises, abrasions) is susceptible to postharvest diseases. Brassicas are prone to bacterial soft rot if nitrogen is applied as foliar feed, thus nitrogen should be applied to the soil. This effect was not observed in pak choi (Jiang and Pearce, 2005); nitrogen above the optimal level did not result in reduced shelf life, while spraying nutrient solution appeared to be beneficial as it retarded yellowing. Potassium sulfate application also enhanced chlorophyll content and extended shelf life of pak choi. Stress during growth has different effects on produce quality and shelf life. Sustained and intermittent water stress had mostly negative effects for pak choi (Jiang and Pearce, 2005). Although shelf life of pak choi could be extended by these stresses, the plant fresh and dry weights were reduced. In Chinese cabbage, water stress did not affect the water content and weight loss during nine weeks of storage at 0°C. On the other hand, low light stress (shading) before harvest resulted in more rapid yellowing and wilting in pak choi, and lowered the leaf sugar, organic acids, and chlorophyll content. Increasing the period of shading before harvest further reduced sugar content and increased moisture loss during storage.

6

AVRDC-ADB Postharvest Projects RETA 6208/6376

Ensuring safety of fresh leafy vegetables also begins in the field. Outbreaks of food-borne illnesses have been traced to contamination of produce in the field (Bachmann and Earles, 2000). Some preventive measures include (1) avoiding application of fresh animal manure or slurries to a field or to an area immediately adjacent to a field nearing harvest maturity, (2) cleaning equipment that has been used to apply manure on one field before moving it to another field, (3) avoiding using irrigation water from a farm pond used by livestock, and (4) avoiding contact of produce with soil during growth (by mulching) or harvest.

Harvesting Harvest maturity Quality cannot be improved after harvest, only maintained; therefore, it is important to harvest at the proper maturity stage and at peak quality. Immature or overmature produce may not last as long in storage as that picked at proper maturity. Common cabbage and Chinese cabbage heads are harvested when firm and mature (Bautista and Acedo, 1987; Boyette et al., 1992; Stephens, 2003; Cantwell and Suslow, 2006). Compactness (firmness, hardness, solidity) of heads may be determined by hand pressure. A compact head can be only slightly compressed with moderate hand pressure. Delaying harvest even a few days beyond maturity can result in split or cracked heads and increased incidence of rots. Immature heads are puffy or have hollow spaces because the inner leaves are not fully developed and hence, loosely arranged, which make them susceptible to damage (Bautista and Acedo, 1987). When harvested immature, yield decreases and shelf life is shorter than that of mature heads. In certain cases, some sample heads of common cabbage or Chinese cabbage are cut longitudinally to observe the internal stem; if the stem is too long, the head is already overmature (Chen, 2007). Harvest maturity of other vegetables such as leafy mustards, amaranth, and kangkong is based on plant size, number of days after planting (usually 25-30 days) and/or tenderness of leaves. They are harvested when they have developed to the fullest size, yet not so advanced in age that the leaves are tough and the flavor is bitter. Physiological age of the vegetable or the leaves within a plant could affect the rate of postharvest quality loss. In pak choi, young leaves (20–25 days after emergence) are more prone to moisture loss and subsequent wilting than older ones (40 days) (O’Hare et al., 2001). However, older leaves turned yellow more quickly than younger leaves. This response was related to initial sugar content, which was higher in younger leaves. In Chinese cabbage heads, intact mature leaves had a greater tendency to yellow than the same leaves, but detached from the head. Young leaves in intact heads began to swell and expand after one month of storage, causing some heads to crack, leading to rapid senescence of whole heads.

Time of harvesting The time of the day when harvesting is done also affects produce quality and shelf life. In general, harvesting during the coolest time of the day (e.g. early morning) is desirable; the produce is not be exposed to the heat of the sun and the work efficiency of the harvesters is higher. If harvesting during the hotter part of the day cannot be avoided, the produce should be kept shaded in the field to minimize product heat, weight loss, and wilting. Research showed that harvest time of day could affect quality of pak choi but not Chinese cabbage. Pak choi harvested at 0400h and 2000h contained the highest initial and final water content. Leaves harvested at these times maintained highest water potential, resulting in a slower rate of wilting than those with lower water potential (Jiang and Pearce, 2005). However, harvesting later in the day has an added advantage because sugar levels were found to be higher as a result of photosynthesis during the day (O’Hare et al., 2001). This could slow down leaf yellowing in pak choi, which has been associated with sugar depletion.

Postharvest Technology of Leafy Vegetables

7

Harvesting method Harvesting is done manually, hence the harvesters have a major influence on produce quality. They should be made aware of the importance of good sanitation practices, proper maturity selection, and careful handling to avoid mechanical injuries. A cabbage head is harvested by bending it to one side and cutting it with a knife, which should be sharpened frequently to reduce effort and lessen picker fatigue. In Yunnan, China, cabbages and other leafy vegetables are harvested and trimmed using a special knife (Fig. 3) (Chen, 2007). The head should not be snapped or twisted, as this method damages the head and results in inconsistent stalk length and trim. Broken stalks are also more susceptible to decay. The stalk should be cut flat and as close to the head as possible, yet long enough to retain two to four wrapper leaves. Extra leaves act as a cushion during handling and may be desired in certain markets. Yellowed, damaged, or diseased wrapper leaves should be removed. Heads with insect damage and other defects should be discarded. It is essential that heads not harvested be left undamaged because fields may be harvested as many as three times for maximum yield. Harvested cabbage can be placed in bags, boxes, wagons, or pallet bins. Harvesting aids can significantly reduce labor costs, improve harvest efficiency and cabbage quality, and speed the harvest and field handling operation. Mustards and Chinese kale are harvested as single leaves or whole plants. Fields are usually harvested several times, so care is needed to prevent damage to the plants. The produce must be handled gently during harvesting and field handling to avoid physical damage.

Field Handling The harvested produce is usually placed in collection containers, which may be plastic crates or bamboo baskets with cotton or paper cushioning or padding (Chen, 2007). Throwing harvested produce into the collection container or vehicle should be avoided to prevent physical injuries. Handling aids such as boxes, farm trailer, or a simple conveyer can be used. Exposure of harvested produce to the heat of the sun is detrimental except in a few cases. Leafy vegetables left in the sun after harvest may reach temperatures as high as 50oC (Kanlayanarat, 2007). High product temperature accelerates quality deterioration due to increased water loss and respiration. If packed and transported without cooling, wilting and other deteriorative processes rapidly set in. Purposive water loss (2–3% water loss) may be imposed on harvested produce. In pak choi, this can be done by laying plants under the sun for 30 minutes immediately after harvest (Jiang and Pearce, 2005). This was found to significantly reduce mechanical damage (snapping of turgid outer leaves) when the produce was packed into bamboo baskets. Subsequent washing to remove dirt was able to rehydrate the produce. Wilted pak choi could be re-hydrated (and cooled) by dipping in water and the general appearance, color, and original weight could be restored if moisture loss was less than 10%. Rehydration and controlled water loss led to a reduction in losses of 14.5%. In Cambodia, the practice of leaving cabbage heads in the field for an hour or two with the cut butt end exposed to the sun may also work for the above purpose (Vanndy and Buntong, 2007). Additionally, this practice would dry out the cut butt end of the cabbage head, thereby depriving soft rot pathogens of water needed for their growth and development. However, the problem of heat accumulation within the produce has to be addressed. After the treatment, prompt transport to the packing shed should be done to dissipate field heat without the use of water for cooling. Washing is not advisable in common cabbage. Other leafy vegetables should be transported to the packing shed as soon as possible as they are particularly susceptible to wilting and other damage from high temperatures.

8

AVRDC-ADB Postharvest Projects RETA 6208/6376

Packinghouse Operations Trimming Leaves that have yellowed, show signs of disease, or have other obvious defects, and some outer or wrapper leaves of common and Chinese cabbages are removed (Fig. 3). Removal of the four outer leaves of pak choi heads increased shelf life to over 14 days (Jiang and Pearce, 2005). In Chinese cabbage, farmers may remove all the outer leaves before transport to market. This is a traditional practice of some Chinese farmers, in which the outer nine leaves of Chinese cabbage are removed at harvest. A second trim of three leaves is made to remove mechanically damaged leaves following transport. A simple change to this practice by removing only six leaves at harvest and leaving the other three outer leaves to protect the head from physical injury was found to reduce losses by 22% without any decline in product quality and increase the profits of the farmers. In common cabbage, outer leaves (wrapper leaves) are also trimmed off except for 3-4 wrapper leaves to protect the head from injury during handling and transport (Bautista and Acedo, 1987). However, wrapper leaves could not fully protect the head from too much force due to impact or compression, which usually results in head bursting. Care during handling is important.

Figure 3. Trimming, cleaning, and bundling leafy vegetables.

Postharvest Technology of Leafy Vegetables

9

Table 1. Chinese cabbage grade standards in Yunnan, China (Chen, 2007) Head Grade Quality Specifications Tolerance Size/Weight (kg) Same variety; head tight or firm, of natural color and luster, fresh, Rotting within 1%; clean, other defects not First and without rot, old stem, yellow over 5% leaf, peculiar smell, bolting, wormhole and physical defect Extra big ≥4.0 Same variety; head tight or firm, Rotting within 1%; Big ≥3.5 of natural color and luster, fresh, weight differences Medium ≥2.5 clean, and without rot, old stem, within 10%; other Second Small ≥1.5 yellow leaf, peculiar smell, defects not over Extra small ≥1.0 bolting, wormhole and physical 10% defect Same variety; head loose, of Rotting within 1%; natural color and luster, fresh, weight differences clean, and within 10%; other Third without rot, old stem, yellow leaf, defects not over peculiar smell, bolting, wormhole 10% and physical defect In Yunnan, China, leafy vegetables are usually trimmed using a special knife (Fig. 3) (Chen 2007). Damaged and senescent leaves are removed and for some vegetables (e.g. cabbage, Chinese cabbage), the butt is trimmed. For leaf mustards, the roots are usually retained and cleaned by washing together with the leaves. The cleaned produce is wrapped or bundled before packaging (Fig. 3).

