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Renewable Energy 98 (2016) 203e215

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Bioconversion of sugarcane crop residue for value added products e An overview Raveendran Sindhu a, *, Edgard Gnansounou b, Parameswaran Binod a, Ashok Pandey a a b

Biotechnology Division, National Institute for Interdisciplinary Science and Technology, CSIR, Trivandrum, 695 019, India Ecole Polytechnique F
ed
erale de Lausanne, Institute of Urban and Regional Sciences, GC A3, Station 18, CH-1015, Lausanne, Switzerland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 January 2016 Received in revised form 16 February 2016 Accepted 19 February 2016 Available online 2 March 2016

Sugarcane is a major crop cultivated globally and the residue left over after the crop harvest and extraction of juice is a good biomass source that can be used for the production of several useful chemicals. The sugarcane bagasse is an excellent substrate for the production of various biochemicals and enzymes through fermentation. Now major interest is focused on the utilization of these residue for biofuel production. The sugarcane crop residue is rich in cellulose and hemicellulose, hence it can be used for the production of bioethanol and other liquid transportation fuels. The present review gives a detailed account of the availability of sugarcane residue and various commercially important products that can be produced from this residue. It also provides recent developments in R&D on the bioconversion of sugarcane crop residue for value added products. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Biomass Bioconversion Value added products Sugarcane crop residue Biorefinery

1. Introduction Sugarcane is a major crop cultivated in tropical and sub-tropical countries like Brazil, China, India, Thailand and Australia [1]. It belongs to grass family, Gramineae and its botanical name is Saccharum officinarum. It was first grown in South-East Asia and Western India. Then the cultivation of sugarcane extended to all tropical and sub-tropical regions. Sugarcane area and productivity differ from country to country. It is cultivated in about 200 countries and Brazil is the world's largest cane producer and contributes to 25% of world's total production. India is the second largest producer of sugar in the world. Its distinguishing features are high biomass yield, high sucrose content and high efficiency in accumulating solar energy. After harvesting of sugarcane, leaves, tops and trash are left in the cane field while the sugarcane stalks are transported to sugar mills for the extraction of cane juice for sugar production [2]. Bio-refinery concept of complete utilization of sugarcane biomass will become a prime component for a sustainable sugarcane industry. Biorefinery involves fractionation and reforming of an input feed stock into multiple product streams. Lignocellulosic

* Corresponding author. Tel.: þ91 471 2515426; fax: þ91 471 2491712. E-mail addresses: [email protected], [email protected] (R. Sindhu). http://dx.doi.org/10.1016/j.renene.2016.02.057 0960-1481/© 2016 Elsevier Ltd. All rights reserved.

biomass offers tremendous biotechnological potential for use as substrate in bioconversion processes and can be effectively exploited for the production of bulk chemicals and value added products. The annual global production of sugarcane is about 328 Tg. Asia is the primary production region which contributes to 44% while South America is the second largest production region producing 110 Tg of sugarcane which contributes to 34% [3]. Sugar production is the major use of sugarcane consuming about 92% of sugarcane. Other uses such as animal feed and so on contribute less than 3%. Studies have indicated that sugarcane crop when harvested comprises of 75% sugarcane stalk and 25% leaves and tops. This waste provides a huge potential fuel resource. Harvesting of sugarcane lead to the production of large amount of post-harvest residues including sugarcane tops which could be an abundant, inexpensive and readily available source of lignocellulosic biomass. This can be used as good substrate for the production of bioethanol as well as for other value added products. In India, it is the most surplus available residue and is usually burnt in the field itself and does not find any suitable application. Burning of sugarcane tops produce fly ash, severely damages soil microbial diversity and raises environmental concerns [4]. Roofing and compost are some of the other uses. It can be used as an animal fodder for a few days before the leaves start rotting. Usually for every 1 MT of sugarcane produced, 0.20e0.30 MT of sugarcane tops