Sorting/Grading Systematic sorting or grading coupled with appropriate packaging and storage, will extend shelf life, maintain wholesomeness, freshness, and quality, and substantially reduce losses and marketing costs. Sorting is done to separate poor produce from good produce, and further classify the good produce based on other quality parameters, such size (Bautista and Acedo, 1987). If this is done following quality standards set by product standards agencies or industry requirements, the process is referred to as grading. Leafy vegetables are usually sorted or graded based on maturity, size, shape, color, weight, and freedom from defects such as insect, disease and mechanical damage. Table 1 shows sample grade standards for Chinese cabbage in Yunnan, China. In many developing countries, implementation of grade standards as well as safety standards for leafy vegetables and other fresh horticultural produce faces formidable difficulties that contribute to the lingering problem of high postharvest losses. Grade standards, if enforced properly, are essential tools of quality assurance during marketing. They provide a common language for trade among farmers, handlers, processors, and marketers; maintain orderly marketing and equity in the marketplace; and protect consumers from inedible and poor quality produce (Kader, 2006).

Washing and sanitizing Most leafy vegetables are washed in clean water to remove dirt and other debris and surface contaminants. This is especially important during rainy weather as the produce often is contaminated with soil. In heading type of cabbages, washing is not advisable as it could favor bacterial soft rot if the heads are not properly dried. The wrapper leaves also keep the inner edible part clean. Sanitation is essential to control the spread of diseases from one item to another and limit the pathogen load in wash water or in the packinghouse air. Waterborne microorganisms, including postharvest plant pathogens and agents of human illness, can be rapidly acquired and taken up on plant surfaces (Kader, 2006). Natural plant surface contours, natural

10

AVRDC-ADB Postharvest Projects RETA 6208/6376

openings, harvest and trimming wounds can be points of entry and provide safe harbor for microbes. Chlorine in the form of sodium hypochlorite (NaOCl) solution (e.g. Chlorox or commercial bleach) or as a dry, powdered calcium hypochlorite, can be used in wash water as a disinfectant. For the majority of vegetables, chlorine in wash water should be maintained in the range of 75–150 ppm (Suslow, 1997; Bachmann and Earles, 2000). The antimicrobial form, hypochlorous acid, is most available in water with a neutral pH (6.5 to 7.5). Concentrations above 200 ppm may injure some vegetables (e.g. leafy greens and celery) or leave undesirable off-flavors. A 100 ppm chlorine solution can be prepared by mixing 4 tablespoons of commercial bleach (5.25% NaOCl) per gallon of water (Bautista and Acedo, 1987). Chlorine is routinely used as a sanitizer in wash, spray, and flume waters used in the fresh fruit and vegetable industry (Beuchat and Ryu, 1997). Antimicrobial activity depends on the amount of free available chlorine (as hypochlorous acid) in water that comes in contact with microbial cells. Chlorinated water can also be used during hydrocooling, and to disinfect packinghouse, packaging and transport facilities. Furthermore, use of sanitized wash water can help to prevent postharvest diseases and food-borne illnesses. E. coli 0157:H7, Salmonella, Cryptosporidium, Hepatitis, and Cyclospera are among the disease-causing organisms that have been transferred via fresh fruits and vegetables. A standard procedure for washing lettuce leaves in tap water was reported to remove 92.4% of the microflora (Adams et al., 1989). Including 100 ppm available free chlorine in wash water reduced the count by 97.8%. Adjusting the pH from 9 to 4.5-5.0 with inorganic and organic acids resulted in a 1.5- to 4.0-fold increase in microbiocidal effect. Increasing the washing time in hypochlorite solution from 5 to 30 minutes did not decrease microbial levels further, whereas extended washing in tap water produced a reduction comparable to hypochlorite. The addition of 100 ppm of a surfactant (Tween 80) to a hypochlorite washing solution enhanced lethality but adversely affected sensory qualities of lettuce. Hydrogen peroxide (food grade) also can be used as a disinfectant. Concentrations of 0.5% or less are effective for inhibiting development of postharvest decay caused by a number of fungi (Bachmann and Earles, 2000). Hydrogen peroxide has a low toxicity rating and is generally recognized as having little potential for environmental damage. Ozone as a sanitizer may not be practical in developing countries because of its high cost.

Other commodity treatments Rehydration by dipping in clean water or water added with chlorine can be done to restore the fresh and turgid appearance of some leafy vegetables such as Chinese kale, kangkong, and mustards. Common cabbage and Chinese cabbage can be applied with antibacterial treatments to control bacterial soft rot. The use of saturated alum solution and lime paste has been found very effective in controlling soft rot in common cabbage (Bautista and Acedo, 1987). Alum has a two-fold function to control bacterial soft rot: as an antimicrobial agent by direct kill, and as a moisture-withdrawing substance that deprives the bacterial pathogens of water. However, alum is phytotoxic and causes black spotting on affected leaf tissues, thus care must be taken to apply alum only on the butt end of cabbage. On the other hand, lime is only moisturewithdrawing and usually cannot control soft rot if the pathogen already has entered into the tissues through wounds. The use of alum is now a commercial practice of common cabbage growers in the Philippines. Lime paste is used by commercial cabbage growers in Indonesia. In Thailand, lime (CaCO3) paste is used in commercial packinghouses for Chinese cabbage (Kanlayanarat, 2007). The paste is applied at the butt end and allowed to dry before packing. Other potential low-cost techniques to control bacterial soft rot in cabbages include the use of botanicals or leaf extracts from plants that are known to be edible or consumed as medicinal plants. One effective treatment is the use of guava leaf extract, the effect of which in common cabbage and Chinese cabbage was comparable to alum treatment (Acedo et al., 1999, 2003;

Postharvest Technology of Leafy Vegetables

11

Acedo and Capuno, 2004). Cabbages treated with guava leaf extract were free of bacterial soft rot, while untreated heads all developed soft rot symptoms after pathogen inoculation, which resulted in trimming losses of more than 30%. Other plants known to be edible or medicinal, such as oregano, milkweed (local medicinal plant used to contain dengue fever) and lemon grass were less effective. After treatment, the applied extract or substance (alum or lime) requires proper drying before packing.

Packaging Proper packing is essential to maintain the freshness of leafy vegetables. Packaging should be designed to prevent premature deterioration in product quality, in addition to serving as a handling unit (Bautista and Acedo, 1987). Packages should prevent or reduce physical injury during transit and handling, provide ventilation to hasten cooling and escape of heat caused by respiration, and reduce water loss from the produce (Gast, 1991). Some packages promote sale of the produce.

Produce packages Different kinds of containers are used for leafy vegetables, depending on the market and value of the produce. For export and high-value leafy vegetables, more rigid and presentable but expensive containers are used, such as foam box and cartons (Fig. 4). For the local markets, bamboo baskets of different sizes and shapes are used. Rigid containers (plastic or wooden crates, cartons) are far much better than non-rigid containers (mesh bags, plastic bags) for protection of produce from damage during handling and transport. Rigid packages are also easier to stack or palletize. The different packages are described as follows: Basket: Usually refers to containers made of woven materials, which may be bamboo, rattan or plastic strips. Box: Usually refers to containers made of corrugated fiberboard or Styrofoam. It may be a two-piece telescoping box, or a carton that closes with top flaps. The contents can be place-packed with liners and layer dividers, or bulk-filled. Crate: Usually refers to a wooden or plastic container. Wooden crates are usually wire-bound and may be collapsible. Plastic containers, a relatively new type of container, have good stacking strength and are water-resistant. Plastic crates for handling and transportation of vegetables wer introduced recently in some developing countries. In Nepal, plastic crates are increasingly used by farmers and traders, particularly in situations where their return and reuse can be guaranteed; the crates have been reported to reduce postharvest losses and improve quality and safety of vegetables (Adhikari, 2006). In Sri Lanka, losses of vegetables were reduced from 30% with the use of poly sacks to 5% with the use of plastic crates (Fernando, 2006).

Reinforcing and handling packages Telescoping construction, dividers, and corner reinforcement are ways that boxes have been made stronger. Container liners and cushions minimize physical injuries. Containers need to be vented to effectively lower and maintain produce temperature for storage (Gast, 1991). Vents allow cold air to be forced more quickly through the containers and produce. Vents also allow the heat built up by respiration to escape. Produce exposed to high temperatures in unvented containers will usually have a shorter shelf life. A well-made container has uniform venting, so when it is stacked the vents will match other containers. Matching is important so cold air can be pulled through a whole stack of containers. Too much venting weakens a container, while too little venting restricts the airflow through it. A good rule of thumb is to have 5% of the container sides and/or ends vented. A few large vertical vents are better than many small round ones.

12

AVRDC-ADB Postharvest Projects RETA 6208/6376

A container with liners and vertical dividers will eliminate the beneficial effects of vents. An alternative way is to wrap the produce individually. Wrapping produce reduces vibration and impact damage. Old newsprint and brown paper can be used as wrapping materials. Some vegetables such as mustard greens and kale may be bundled before wrapping and packing. Palletization or unitized handling (stacking containers on standard size pallets) reduces the number of times an individual container is handled, and reduces damage to the contents. Container sizes should fit standard pallet sizes.

Figure 4. Packaging containers for leafy vegetables.

Postharvest Technology of Leafy Vegetables

13

Figure 5. Protective packaging practices. In GMS countries, some techniques to reduce damage and improve shelf life of packed leafy vegetables include the use of newsprint or sack liners and cotton-cloth covers, use of paper and stretchable foam cups to wrap cabbages individually or in groups, and special arrangements of Chinese kale or mustard inside the container to protect the leaves from damage and water loss (Fig. 5) (Buntong and Vanndy, 2007; Chen, 2007; Thanh, 2007).

Modified atmosphere packaging (MAP) MAP is very effective in retaining freshness and extending shelf life of fresh produce by maintaining the green color, inhibiting water loss, reducing loss due to product respiratory heat, and maintaining the natural fresh taste of produce. MAP is exemplified by the use of polymeric film as packing material, which can be employed during transport and storage. Plastic films can be used to pack specific volumes of produce, as individual wrapping, or as container liners (Fig. 6). Low density polyethylene (PE) film is generally used for packing fresh vegetables and fruit owing to its high permeability and softness compared with high density PE (Somjate, 2006). PE can be sealed easily, has good O2 and CO2 permeability, low temperature durability, good

14

AVRDC-ADB Postharvest Projects RETA 6208/6376

tear resistance, and good appearance. It is therefore used for the production of MAP, which can be manipulated to match the characteristic respiration of produce by reducing O2 levels to slow down the rates of respiration and senescence. However, high-density PE also has been found to markedly reduce water loss of produce (Ben Yehoshua, 1978). Since PE bags are non-rigid, product volume per bag should be limited. In a study on packing 5, 10, and 15 kg Chinese kale per PE bag, it was found that losses due to weight loss, bruising and trimming increased with increasing product volume and were about 5.6%, 6.9% and 13.1%, respectively (Amuttiratana and Passornsiri, 1992). Plastic film packaging in pak choi effectively reduced moisture loss and wilting and was considerably more effective than manual misting or treating leaves with anti-transpirant chemical (O’Hare et al., 2001). Plastic packaging maintains a very high RH, which necessitates sanitary washing before packing to avoid bacterial rot. In another MAP trial under supermarket conditions (ambient temperature of 28°C) using plastic film wrap (clingwrap), it was found that semi-packed pak choi (two-thirds of leaves exposed) performed better than fully packed and non-packed pak choi (Jiang and Pearce, 2005). Although the fully packed produce had less water loss, it tended to have more rot. Supermarkets preferred the semipack option from an aesthetic standpoint, as the fully packed produce tended to fog due to moisture condensation. In Chinese cabbage, plastic film wrap was similarly effective in reducing moisture loss from outer leaves. However, rot develops if the heads are mechanically damaged. In Thailand, commercial supermarket MAP practice for cabbage and Chinese cabbage includes the use of perforated plastic bags (4-8 holes at 5 mm diameter) or individual o wrapping with polyvinyl chloride (PVC) film at shelf temperature of 7 C (Kanlayanarat, 2007). For Chinese kale, perforated plastic bags (4-8 holes) or PE bags with one open end are used. If perforated plastic bags are used, the number of holes should not be excessive, as the leafy vegetable will still easily wilt. If the number of perforations is too few, water will condense at the surface of the plastic bag, favoring disease development.

Cooling and Storage Cooling is the foundation of produce quality protection. It extends shelf life by reducing the rate of physiological change (i.e. rate of respiration and transpiration) and retarding the growth of spoilage microorganisms. Because every degree of reduction from ambient temperature increases storage life, every form of cooling is beneficial, even if it is not optimum; simple lowcost cooling or refreshing the product is better than no cooling at all. Ways of cooling fresh produce include (1) keeping out of direct sun; (2) using natural cooling, such as harvesting during the cool early morning hours, opening stores for ventilation during the cool of the night, using the cool temperature of high altitudes or a natural source of cold water when available; (3) evaporative cooling obtained by drawing dry air over a moist surface; (4) mechanical refrigeration; and (5) cooling promptly after harvest by appropriate precooling methods. Some cooling and storage recommendations and simple techniques are described by Kitinoja and Kader (2003).

Postharvest Technology of Leafy Vegetables

15

Figure 6. MAP practices for leafy vegetables. Table 2. Postharvest cooling methods and suitable commodities (Sullivan et al., 1996). Cooling method

Commodities

Hydrocooling

Most leafy vegetables, fruits Very fast cooling; uniform cooling in bulk if properly used, and fruit-type vegetables, but may vary extensively in packed shipping containers; sweet corn, snap beans daily cleaning and sanitation measures essential; product must tolerate wetting; need water-tolerant shipping containers

Forced-air cooling (pressure cooling)

Most fruits, berries, fruittype vegetables, tubers, and vegetables not susceptible to chilling injury

Much faster than room cooling; cooling rates uniform if properly used. Container venting and stacking requirements are critical to effective cooling. Economical and efficient.

Package icing

Most vegetables

Fast cooling; limited to commodities that can tolerate water-ice contact; water-tolerant shipping containers are essential. Economical and efficient.

Room cooling

All commodities

Too slow for many perishable commodities. Cooling rates vary extensively within loads, pallets, and containers.

Vacuum cooling Leafy vegetables, iceberg lettuce

Comments

Commodities must have a favorable surface-to-mass ratio for effective cooling. Causes about 1% weight loss for each 6°C cooled. A procedure that adds water during cooling prevents this weight loss, but equipment is more expensive, and water-tolerant shipping containers needed.

Transit cooling

16

Mechanical refrigeration

All commodities

Cooling in most available equipment is too slow and variable; generally not effective for field heat removal.

Top-icing and channel-icing

Most vegetables

Slow and irregular, top-ice weight reduced net payload; water-tolerant shipping containers needed.

AVRDC-ADB Postharvest Projects RETA 6208/6376

Precooling Precooling has been reported as among the most efficient quality enhancements for commercial producers and was found to rank as the most essential of the value-added marketing activities, especially if cold storage facilities are available (Sullivan et al., 1996). Research confirms that lowering the respiration rate of fresh vegetables is essential to preserving market quality and the most important technology for lowering respiration rates remains proper precooling of produce within hours of harvest. Proper precooling preserves product quality by: (1) inhibiting the growth of decay producing microorganisms; (2) restricting enzymatic and respiratory activity; (3) inhibiting water loss; and (4) reducing ethylene production. There are different precooling methods (Table 2) and among these, forced-air cooling and hydrocooling were found to be the most effective and economical in preserving optimum quality and increasing market life. Rapid cooling either by hydrocooling alone or in combination with package icing (ice packing) is essential to maintain the quality of leafy vegetables. Hydrocooling by dipping in cold water is simpler, but hydrocooled produce must be kept cool in order to prolong shelf life. Hydrocooling o o Chinese kale in 4 C water for 5–10 minutes prior to 7 C storage was found to reduce water loss and yellowing and extend shelf life (Kanlayanarat, 2007). In pak choi, ice packing is a cheap form of cooling to extend shelf life but has not been widely adopted because growers were seldom in a position to easily access the loose ice and plastic packing containers required, which would lead to additional costs. Furthermore, the effect of ice is transitory; without proper insulating material, it melts quickly and the temperature returns to near ambient (Jiang and Pearce, 2005). Some of these constraints have been addressed through the use of ice bottles (see below). Cabbages can be precooled to 400F before transport to improve shelf life and reduce rot (Sanders, undated).

Optimum storage conditions If produce is to be stored, it is important to begin with a high quality product. Damaged or diseased produce must be separated or discarded and containers must be well ventilated and strong enough to withstand stacking. Damaged produce will spoil and induce spoilage in the rest of the crop. Proper storage practices include temperature control, relative humidity control, air circulation and maintenance of space between containers for adequate ventilation, and avoiding incompatible product mixes. Temperature is the most important environmental factor that influences the deterioration of harvested commodities (Kader, 2006). The optimum storage temperature for most temperate or semi-temperate/subtropical leafy vegetables, such as many brassicas, is close to 0°C while o tropical produce, >10 C. Relative humidity (RH) can influence water loss, decay development, and incidence of some physiological disorders. Condensation of moisture on the commodity (sweating) for a long time favors decay development. For most leafy vegetables, RH requirement usually ranges from 90-98%. (Kader, 2002; Kader and Rolle, 2004). Optimum temperature is achieved by mechanical refrigeration. In refrigerated chambers, RH can be increased by (1) adding moisture (water mist or spray, steam) to air by humidifiers; (2) regulating air movement and ventilation; (3) maintaining temperature of the refrigeration coils within about 1oC of the air temperature; (4) providing moisture barriers that insulate walls of storage rooms and transit vehicles; or (5) wetting the floor. Cabbages and other semi-temperate brassicas can be stored at 0-2.5°C with 95-98% RH (Boyette et al, 1992; Cantwell and Suslow, 2006; Cantwell and Reid, 2006; Sanders, undated); for tropical produce (e.g. mustard greens, kangkong), a temperature above 10°C, usually 13°C, is recommended. Higher temperatures accelerate physiological deterioration and quality loss. In lettuce, higher storage temperature (10°C) hastened chlorophyll and carotenoid loss compared with low temperature (4°C) while anthocyanin and phenolics contents were unaffected (Ferrante and Maggiore, 2007). Chlorophyll-a fluorescence was used to rapidly and non-destructively determine the effects of storage time and temperature on lettuce leaf quality.

Postharvest Technology of Leafy Vegetables

17

Exposure to alternating cold and warm temperatures may result in moisture accumulation on the surface of produce (sweating), which may hasten decay. Different commodities stored together should be capable of tolerating the same temperature, RH and level of ethylene in the storage environment. High ethylene producers, such as ripe bananas, apples, cantaloupe, can stimulate undesirable physiological changes in color, flavor, and texture in ethylene sensitive commodities, such as leafy vegetables. A farmer who can cool and store produce has greater market flexibility because the need to market immediately after harvest is eliminated. The challenge, especially for small farmers, is the set-up cost.

Cooling methods in developing countries Use of ice. In developing countries, ice packing or top icing is increasingly used for leafy vegetables due to increasing availability of ice (Fig. 7). Ice packing can be used to cool vegetables during transport, distribution and storage. In tropical climates, the temperature in a box of leafy vegetables may increase to 35-40°C when sealed in the afternoon and transported the following morning. Ice packing can lower the temperature to 20-25°C (Huang, 2006). It was also found effective in reducing leaf yellowing, wilting, and trim loss. For vegetables sensitive to chilling injury or that are damaged when in direct contact with ice, such as amaranth, a layer of newspaper can be placed between layers of vegetables and ice. Thickness of the alternating layers of vegetables and ice depends on the type of vegetable, ambient temperature, and distance or the time to the market. During transport and sale at the market, the ice melts. Leafy vegetables are sprayed repeatedly with water, especially at destination markets, to maintain low temperatures and prevent wilting or softening. In Yunnan, China, ice bottles are used by commercial growers as a simple technique for cooling produce in containers and preventing direct contact of produce with the ice (Fig. 7) (Chen, 2007). A simple refrigerating unit also has been developed for fresh vegetables (Fig. 8). Evaporative cooling storage. Refrigerated facilities are expensive in terms of set-up and operational cost. Alternative storage methods are therefore important in developing countries and one of these is evaporative cooling storage. An evaporative cooler (EC) developed in India, the Zero Energy Cool Chamber, is an on-farm, low-cost, environmentally friendly cool chamber, was made from locally available material (Ahsan, 2006). Temperatures within the chamber were reduced by as much as 17–18°C, with more than 90% RH during peak summer periods. It increased shelf life and reduced wastage of fruit (banana, mango, oranges, limes, and grapes) and vegetables (tomato and potato). A similar zero-energy storage structure was developed in Nepal (Adhikari, 2006). The structure is constructed using brick and sand, rectangular in shape, and has dimensions of 75 cm x 50 cm x 75 cm. Its outer and inner walls made of bricks are separated by a 10 cm space filled with sand, which is frequently watered to maintain a temperature of 7–10°C and RH of >85%. It increased shelf life and reduced losses of vegetables including leafy type such as cabbage, capsicum, and leafy vegetables. In Sri Lanka, a low cost evaporative cooler was also developed (Fig. 9) and introduced to retail o traders (Fernando, 2006). Temperatures inside the cooler are 5-7 C lower than ambient and RH ranges from 90–95%. It has a capacity of 100 kg vegetables and can be used for temporary storage of unsold produce. It reduced losses from 20% to 5%.