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is generated. Sugarcane bagasse, the largest agro-industrial residue is a fibrous residue of cane stalks left after the crushing and extraction of juice from the sugarcane. This by-product of the sugar industry is mainly used by sugar factories as fuel for boilers [5]. Comparing to other agricultural residues, bagasse can be considered as a rich solar energy reservoir due to its high yields and annual regeneration capacity. Currently several processes and products have been reported using sugarcane bagasse as a raw material. This include electricity generation, pulp and paper production and various products based on fermentation like industrially important enzymes, bioethanol, organic acids, alkaloids, protein enriched cattle feed, antibiotics etc. Bagasse in most case is used for co-generation of heat and power or sometimes used for manufacture of building materials. Paper plants also purchase bagasse from sugar plants. Sugarcane molasses are a dark, viscous and sugar rich byproduct of sugar extraction from sugarcane. It is used as a feed ingredient, binder and as an energy source. Around 3e7 tons of molasses were generated from 100 tons of sugarcane. The composition of the molasses varies depending on cane variety, climate and processes employed for sugar extraction. Molasses contain approximately 34% of sucrose, 11% of reducing sugars (glucose and fructose) and several minerals. It can be used as animal feed, for yeast cultivation, for the production of ethanol, rum, other alcohols and organic acids. Vinasse is a by-product of sugar-ethanol industry and is acidic compost with a pH of 3.5e5.0 with a high organic content and unpleasant odor. On an average 10e15 L of vinasse is generated while preparing each liter of ethanol [6]. Inadequate and indiscriminate use of vinasse in soils and water bodies leads to several environmental hazards. Several studies have been carried out for finding adequate uses and treatments of vinasse. It can be used for fertirrigation, yeast production, energy production and as a raw material for the production of livestock and poultry feed [7]. The chemical composition of vinasse varies depending upon the source used for ethanol production and distillation. The study revealed that fertirrigation or the use of vinasse as a fertilizer is the best alternative for vinasse reuse and disposal. Several new green methods need to be explored for developing novel uses of vinasse [8]. Cortez et al., 2007 [9] carried out anaerobic digestion of vinasse for the production of biogas. The anaerobic digestion was carried out in two stages-the acidogenic phase and the methanogenic phase. In the acidogenic phase the complex chains of carbohydrate, lipids and proteins were hydrolyzed to organic acids and in the methanogenic phase these acids were converted to methane and carbon dioxide. Laime et al., 2011 [10] utilized vinasse for the production of yeast. Additional supply of ammonium and magnesium salts as well as high energy consumption for water removal from the process made it economically unviable. Chemical compositions of the bagasse may vary for different sugarcane varieties depending upon the genotype. Several other factors like location, age of crop, environmental and cultivation parameters also affect the composition of the biomass. A study conducted by Benjamin et al., 2014 [2] showed wide variation in agronomic parameters, chemical composition and sugar released after pretreatment of sugarcane varieties harvested for two growing seasons. A significant difference was observed among varieties over harvest years. The study revealed severe drought negatively influenced the performance in cane yield except for variety containing the highest lignin. Leaves and tops contain higher amounts of salts and nutrients. The sugar contained in the stem is 90% sucrose and small amounts of glucose and fructose. The greatest difference in composition of sugarcane is seen in the moisture content which varies between 13.5% in the dry leaves and 82.3% in the tops. The content of carbon,