18

AVRDC-ADB Postharvest Projects RETA 6208/6376

Figure 7. Ice packing and use of ice bottles for cooling leafy vegetables.

Figure 8. Simple refrigeration equipment for the storage of leafy vegetables.

Postharvest Technology of Leafy Vegetables

19

Figure 9. Low-cost evaporative cooler developed in Sri Lanka (Fernando, 2006). Other simple and low-cost evaporative cooling structures were described previously (Acedo and Thanh, 2006), some of which were tested by partners in Cambodia, Laos, and Vietnam for tomato and chilli storage. The box-type evaporative coolers with moist jute sack walling and the other with moist rice hull wall inserts, as described in this report, were found to be equally effective in inhibiting wilting and reducing weight loss, resulting in doubling of shelf life of pechay (Brassica napus var. chinensis) (Acedo, 1997). Intermittent exposure to light was found to reduce leafy yellowing, which is a problem of continuous holding inside the evaporative cooler. Later, postharvest lighting also was found to affect the shelf life of pak choi (Jiang and Pearce, 2005). Leaves stored at 10°C under normal fluorescent lighting had a shelf life of 10 days, compared with 8.2 days for leaves stored in the dark. However, high intensity lighting (metal halide and high-pressure sodium) reduced shelf life to about 6 days due to heat damage. Evaporative cooling principle can also be employed during packaging and transport. For example, Chinese kale dipped in water for rehydration is packed right away in the container while still wet to provide water for evaporation and cooling (Fig. 10). This technique can be used during transport of produce and after arrival at the destination market; however, the produce has to be taken out from the container because prolonged exposure to wet condition favors decay development. Covering the container with wet cloth (Fig. 10) can also cool the produce. In addition, the transport load can be transformed into an evaporative cooling and/or modified atmosphere chamber by lining it with wet cloth and/or plastic film (Fig. 10) (Chen, 2007).

20

AVRDC-ADB Postharvest Projects RETA 6208/6376

Figure 10. Application of evaporative cooling and/or modified atmosphere in packing and transport of leafy vegetables.

Transport Techniques Minimizing losses during transport necessitates special attention to vehicles, equipment, infrastructure, and handling. Fresh produce is transported using both refrigerated and nonrefrigerated vehicles. Non-refrigerated vehicles are generally open-sided trucks, with wire mesh frames. This type of transport is inexpensive and convenient, and usually is used in developing countries. Fresh produce must not be watered prior to loading, as this will lead to decay, rotting, and extensive losses. Major causes of losses are improper handling during loading and unloading; over loading without separation of produce, which leads to overheating and mechanical injury to produce at the bottom of the stack; rough roads; and lack of

Postharvest Technology of Leafy Vegetables

21

ventilation of the produce. Postharvest operations must address these problems. Refrigerated transport facilities become necessary for fresh produce destined for niche and international markets. For maximum shelf life and quality, leafy vegetables should not be stored or transported in trucks where there are mixed loads containing ripening fruit such as apples, pears, mangoes, tomatoes, or bananas. Because of their soft and delicate texture, leafy vegetables should be handled gently to minimize mechanical injury. Stacking of containers in the transport load provides one of the major challenges to reduce mechanical injury. This is a particular problem for vegetables such as cabbages packed in baskets or big plastic bags (Fig. 11). An innovative but very simple technique to reduce damage due to compression of produce is the provision of wooden dividers separating the layers of containers in the transport load (Fig. 11).

Market Handling At destination markets, the produce may again undergo cleaning, sorting, packing and storage. Most postharvest measures described above could be applied at this stage.

Other PHT Developments Ethylene removal from postharvest chain Ethylene, being a senescence hormone, is destructive to the quality of leafy vegetables as it accelerates different deteriorative processes. Ethylene removal from the postharvest chain, therefore, has far-reaching benefits. Preventing ethylene buildup around the product in packages and during storage, transport, and marketing is often the simplest method of reducing the damaging effects of ethylene. For ethylene-sensitive products such as leafy vegetables, it is important to avoid storing them with products that produce high levels of ethylene. Increasing the ventilation rate of the storage area is another way of reducing ethylene around fresh produce. Ethylene can be removed through different chemical processes. Potassium permanganate is usually used because it reacts with ethylene to produce carbon dioxide and water. To scrub the air efficiently, it is best to spread the potassium permanganate over as large a surface area as possible, either in trays or within highly permeable bags. An ethylene scrubber made of potassium permanganate impregnated onto clay-ash chip (a propriety Philippine product) has been developed. Ayoub et al (1987) also tested ethyleneabsorbing blankets containing alumina coated with potassium permanganate in two mixed loads of fruits and vegetables in two marine containers shipped from California to South Korea. The total produce lost in the container without ethylene scrubbing was 2,645 lbs (out of 16,070 lbs) valued at $928, which is much higher than the $160 cost of the ethylene scrubbers. The technical and economic feasibility of preventing ethylene damage was similarly demonstrated in lettuce using ethylene scrubber (Thompson et al., 1989) and by separating ethylenegenerating produce from ethylene-sensitive produce during transport (Jordan et al., 1987).

22

AVRDC-ADB Postharvest Projects RETA 6208/6376

Figure 11. Traditional and innovative stacking procedures for containers of leafy vegetables.

Cold chain system As with refrigerated storage, the cold chain system is not a low-cost technology, but is introduced here for future consideration as a joint government-private sector initiative. Cold chain systems preserve the freshness of produce from harvesting through marketing and delivery to the consumer and have a tremendous impact on fresh produce marketing (Ho, 2006). The economic impact of cold-chain systems is due to (1) increased consumer satisfaction as a result of improved freshness and keeping quality of produce; (2) price stabilization and continuity of supply; (3) reduced total marketing expenses due to reduced product losses; increased net quantities of fresh produce and reduced unit marketing and garbage disposal costs; and (4) improved quality and competitiveness of farm produce, thereby contributing to increased farmer income. Some developing countries, such as Indonesia and Philippines, are starting to adopt the cold chain approach and adapt it to their needs. In the Philippines, small vegetable growers in different villages of a highland province in Bukidnon (located in the southern islands collectively called Mindanao) penetrated institutional markets (e.g. a fast-food chain) in Cebu

Postharvest Technology of Leafy Vegetables

23

(one of the islands in the central part collectively called Visayas) and Manila (the country’s center located in the northern part collectively called Luzon) through clustering and application of the cold chain system (Rapusas, 2006). Previously, selling lettuce to local traders was difficult because of low prices and 25% of the weight deducted as an allowance for trimmings, despite the fact that the lettuce was of good quality. An alternative market was sought by the grower, who began supplying 200 kg of lettuce on a weekly basis to fast-food outlets in Cebu, and later, in Manila—the latter requiring air transport of 400 kg lettuce weekly. Apart from the high cost of air freight, lettuce delivered to the processor did not meet the 61% yield specified in the marketing contract, owing to the need for 16-20% trimming. Attaining the high quality standards of the fast-food processor was a formidable challenge for the grower. A further challenge was that of supplying a 20-foot refrigerated van with 3.5 metric tons of lettuce on a weekly basis. Clusters of lettuce growers were then formed and shared production technologies and quality standards. With the use of refrigerated transport, the trimmings were significantly reduced to a maximum of 10% and the processor’s yield recovery specification of 61% was successfully met. To further improve the lettuce supply chain, government agencies provided assistance in terms of equipment support—a 10-foot refrigerated truck, a 20-foot refrigerated van or container, and a precooler—to complete the cold chain system. Operational steps are as follows: (1) harvested lettuce heads are immediately brought to the packinghouse for cleaning (wiping with a cloth to remove soil and other dirt particles), sorting, air drying (about 2 hours) and anti-browning treatment (using citrus or “calamansi” juice, alum, or ascorbic acid applied to the cut portion of the lettuce); (2) selected heads are carefully arranged into nestable and vented plastic crates (11.5 kg capacity) with a brown paper lining for every two layers with each layer consisting of 12 heads; (3) cluster growers transport the packed produce to a consolidation area using a rented 20-foot refrigerated van, especially for growers located far from this area; (4) the consolidated packed produce is transported in refrigerated containers to a city pier for loading into the ship en route to Manila (shipping time from the consolidation area up to the buyer/processor takes 40 hours). The achievements of the five-grower lettuce cluster have provided the impetus for other independent, small lettuce growers to join the cluster. This development has given the cluster a window of opportunity to expand its production volume and, in turn, its captive market.

Supply chain management A system approach to producing and marketing fresh leafy vegetables is essential to raise farm productivity and profitability and ensure the sustainability and reliability of supply chains. Different supply chain systems exist in developing countries, and in general, they can be grouped into traditional and progressive supply chains. Traditional supply chain. Many supply chains that involve small farmers in rural areas fall under this category. Farmers are at the mercy of middlemen who usually dictate product prices and who may have contract-like agreements with the farmers. The middleman receives the crop and sells it in a wholesale market to wet markets and to supermarkets (Fig. 12) (Kanlayanarat, 2007). The farmer also may sell the crop directly to the market. This supply chain is a low-technology system, usually with no temperature control, and relies on selling the produce within one day after harvest. Product losses may be very high, particularly during adverse weather conditions. Progressive supply chain. The cold chain system described above is one example of progressive supply chains, which harness technological developments and market requirements to create marketing advantage and opportunities. Food corporations (e.g. supermarket chains) and multinational companies develop their own supply chain system to serve better the needs of their customers and therefore ensure profitability of their business ventures. In Thailand, for example, a food corporation supplies vegetables to its chain of supermarkets by getting supplies from its contract farmers, who grow vegetables following recommended production practices (Fig. 13) (Kanlayanarat, 2007). Pricing of farmers’ produce depends on prevailing market price. Depending on the crop, the