hydrogen, nitrogen and sulfur showed similar values in dry leaves and in tops. Bagasse contains 50% of cellulose, 25% each of hemicellulose and lignin. Chemically it contains about 50% a-cellulose, 30% pentosans and 2.4% ash [5]. Bagasse offers numerous advantages over other crop residues like rice straw and wheat straw because of its low ash content. Rice straw and wheat straw have 17.5% and 11.5% of ash respectively. Bagasse is the raw material for 20% of total paper production. Sugarcane tops contain 29.85% of cellulose, 18.85% of hemicelluloses and 25.69% of lignin [11]. The composition may vary depending on the geographical location, variety etc. The present review addresses the potential of sugarcane crop residue for the production of various value added products. 2. General conversion methods Native form of lignocellulosic biomass is a tough feed stock for hydrolysis due to crystallinity of cellulose and due to the compact packing of cellulose, hemicelluloses and lignin. Due to recalcitrant nature of the lignocellulosic biomass a pretreatment process is essential for the removal of hemicelluloses and lignin and to increase cellulose conversion efficiency. The basic objective of the pretreatment is to make cellulose accessible by the action of cellulases which is achieved by removal of hemicelluloses or lignin from the biomass. A wide range of physical, mechanical, chemical, biological, combination and alternative strategies were reported for achieving these goals. In addition to pretreatment, an effective cellulase cocktail, enzyme loading and hydrolysis conditions and nature of the lignocellulosic material are critical for maximum hydrolysis. Several reports were available for the pretreatment of sugarcane tops like acid [11], alkali [12], surfactant assisted acid pretreatment [13], surfactant assisted ultrasound pretreatment [14] and sequential pretreatment [15]. Among these methods the highest reducing sugar yield was observed with sequential pretreatment (0.796 g/g) followed by alkali pretreatment (0.775 g/g). But there were generation of inhibitors during acid and alkali pretreatment. The alternative strategies like surfactant assisted ultrasound pretreatment and surfactant assisted acid pretreatment strategies did not generate any inhibitors. Compared to other pretreatment strategies employed for sugarcane tops, sequential pretreatment was found to be better in terms of improved reducing sugar yield without any inhibitor generation as well as better removal of hemicelluloses and lignin from the biomass compared to conventional acid and alkali pretreatment as well as other alternative strategies of pretreatment. Selection of the pretreatment strategy will be based on the economic feasibility as well as the targeted product. Pretreated bagasse serves as an efficient inert support material for fungal cultivation in SSF. Several pretreatment strategies were reported for bagasse like acid [2], alkali [16], combined [17], organo-solvent [18], organic acids [19] and physical [20]. Though several pretreatment strategies were available only a few seems promising. One of the most important challenges associated with pretreatment is to identify the composition of the feed stock and to device the best pretreatment strategy of the selected item. Proper pretreatment can improve the biomass digestibility and increase accessibility of enzymes to the materials. Cellulose crystallinity, accessible surface area, degree of cellulose polymerization, lignin and hemicelluloses seal as well as degree of acetylation of hemicelluloses are the critical factors to be considered for developing a suitable strategy for pretreatment of a specific biomass. Composition plays an important role. Hence fine tuning of specific pretreatment strategies to be developed for each biomass which will

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make the process economically as well as ecologically viable. Intensive R and D efforts are going on in this direction. Development of a proper pretreatment strategy will minimize the capital and operating costs. Most of the commercial plants use dilute acid pretreatment. The main advantage of this strategy is that a separate pentose and hexose stream will be generated. The pentose stream can be utilized for the production of various value added products while the hexose stream can be used for the production of bioethanol. Dilute acid hydrolysis gives high reaction rates and improves the cellulose hydrolysis rate. Under optimized conditions 95% of hemicelluloses can be recovered from the lignocellulosic biomass. 3. Value added products from sugarcane crop residue Several value added products can be produced by utilization of various crop residues and by-products of sugarcane like bagasse, sugarcane tops, molasses and vinasse. This include bioethanol, biodiesel, biobutanol, 2, 3-butanediol, biohydrogen, bioelectricity, biopolymer, different enzymes, organic acids, amino acids, pigments, animal feed, composite, chelating agents, alkaloids etc. Table 1 shows different value added products produced from sugarcane crop residue. Schematic representation of value added products from sugarcane crop residue is presented in Fig. 1. 3.1. Bioethanol Increasing energy demand and depletion of fossil fuels leads to increase interest on alternative fuels. The requirement to reduce carbon dioxide emissions leads to use of many types of biomass as alternative energy sources. Since the biomass is produced by photosynthetic reduction of carbon dioxide, utilization of biofuels can be carbon neutral with respect to build-up of atmospheric greenhouse gases. Bioethanol is the most abundant biofuel for automobile transportation. It is a renewable fuel and contains 37% of oxygen by weight. Oxygen enhances the combustion of petrol in engines and contributes to reductions in exhaust emissions of