24

AVRDC-ADB Postharvest Projects RETA 6208/6376

harvested produce is sorted and packed on-farm and brought to the company’s headquarters for subsequent distribution. Other crops are brought to the company’s packinghouse for grading using the company’s quality standards, packing, and holding or storage. The packinghouse is located near the company’s headquarters and distribution center. If distribution cannot be done on the same day of arrival, storage is done in the company’s central cold room. The produce is distributed to supermarket outlets in refrigerated trucks and displayed on refrigerated shelves. Another Thailand initiative to improve leafy vegetable supply chain management in the northern highlands of Chiang Mai capitalizes on the cooperative system with outside support (Royal Project Foundation) (Kanlayanarat, 2007). The vegetables include cabbages and other temperate types produced following research-based recommendations. The farmers harvest their own vegetables and deliver them to 37 fully equipped collection centers (Fig. 14). The vegetables then go through the following processes: checking for quantity and quality; cleaning, trimming, checking for chemical residues such as fertilizer and pesticides (if found to be unsafe, the produce is rejected); packing; pre-cooling (if needed); and storage in cold rooms. Each postharvest center has fairly extensive infrastructure and equipment, such as conveyors, carts, crates, measuring devices, displays showing quality guidelines, and cutting and trimming devices. At least one pre-cooling facility is used. Crops are cooled in different ways. The center has also a cold room for storing perishable crops before transport. Small refrigerated trucks then collect the produce from each center and take it to the packinghouse in Chiang Mai. This is a large, central collection point usually employing more than a hundred people. Produce is processed and packed to a high standard of efficiency and hygiene. At the packinghouse, produce is checked for quality, trimmed, washed, checked for all chemical residues, and then packed again. Low grade or excess produce is usually sent for food processing. The packed produce is transported to Chiang Mai, Bangkok, or regional markets. Like the local postharvest centers, the packinghouse is fully equipped and has cold storage facilities. The packinghouse itself is temperature and humidity controlled to reduce crop wastage. Whether the produce is destined for Chiang Mai, Bangkok or regional markets, the produce is transported by large refrigerated trucks. When the produce arrives at the Bangkok Distribution Center, the delivery is again checked for quantity and quality, as some produce may have been damaged in transit. After quality check up, the produce is stored in cold rooms. From there, the produce is transported to Doi Kham Stores or to third-party retailers and wholesalers.

Figure 12. Traditional supply chain system for leafy vegetables.

Postharvest Technology of Leafy Vegetables

25

Figure 13. Corporate supply chain system for leafy vegetables.

Figure 14. Cooperative system of supply chain management. In Myanmar, a government initiated and owned corporation operates a supply chain for vegetables for export (Kyaw, 2007). It has a packinghouse equipped with three cold rooms each with 10-ton capacity, hydrocooling facility and other postharvest equipment including packaging area (Fig. 15). Packaging containers, such as cartons are fabricated and supplied by a contracted company. Upon arrival in the packinghouse, the vegetables produced by contract growers, such as lettuce and broccoli, are sorted, precooled with 1oC water containing a disinfectant, air-dried, packed in air-tight plastic bags before putting into cartons, stored in the cold room (if delivery within the day could not be done), and transported in refrigerated trucks for cargo flights.

Economic Analysis of Postharvest Technologies The development, introduction, and use of a certain technology have economic, environmental and social impacts (Jiang and Pearce, 2005). Economic impacts are usually changes in profitability due to higher demand and/or bigger markets, lower costs, higher yields, and/or better quality. Environmental impacts are effects on the natural system, such as reduced waste and pollution or improved environmental quality. Social impacts may include enhanced networking, empowerment of the most disadvantaged groups, recognition of gender contributions, and the development of human and social capital. Environmental and social impacts may be quantified in monetary terms, but in many cases they do not have market values. Some impacts cannot be realized immediately after an intervention has been developed or introduced. Only potential impacts can be evaluated and so, certain assumptions and projections have to be made.

26

AVRDC-ADB Postharvest Projects RETA 6208/6376

For postharvest technologies developed or technological recommendations introduced, economic impacts are mainly longer shelf life, better quality and/or lower losses of the produce, which lead to higher profitability. For these techniques, cost-benefit analysis can be performed and from this, the return on investment or the cost/benefit ratio can be estimated. Three examples are given below.

Example 1: postharvest technical advice The postharvest intervention was given as simple advice to retain 2-3 wrapper leaves in Chinese cabbage instead of removing all outer leaves as traditionally practiced. The technique reduced losses from 28.5% for the traditional practice to 6.3% for the introduced technique, which resulted to a net profit that far exceeded the additional cost (Table 3).

Example 2: postharvest technique from exploratory investigation The use of botanical extracts as alternative to alum for cabbage soft rot control was explored. Experiments were conducted and the most effective treatment (guava leaf extract, 1:1 extract:water ratio) showed complete control of the disease in contrast to 100% infection of untreated heads that resulted in trimming losses of 34.8%. (The cost and benefit are only potential.) The potential net benefit again far exceeded the cost of the technique (Table 4).

Example 3: introduction of better postharvest material The use of plastic crates as packaging material for vegetables was introduced and reduced losses to 5%, down from 30% for the usual practice of using polyethylene sacks (Fernando, 2006). Cost benefit analysis is shown in Table 5, which illustrates that using plastic crates can increase profitability.

Figure 15. Myanmar government-initiated supply chain for export vegetables produced by contract farmers and brought to the packinghouse for sorting, hydrocooling, packing in carton box, cold storage, and transport in refrigerated trucks for cargo flight.

Postharvest Technology of Leafy Vegetables

27

Table 3. Cost benefit analysis of keeping 2-3 outer leaves of Chinese cabbage during transport compared to removing all outer leaves (modified from Jiang and Pearce, 2005). Particulars Losses -2nd day -3rd day -Average Weight of Chinese cabbage -Wasted -Sold -Revenues @ 0.60 yuan/kg Cost -Transportation cost -Additional labor (loading etc.) -Total costs Net profit (revenues-total cost)

Unit

Traditional practice

% % % kg kg kg yuan

28.5 28.6 28.5 1000.00 285.35 714.65 428.79

yuan yuan yuan yuan

33.00 6.00 39.00 389.79

Keeping 2-3 outer leaves 6.2 6.4 6.3 1000.00 (1156 kg shipped) 62.00 938.00 562.80 37.50 6.00 43.50 519.30

1 USD = 8 Chinese yuan or RMB (Renminbi)

Table 4. Potential cost and benefit of using guava leaf extract and alum for bacterial soft rot control in common cabbage (Acedo et al., 1999). Particulars

No treatment Guava leaf extract

% Trimming losses due soft rot -Trial 1 -Trial 2 -Average Weight of Chinese cabbage, kg -Wasted -Sold -Revenues @ 30 pesos/kg Cost, pesos -Treatment cost (materials, labor) -Trimming cost (labor) -Total costs, pesos Net profit (revenues-total cost), pesos

30.9 38.6 34.8 1000 348 652 19,560

0 0 0 1000 0 1000 30,000

0 0 0 1000 0 1000 30,000

0 150 150 19,410

100 0 100 29,900

120 0 120 29.880

1 USD = 52 Philippine peso (PHP)

28

Alum treatment

AVRDC-ADB Postharvest Projects RETA 6208/6376

Table 5. Cost-benefit analysis of using poly sacks and plastic crates for transporting vegetables from the collecting center, Keppetipola to the central (Manning) market in Colombo, Sri Lanka (Fernando, 2006). Particulars 1) Capacity per truck load - Number of units transported - Average weight of vegetables per unit - Total capacity 2) Unit price of a package LKR 3) Lifespan of package 4) Farm-gate purchasing price, LKR 5) Transport cost LKR - Keppettipola to central market LKR - Return journey LKR 6) Handling charges LKR 7) Selling price of vegetables LKR

Use of poly sacks

Use of plastic crates

80 bags 50 kg 4,000 kg 30.00/bag 2 journeys 25.00/kg

125 crates 20 kg 2,500 kg 527.00/crate 240 journeys 27.00/kg

2,500.00 6.00 30.00

2,500.00 625.00* 6.00 37.00

2,400.00

65,876.00

Capital cost -Total cost of packages LKR Fixed cost -Depreciation of packages LKR Variable -Total transport cost LKR -Loading and unloading cost LKR -Cost of vegetables LKR

1,200.00

274.00

2,500.00 960.00 100,000.00

3,125.00 1,500.00 67,500.00

Total cost LKR Total revenue LKR

104,660.00 120,000.00

72,399.00 92,500.00

Net profit LKR

15,340.00

20,100.00

* Empty crates occupy 1/4 of the total truck capacity 1USD = 100 LKR. Losses of vegetables were reduced from 30% with poly sacks to 5% with plastic sacks.

Postharvest Technology of Leafy Vegetables

29

4 PROCESSING TECHNOLOGIES FOR LEAFY VEGETABLES Importance of Processing Processing is an important value-added activity that stabilizes and diversifies food supplies and creates employment and income opportunities. It can minimize the high perishability problem of leafy vegetables. Many advantages of processed products have been reported, particularly the extended availability of seasonal, perishable products beyond the growing season. Processed products are also more stable, have improved digestibility, and permit great diet diversity, giving consumers access to a wider choice of products and a better range of vitamins and minerals. Drying, salting, fermenting, and pickling leafy vegetables, the focus of this section, are the simplest processing methods that do not require sophisticated equipment. However, shelf life of salted or fermented vegetables is short, from several days to a few weeks. To increase shelf life, preservative solution is used, together with vacuum packing or bottling with heat processing.

Commodity Considerations Leafy vegetables are low in acid and sugar compared with most fruits and so, they vary in processing requirements for salting, fermenting, pickling, drying, packing, and freezing (Diamante, 2007). For example, fermented leafy vegetables are mixed with salt to promote the growth of lactic acid bacteria (LAB), which give the characteristic sourness to the product. Leafy vegetables need to be blanched (with water or steam) prior to drying, canning/bottling or freezing. The purpose of blanching is to inactivate the enzymes catalase and peroxidase, which cause deterioration of vegetables during further processing. Blanching must be properly done to preserve the green color (chlorophyll), otherwise chlorophyll is broken down to phaeophytin, which imparts brown color to over-blanched vegetables. Dehydrated leafy vegetables are usually dried to very low moisture (5% or less) to slow down deterioration during subsequent storage. Canned leafy vegetables are processed with high heat processing o (above 100 C) because of their low acidity compared with most canned fruits, which are acidic and require temperatures lower than 100oC. There is little difference in processing requirements for fruit and vegetables with respect to freezing.

Preprocessing Operations Washing Vegetables may be washed with water in three different ways: soaking, washing by agitation, and spraying (Diamante, 2007). Washing vegetables with water can be manual or mechanized, depending upon the scale of operation. Soaking is not in itself an effective means of removing dirt but it is useful as a preliminary treatment to washing by spray or agitation. If the vegetables are agitated in water, the efficiency of the soaking process is greatly enhanced. Washing by means of water spray is by far the most satisfactory method. Vegetables that are heavily contaminated with soil or other objectionable material should first be soaked thoroughly to loosen adhering soil before washing under spray. The efficiency of a spray of water for washing depends upon the pressure of the water, its volume, and also the distance of the spray nozzle from the vegetable to be washed. Spray washer pressures vary between 60 to 200 psi. The process removes most of the soil and many of the insects from the leaves.

30

AVRDC-ADB Postharvest Projects RETA 6208/6376

Cutting Leafy vegetables require different cutting methods. For example, cabbage heads may be sliced into quarters as in the case in Cambodia (Buntong and Vanndy, 2007). In some instances, cabbages for fermenting and drying are shredded into fine slices (1/32 to ¼ inch wide). Generally when the leafy vegetables readily soften after blanching, there is no need to cut or shred them prior to fermenting or drying. This is the case for leaf mustards as practiced in many Greater Mekong Subregion countries such as Yunnan, China (Hong, 2007), Laos (Chanthasombath, 2007), Thailand (Srilaong, 2007), and Vietnam (Thanh, 2007). However, there are fermentation procedures that require slicing of mustard leaves (Thanh, 2007).

Blanching Leafy vegetables must be heated to a minimal temperature to inactivate natural enzymes before processing or storing, even when processed as frozen product (Diamante, 2007). This special heat treatment to inactivate enzymes is known as blanching. Blanching is not indiscriminate heating. Too little is ineffective, and too much damages vegetables by excessive cooking, especially when the fresh character of the vegetable is to be preserved by further processing. Two of the more heat-resistant enzymes in vegetables are catalase and peroxidase. If these are destroyed, then other enzymes that contribute to deterioration will be inactivated also. Effective heat treatments for inactivating catalase and peroxidase in different vegetables are known, and sensitive chemical tests have been developed to detect the amounts of these enzymes that might survive the blanching treatment. o Cabbage for canning is water-blanched at 100 C for 1-1.5 minutes while cabbage for drying is steam-blanched for about 4-5 minutes. Spinach for canning is water-blanched at 76.7oC for about 6 minutes or at 100oC for 1.5 minutes, while spinach for drying is steam-blanched for about 3-5 minutes. For other leafy vegetables such as Chinese kale and Chinese mustard, blanching may be done in 100oC water for about 1.5 minutes or by steaming for 3-5 minutes. Other blanching techniques require water temperatures lower than 100oC.

Salting Technology Salting is done to draw water from the vegetables, impart a salty taste, inhibit or kill some of the microorganisms on the vegetables, and permit the survival of useful microorganisms. Useful microorganisms (e.g. LAB) produce acids and flavor compounds by fermentation of sugars in the vegetables. A salting technology for Chinese cabbage was developed in Australia to supply the Japanese market with a raw material for pickled vegetable (Thompson et al., 2001). This was done to avoid the use of complex packaging and storage requirements for live, fresh product. The value of whole, fresh vegetables often does not justify the use of quicker, but far more expensive, air transit. The salted Chinese cabbage produced by this technology conformed to Japanese market requirements and trial export shipments were well received. The technique involves the following procedure:    



o Cabbages are cooled to 5 C; overmature and damaged wrapper leaves are removed. The produce is cut into half longitudinally through the stem and leaf petioles, but not the majority of leaf-blade material. The halves are torn apart and immersed for 5 minutes in a 200–300 ppm sodium hypochlorite solution at 5oC. Following chlorine disinfection, the halves are rinsed in 5oC tap water and shaken to remove excess water before transfer to salting tubs (inner tub dimensions: length-upper 56 cm, base 52 cm; width-upper 36.5 cm, base 32.5 cm; depth-20 cm). During packing and layering into tubs, salt is sprinkled onto the stem/leaf-petiole area at the rate of 5% by weight.

Postharvest Technology of Leafy Vegetables

31

      

5% salt solution at 5oC is added to tubs at the rate of 15% cabbage weight. Cabbages are placed under pressure with surface weight for 16 hours at an air temperature of either 4oC or ambient (i.e. 15o to 25oC). The pressure employed at 4oC is 175% of total cabbage weight per tub, or approximately 1.2 kPa pressure. If higher room temperatures are used, the pressure should be higher, approximately 1.6 kPa (processors can determine if low temperature or high pressure is easier to achieve in their factory). After 16 hours, 25% of surface weight is removed and approximately 10% (by cabbage weight) additional volume of salt solution is added. After 4 days, the salt solution is drained and the cabbage halves are vacuum-sealed in barrier-film bags. The product is now ready for immediate consumption or additional processing. o Otherwise, storage should be continued at 4 C.

The abovementioned salting technology could be adapted by Asian countries, such as those in the Greater Mekong Subregion, that have a competitive edge in serving international markets such as Japan and South Korea.

Fermentation Technologies Fermented vegetables are foods in which the acid is produced from sugar in the food product by fermentation with lactic acid bacteria (LAB) (Diamante, 2007). Nearly all vegetables can be fermented by LAB. They contain sugars and are nutritionally adequate as substrate for growth of LAB and other microorganisms. Relatively few species of bacteria are responsible for the fermentation of the majority of vegetable products. They develop in a natural sequence of species. The relative role of each species is governed primarily by environmental conditions. Lactic acid fermentation enhances the nutritional value of a food product through increased vitamin levels and improved digestibility. It is extremely important in meeting the nutritional requirements of a large proportion of the world’s population.

Basic information Kyi (2007) reviewed some basic information on the fermentation process: Desirable fermentation. It is essential with any fermentation process to ensure that only the desired bacteria, yeast or mold start to multiply and grow on the substrate (the produce). These organisms suppress other microorganisms, which may be pathogenic, may cause food poisoning, or may spoil the fermentation process, resulting in an end product which is neither expected nor desired. Most food-spoilage organisms cannot survive in either alcoholic or acidic environments. Therefore, the production of both these end products can prevent food from spoilage and extend shelf life. Lactic acid fermentation is carried out under three basic types of conditions: dry salted, brined, and non-salted condition. Salting provides a suitable environment for LAB to grow, which imparts the acid flavor. Any variety of common salt is suitable as long as it is pure; impurities or additives can cause problems. Salt with chemicals to reduce caking should not be used as they make the brine cloudy. Salt with lime impurities can reduce the acidity and shelf life of the final product; iron impurities can result in blackening of product; magnesium impurities impart a bitter taste; and carbonate impurities cause soft texture of product. Dry-salted fermentation. Dry salt is used and extracts the juice from the vegetable, producing brine. The vegetable is prepared, washed in potable cold water, and drained. For every 100 kg of vegetables, 3 kg of salt is needed. The vegetables are placed in a layer about 2.5 cm deep in a fermentation container. Salt is sprinkled over the vegetables. Another layer of vegetables is added and more salt is added. This is repeated until the container is threequarters full. Cloth is placed over the vegetable and a weight is added to compress the vegetable and assist in the formation of brine, which takes about 24 hours. As soon as the brine is formed, fermentation states and bubbles of carbon dioxide begin to appear.

32

AVRDC-ADB Postharvest Projects RETA 6208/6376

Fermentation takes between 1-4 weeks, depending on the ambient temperature. Fermentation is complete when no more bubbles appear, after which the product can be mixed with vinegar, oil, and spices. Brine-salted fermentation. Brine is used for leafy vegetables that contain less moisture. A brine solution is prepared by dissolving salt in water (15-20% salt solution). Fermentation takes place well in a brine of about 20° salometer. As a rough guide, a fresh egg floats in a 10% brine solution. Properly brined vegetables will keep well in vinegar for a long time. The duration of brining is important to the overall keeping quality. The vegetable is immersed in the brine and allowed to ferment. The strong brine solution draws sugar and water out of the vegetable, decreasing the salt concentration. It is crucial that the salt concentration does not fall below 12%, otherwise the condition does not allow for fermentation. To achieve this, extra salt is added periodically to the brine mixture. Once the vegetable has been brined and the container sealed, microorganisms will rapidly develop in the brine. Natural controls that affect microbial population during fermentation include the concentration of salt, temperature of the brine, availability of fermentable materials, and the numbers and types of microorganisms present at the start of fermentation. The duration of fermentation is correlated with the concentration of salt in the brine and its temperature. Most vegetables can be fermented at 12.5-20°C. At higher salt level of up to about 40°C salometer, the sequence is skewed towards the development of homo-fermentation, dominated by Lactobacillus plantarum. At the highest concentration of salt (about 60°C salometer), lactic fermentation ceases to function and if any acid is detected during brine storage, it is acetic acid, presumably produced by acid-forming yeasts that are still active at this salt concentration. Non-salted fermentation. Some vegetables are fermented by LAB without prior addition of salt or brine. Non-salted products include gundruk (consumed in Nepal), sinki, and other wilted fermented leaves. The fermentation process relies on the rapid colonization of produce by LAB, which lower the pH and make the environment unsuitable for growth of spoilage organisms. Oxygen is also excluded. Restriction of oxygen ensures that yeasts do not grow.

Sauerkraut processing Sauerkraut or kraut, originally a German fermented cabbage, is prepared from sound, well matured common cabbage heads (Diamante, 2007; Sasitorn, 2007). Cabbages with 24% total sugars are suitable for sauerkraut production. Generally, good quality raw material contains up to 30-60 mg/100 g of vitamin C. Sauerkraut can be prepared whole or shredded. However, shredded sauerkraut is more common for the industry due to its good quality and uniform fermentation. The sauerkraut processing flow is shown in Figure 16. After delivery, cabbages are transported via conveyer to the coring machine. Then the cored head is trimmed (removing outer leaves and bad spots) and cut/shredded before transport to fermentation vat. Salt (23%) is added evenly as the shredded cabbages are distributed in the vat. This allows the product to be cured by natural fermentation. Immediately after adding salt, juice is released from cabbage. This “early brine” may be withdrawn from the vat to assure a maximum filling of cabbage into the vat. After the vat is filled, it is covered with a plastic sheet that is weighed with water. After four or more weeks, the fermentation is considered complete, when the finished product contains not less than 1.5% of acid, expressed as lactic acid. Fermentation temperature is at 20-25°C in the first phase and needs to be lowered then to 14-18°C. In the early stage of fermentation, most LAB are hetero-fermentative (gas-forming) species such as Leuconostoc mesenteroides. The carbon dioxide creates an anaerobic environment that promotes the desirable lactic acid bacteria. After 8 days of fermentation, most LAB are homofermentative (non-gas-forming) species such as Lactobcillus plantarum. Other LAB species in sauerkraut production are Lactobcillus brevis and Pediococcus cerevisiae.

Postharvest Technology of Leafy Vegetables

33

Cabbage head Trim green, broken, and dirty leaves Remove core Shredding Add salt on the shreds Place shreds in a jar Cover jar tightly (airtight) Ferment Sauerkraut

Figure 16. Process flow for sauerkraut production (Diamante, 2007; Sasitorn, 2007). Shelf life of the finished product after completion of fermentation (i.e. after the estimated six weeks period) depends on the storage temperature. The finished product shelf life at 15°C is about six months while at 20°C, 2-3 months. The product maybe canned using sufficient thermal processing to assure preservation in hermetically sealed containers (cans, jars, or flexible pouches), or it may be packaged in sealed containers and preserved with or without the addition of sodium benzoate or any other ingredient approved by food safety authorities. At small scale and in traditional processing, shredded sauerkraut can be obtained by using simple available glass jars or rigid receptacles (plastic jars, earthenware jars) from a minimum size of 2-3 kg up to practical commercial size of 10-15 kg. For whole sauerkraut production, prepared whole cabbages are put into fermentation vats and 5-6% salt concentration brine is poured on top. The fermentation conditions are the same for shredded sauerkraut. To assure a uniform fermentation and to avoid a strict anaerobic (butyric) fermentation, it is necessary to apply "aeration" by turning over the cabbages every 2-3 days at the beginning of the fermentation, and then every 5-7 days thereafter. In some countries, sauerkraut juice is produced for its dietetic value (lactic acid and vitamin C content) and its refreshing taste. The juice, which is the result of the fermentation of lactic acid from cabbage mainly from sliced sauerkraut, is used. The juice must be the result of a normal lactic fermentation, i.e. without butyric fermentation or other deterioration. A good quality juice must have an acidity of 1.4% lactic acid and a content of maximum 2.5% salt. This is obtained by mixing various sauerkraut qualities. The collected juice is heated slightly to eliminate CO2 gas and to obtain protein coagulation. Filtration of juice is the next step, followed by filling in container, closing and pasteurization at 75-80°C for 4-5 minutes.

34

AVRDC-ADB Postharvest Projects RETA 6208/6376

Kimchi processing Kimchi is a Korean fermented Chinese cabbage, which is usually sliced thicker than sauerkraut (Diamante, 2007; Sasitorn, 2007). It is probably the most important processed food product in Korea as it is eaten with most meals. Production is estimated at over one million tons, mainly at household level. Daily consumption is estimated at 150 to 250 grams per person. There are almost 200 types of kimchi available in the market. However, the most popular type of kimchi is baechu kimchi, which is made from Chinese cabbage. Baechu kimchi can be prepared using whole cabbage (tongbaechu kimchi) or chopped/cut cabbage (matbaechu kimchi). Essential processing steps that can affect kimchi quality are selection and formulation of raw ingredients, salting, rinsing and draining, pretreatment of subingredient and mixing process, placing in fermentation vessel, and fermentation (Fig. 17) (Diamante, 2007; Sasitorn, 2007). Good quality Chinese cabbage, with light-green colored soft leaves and compact structures with no defects, are required for production of kimchi. After grading and washing, the cabbage is cut lengthwise into 2 to 4 parts and treated with dry salt for several hours or with 10% brine for 10 hours. For chopped kimchi, the cabbage is cut 3-5 cm and macerated in 8-15% salt solution for 2-7 hours. Maceration is the most important step for taste, texture, fermentation, and preservation. The macerated cabbage is rinsed several times with fresh water to remove excessive salt and drained to remove extra water. A pre-mixture of spices and other ingredients (Table 6) are packed between layers of cabbage leaves. The stuffed cabbages are placed in a jar (crock) to allow facultative anaerobic condition for fermentation. Traditionally, to maintain consistent low temperature, the crock is buried underground up to the neck and the lid is covered with straw until the kimchi is needed. Kimchi fermentation is carried out by various microorganisms present in the raw materials and ingredients used, especially LAB. Among the 200 bacteria isolated from kimchi, the important microorganisms in fermentation are Lactobacillus plantarum L., Brevis, Streptococcus faecalis, Leuconostoc mesenteroides and Pediococcus pentosaceus. After fermentation, the product can be left to mature for several weeks if refrigeration is available. If stored under warm conditions, the kimchi deteriorates rapidly. To assure good quality of kimchi, the final salt concentration of the product is adjusted to 2.2-3.0% (w/w).

Fermented mustards and cabbage Fermented mustard leaf is a well-known indigenous fermented food product found in many countries including GMS countries. In Taiwan, most of harvested leaf mustards are dry-salted in wells or vats for fermentation to prepare the pickle product. After fermentation, the yellowish pickles with crispy texture and sound and pickle flavor, are called Hum-choy. General processing steps of leaf mustard pickles are shown in Figure 18 (Sasitorn, 2007). The mature leaf mustard are washed, cut, and wilted in the sun for a day. Then they are trimmed and placed in fermentation wells or vats. Prior to deposition of the first layer of leaf mustard in an upright position, dry salt is spread at the base of the well. For the following layer, the leaf mustard is deposited at inverted position and each layer is spread with dry salt and pressed tightly. At the top of the well, the leaf mustard is covered with a heavy-duty plastic film and weights. After 3 days, water is drawn out of vegetable and the level of leaf mustard is lowered. Further deposition of mustard and salt are repeated 2-3 times and finally covered and sealed with a heavy-duty plastic film and pressed with stones for long term fermentation (2-6 months).

Postharvest Technology of Leafy Vegetables

35

Chinese cabbage Remove green, broken and dirty leaves Remove core Slice

Dissolve salt in water

Cabbage slices

Mix cabbage slices in a jar

Cover jar tightly Drain the brine solution Other ingredients

Mix cabbage with other ingredients

Ferment Kimchi

Figure 17. Process flow for Kimchi production (Diamante, 2007; Sasitorn, 2007). Table 7. Sample recipe for Kimchi production (Diamante, 2007; Sasitorn, 2007). Particular Description A. Composition B. Process 3 tablespoons plus 1 teaspoon pickling salt 6 cups water 2 lbs Chinese cabbage, cut into 2-inch squares 6 scallions, cut into 2-inch lengths, then slivered 1 1/2 tablespoons minced fresh ginger 2 tablespoons ground dried hot pepper (or other mildly hot ground red pepper) 1 teaspoon sugar

1. Dissolve the 3 tablespoons salt in water. Put the cabbage into a large bowl, a crock, or a nonreactive pot, and pour the brine over it. Weigh the cabbage down with a plate. Let the cabbage stand for 12 hours. 2. Drain the cabbage, reserving the brine. Mix the cabbage with the remaining ingredients, including the 1 teaspoon salt. Pack the mixture into a 2quart jar. Pour enough of the reserved brine over the cabbage to cover it. Push a freezer bag into the mouth of the jar, and pour the remaining brine into the bag. Seal the bag. Let the Kimchi ferment in a cool place, at a temperature no higher than o 20 C (68°F), for 3 to 6 days, until the Kimchi is as sour as you like. 3. Remove the brine bag, and cap the jar tightly. Store the kimchi in the refrigerator, where it will keep for months.

36

AVRDC-ADB Postharvest Projects RETA 6208/6376

Mature leaf mustards  Washing/cutting  Sun-drying (1 day)  Trimming and shipping to fermentation vats  Spreading dry salt at the base of vats  (A)Deposition of leaf mustard (upright position)  (B)Spreading dry salt onto leaf mustard (9-15% w/w)  (C) Deposition of leaf mustard (inverted position)  (D)Coverage with plastic film and weights (3 days)  Repeat (A)-(D) 2 or 3 times  Coverage and sealing with heavy-duty plastic film and weights  Fermentation (2-6 months)  Fermented leaf mustard  Partially dried (sun drying)  Inner parts of leafs  Cutting into stripes  Packing into jars or bottles  Pressing/capping /sealing  Aging bottom up (2 months)  Fu-choy

Outer parts of leaf  Further sun drying  Wrap wilted leafs a balls  Packing in plastic bag  Mei-kan-choy

Figure 18. Flow chart of fermented leaf mustard processing (Sasitorn, 2007). Microorganisms associated with fermentation include Lactobacillus spp., Pediococcus spp. and Leuconostoc mesenteroides. The products can be either marketed for consumer demand or hermetically sealed in sterilized cans or pouches for local and overseas marketing. Fermented leaf mustard can be used as raw material for fu-choy and mei-kan-choy production (Figure 18). Fu-choy is a product with unique flavor and aroma that went through an incontainer secondary fermentation. For fu-choy production, fermented mustard leaf is partially dried (sun drying), then the stems and the inner leaves are cut into thin strips and packed tightly in glass bottles or other types of containers. The neck of container is filled with the partially dried leaves originating from outer leaves and followed by sealing. The bottles or jars undergo secondary fermentation in a bottom-up position for 2-3 months. The products are

Postharvest Technology of Leafy Vegetables

37

stable at ambient temperature with shelf life of several months to a year. Mei-kan-choy, an intermediate moisture fermented product, is usually made from the sun-dried outer leaves of leaf mustard pickle. Figure 19 shows another process for leaf mustard fermentation (Diamante, 2007). The mustards are first washed very well and the roots and old leaves are removed. Salt is then sprinkled to the leaves to induce wilting. For a kilogram of mustard leaves, boil 1 cup of rice water (from washing) with ¼ cup salt and cool. The wilted mustard leaves are placed in a jar and the rice water washing mixture is added. The leaves are weighted down so that they are thoroughly submerged in the salt solution and stored at room temperature. After a week or so, the fermented mustard leaves should be sour. This processing procedure is similar to the Lao technique for fermented Chinese mustard production with some modifications (Fig. 20) (Chanthasombath, 2007). After adding salt to the mustards, they are mixed with hand pressure to squeeze out the water from the mustards, which is then discarded. In addition, the rice wash water is not cooked and chili is added prior to fermentation in jars. The fermented product is usually marketed immediately due to short shelf life of 1-3 days. In Yunnan, China, mustards are exposed to the sun for 1-2 days to reduce water content and make them soften or flaccid in order to avoid breaking or tearing of leaves and stems during subsequent operations (Hong, 2007). Washing in fresh water is then done to remove dust and other extraneous materials. Excess water on the leaf surface is removed by shaking manually. The vegetable is placed into a suitable container, added with salt (about 15% of the total volume of produce), hot pepper powder, ginger, and monosodium glutamate. They are then mixed, twisted and kneaded to allow maximum absorption of the condiments. The vegetable is put into prepared a pond (Fig. 21A) or plastic jar disinfected with alcohol (Fig. 21B), covered and pressed with weights for fermentation. The fermentation period varies from 3–4 days in summer and 6–7 days in winter. The vegetable has to be turned periodically to allow uniform fermentation and dissipate the heat and bad smell. Turning the vegetable is done 3-4 times during the fermentation period. The fermented vegetable is transferred from the fermentation pond to pots or plastic barrels and covered with the salty liquid (Fig. 21C). The pot must be airproof and placed in a shaded area or cold room. In Thailand, another processing procedure for leaf mustard fermentation is employed by a SME company that is GMP certified (Srilaong, 2007). The mature leaf mustard (60-70 days after cultivation) is cut and transported to the production plant or left in the field for a day to dry (especially during the rainy season). The mustards are then trimmed and transferred to fermentation well or tank in a layer-by-layer arrangement, brine solution is aded, and the vegetable is pressed tightly. A heavy plastic sheet with weights is placed on the top layer as a cover. In general, the longer the fermentation period, the higher the salt concentration used. The concentration of brine solution is about 20% and the duration of fermentation depends on the desired characteristic of final product. After fermentation, the mustards are washed and while washing, cleaning and sorting are done. The selected product is then weighed, placed in plastic pouches or bags, sauce with preservative is added, and the product is vacuum-packed. Shelf life is one year. Figures 22-23 show the processing procedures.

38

AVRDC-ADB Postharvest Projects RETA 6208/6376

Harvested mustard Washing Roots/old leaves removed Add salt on the leaves Place wilted leaves in jar

Cooked rice water washing with salt Cooled salt solution

Cover jar tightly (airtight) Ferment (duration varies) Fermented mustard leaves Figure 19. Process flow for producing fermented mustard leaves (Diamante, 2007).

Figure 20. Lao process for producing fermented Chinese mustard (Chanthasombath, 2007).

Postharvest Technology of Leafy Vegetables

39

Figure 21. Fermentation pond/plastic barrel and earthen jar for fermented product (Hong, 2007).

Cut mature leaf mustard and transport to processing plant Trim and place in fermentation tank Add 20% of brine solution or dry salt Cover with plastic sheet with weights 1 night

3 months – 1 year

Transfer to sauce, 4-5 days*

Wash 3 times with tap water

Pack in plastic bag or vacuum pack

Adjust pH for 1 night

Market

Wash again with tap water and drain Filling in plastic bag with sauce Vacuum pack Pack in transportation box Market

Figure 22. General flow chart of fermented leaf mustard processing (Srilaong, 2007). *Composition of sauce depends on the processor. In general, it contains monosodium glutamate (MSG), acidifying agent (citric acid), firming agent (calcium chloride), sugar, herbs, spice (chili), vinegar, and soy sauce or fish sauce.

40

AVRDC-ADB Postharvest Projects RETA 6208/6376

Figure 23. Company processing operation for fermented mustards in Thailand. Quality standards for fermented leaf mustard have been developed by Thai Industrial Standard Institute (1980). Standards specifications are as follows: Definition: Leaf mustard pickle is a product from green leaf mustard (Brassica juncea) Ingredients of sauce: Water, soy sauce or fish sauce, vinegar, salt, sugar and additives such as chili, herbs, etc. pH of finished product: < 4.5 Additives: MSG < 0.25%; no artificial coloring; no preservatives Acidifying agent: acetic acid, citric acid, malic acid, tartaric acid, lactic acid, ascorbic acid Firming agent: NaCl < 1,000 mg/kg or CaCl2 < 350 mg/kg Contamination: tin < 250 mg/kg Microorganism: Total plate count < 1×103 colonies/ g of sample; Coliform < 3/ g sample; Yeast and fungi < 100 colonies/ g of sample Fermented mustards may be displayed bare in the market or packed in different kinds of containers (Figure 23). Packaging materials may include plastic bags tightened with rubber bands for local market, plastic bags under vacuum and heat seal, tin cans with easy open lids, and retort pouch for thermal processing. The main problem of fermented leaf mustard production is the quality of raw material, especially during the rainy season. Leaf mustards produced in the rainy season develop a soft texture in contrast to mustards produced in winter. Some processors solve this problem by adding chemicals during the fermentation process. In Vietnam, RIFAV has developed a fermentation technique for Chinese mustard (Fig. 24-25) (Thanh, 2007). The specifications of the technique are as follows: a) Chinese mustard is usually harvested at late stage of maturity to ensure the soluble solids of 8-10%, of which sugar is about 4%, protein 1-2%—the most appropriate condition for lactic fermentation. Too old outer and too young leaves as well as rotten and heavily damaged leaves are sorted out. The upper parts of the leaves are cut and sorted aside. Too long leaves are cut into shorter sections. The leaves are washed with clean water to remove foreign matter, dust, and surface microorganism. The leaves are then dried by leaving in shade with natural air circulation until there is no excess water on leaf surface. Proper weighing is done for accurate preparation of mixture recipe. The weighed vegetable is loaded into a container such as plastic, glass, porcelain, or earthenware jars. b) Dry salt without contaminating matter is used to prepare a 12-15% solution.

Postharvest Technology of Leafy Vegetables

41

c) After loading into containers together with the salt solution, the vegetable is covered by a plastic or bamboo grid to keep it soaked in the solution. d) The vegetable is pressed by putting a load or weight to ensure full immersion in the salt solution. The load weight is roughly 50% of total vegetable weight. e) Fermentation takes about 6-8 days. Care must be taken to ensure that foreign matters and other contamination sources do not fall into the container. If there are black scum on product surface during fermentation and storage, remove it immediately and clean the container accordingly. The above procedure could also be followed for common cabbage fermentation.

Chinese mustard

Sorting and grading

Trimming

Washing

Surface drying

Weighing

Container loading

Covering

Filling

Pressing

Storage

Immediate consumption

Semi-produce for further processing

Figure 24. Process flow for Chinese mustard fermentation technique (Thanh, 2007).

42

AVRDC-ADB Postharvest Projects RETA 6208/6376

Figure 25. Process flow for Chinese mustard and cabbage fermentation (Thanh, 2007). RIFAV has also developed a canning technique for both fermented Chinese mustard and common cabbage to improve product shelf life (Thanh, 2007). Fermented leafy vegetables usually have a very short shelf life, up to 3-4 days. Packing in airtight pasteurized/sterilized containers like jars, cans, or even plastic pouches can increase shelf life for up to six months. The canning technique shown in Figure 26 has the following specifications: a) Good quality fermented head cabbage and Chinese mustard must be used. b) Washing is done by soaking in clean water for 5-10 minutes to remove possible contaminating matters. c) Surface drying is done by natural air by leaving the fermented vegetables in a shaded place until there is no water on the product surface. d) Containers are cleaned and pasteurized. Empty jars and plastic pouches are cleaned by brushing and sterilizing using steam. e) The jars/pouches are carefully loaded to full capacity with the fermented vegetable. f) Ingredients (salt, food-grade acid and preservatives): The concentration of solution depends on the acidity and salt content of the fermented vegetables. In most cases, the solution contains 0.5% acidity and 4% salt. Dissolve the ingredients in water, mix well, filter to remove dust and other foreign matter, and boil. The containers with fermented vegetables are filled with filtered solution. The solution temperature at filling o should be around 80 C. The filling volume should be as much as 40% compared to the container net weight. To avoid re-contamination of products, the containers should be capped/sealed as soon as possible. In most cases, capping is done manually. For plastic pouches, sealing is done by vacuum plastic sealer. g) Pasteurization: The pasteurization regime for glass jars (500 ml volume) is as follows: 20’ – 20’ – 30’ ---------------78oC The pasteurization regime for plastic pouches (500 g) is as follows: 15’ – 20’ – 30’ ---------------78oC

Postharvest Technology of Leafy Vegetables

43

h) Cooling: After pasteurization, the products should be cooled down as soon as possible until the containers’ surface temperature reaches about 40oC. i) Temporary storage: As usually practiced, the products are subjected to temporary storage for at least two weeks to examine any possible latent defects. j) Labeling and packing: After temporary storage, defect-free containers can be labeled and packed into carton boxes for distribution and marketing. In Cambodia and Myanmar, fermented common cabbage is also produced, but there is a problem with short shelf life (Buntong and Vanndy, 2007; Kyi, 2007). The cabbage is cleaned and washed, good quality heads selected, sliced or shredded, salt added at 2.5%, and placed in jars for fermentation for 2-4 weeks. After fermentation, lactic acid concentration should range from 1-2% (Kyi, 2007). The fermented product is then packed in glass jars or marketed and displayed in open basins.

44

AVRDC-ADB Postharvest Projects RETA 6208/6376

1. Fermented vegetables

2. Washing

3. Surface drying

4. Loading in jars

3a. Cleaning and sterilization

or plastic pouches

5. Filling

6. Capping or sealing

5a. Mixing and filtration

3a1. Plastic pouches

5b. Ingredients

3a2. Jars and caps

7. Pasteurization

8. Cooling

9. Temporary storage

10. Labeling and packing

11. Marketing

Figure 26. Process flow for fermented cabbage and mustard canning (Thanh, 2007).

Postharvest Technology of Leafy Vegetables

45

Pickling Technologies Pickled vegetable may be defined as a food to which an edible acid, for example, lactic acid or acetic acid in the form of vinegar, has been added (Diamante, 2007). It does not require fermentation treatment. For cabbages, pickling may first involve shredding of selected leaves to produce 1/32 inch leaf shreds (Figure 27). The shreds are then acidified with citric acid and/or vinegar to about 0.5-0.7% acid expressed as lactic acid, sweetened to a desirable degree (5-10% added sugar), and heated in boiling water to attain a closing temperature of 74-77oC. The usual procedure is to place the acid-sugar solution in water in the bottom of each bottle and then add the blanched shredded cabbage to fill the bottle. This is followed by heat processing and subsequent cooling with tap water.

Cabbage head Remove green (wrapper), broken and dirty Leaves Remove core Shredding Cabbage shreds

Acid-sugar solution Bottling cabbage shreds in solution Heat-process in boiling water Pickled cabbage

Figure 27. Process flow for producing pickled cabbage (Diamante, 2007).

Drying/Dehydration Technologies Basic principles Dehydration or drying is the simplest and most natural form of food processing (Srilaong, 2007). It preserves fresh produce by removing most of its free water. Reducing the water content of the produce slows the rate of respiration, enzymatic action, and overall deterioration rate, making the product less susceptible to decay and facilitating transport and long-term storage. It also reduces the cost of packaging, handling, storing, and transporting the material by converting it to a dry solid, thus reducing its weight and volume. While all vegetables can be dried, not all would be of high quality and good taste when dried. Most vegetables are dried to