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carbon monoxide. Ethanol can be produced from fermentation of sugars obtained from lignocellulosic biomass which serves as the future feed stock for bioethanol production because of its low cost and huge availability. Its high carbohydrate content and low lignin content makes it a suitable substrate for bioconversion to ethanol. Sugarcane is the most efficient raw material for bioethanol production [21]. Ethanol production from sugarcane bagasse by Zymomonas mobilis using simultaneous saccharification and fermentation was reported by dos Santos et al., 2010 [22]. The optimum conditions were biomass loading of 30%, enzyme loading of 25 FPU/g and cell concentration of 4 g/L. Maximum ethanol concentration and productivity were 60 g/L and 1.5 g/L/h respectively. Few reports were available on exploitation of sugarcane tops for the production of bioethanol [11]. Fermentation of the hydrolyzate obtained after acid pretreatment and enzymatic saccharification with Saccharomyces cerevisiae yielded 11.365 g/L of bioethanol with a fermentation efficiency of 50%.

3.2. Biodiesel Depletion of petroleum reserves and the impact of environmental pollution lead to search for new alternative fuels for use in diesel engines. Biodiesel are monoalkyl esters of fatty acids derived from vegetable oil or animal fat. Trans-esterified renewable oil has been proven to be a viable alternative diesel engine fuel with characteristics similar to diesel. The energy density of biodiesel is comparable to petroleum diesel. Biodiesel has a number of advantages. Since it is derived from biomass it does not contribute to atmospheric CO2 emissions, low toxicity and biodegradable and can be used in existing diesel engines blended with petroleum diesel or can be run unblended in engines with minor modifications. Utilization of low cost agricultural residues of pineapple peels and sugarcane bagasse for lipid accumulation and biodiesel production in Scenedesmus acutus PPNK1 was carried out by Rattanapoltee and Kaewkannetra, 2014 [23]. The maximum biomass concentration, productivity, lipid content and lipid yield using sugarcane bagasse were 3.85 g/L, 160.42 mg/L/day, 40.89% and

Table 1 Value added products from sugarcane crop residue. Sugarcane residue

Product

Microorganism

Reference

Bagasse Sugarcane tops Bagasse Bagasse Bagasse Bagasse Bagasse Molasses Bagasse Sugarcane tops Bagasse Bagasse Bagasse Bagasse Bagasse Bagasse Bagasse Bagasse lignin Molasses Molasses Bagasse Bagasse Bagasse Sugarcane pith bagasse Bagasse

Bioethanol Bioethanol Bioethanol 2,3- butanediol 2,3-butanediol Biohydrogen Biohydrogen Biohydrogen Polyhydroxyalkaonates (PHA) Poly-3-hydroxybutyrate (PHB) Composite Composite Xylitol Xylitol Xylitol Xylitol Xylitol Chelating agent Carotenoides Carotenoides Modified catalysts L-glutamic acid Animal feed Ergot alkaloides Penicillium

Zymomonas mobilis Saccharomyces cerevisiae Scenedesmus acutus PPNK1 Klebsiella pneumoniae CGMCC 1.9131 Klebsiella pneumoniae Clostridium butyricum TISTR 1032 Thermoanaerobacterium aotearoense Clostridium butyricum Ralstonia eutropha Comomonas sp. e e e e Debaromyces hansenii Williopsis saturnus Candida tropicalis IEC5-ITV

Santos et al., 2010 Sindhu et al., 2011 Rattanapoltee and Kaewkannetra, 2014 Zhao et al., 2011 Song et al., 2012 Plangklang et al., 2012 Lai et al., 2014 Whiteman and Kane, 2014 Jian and Heiko, 2008 Prabisha et al., 2014 Acharya et al., 2011 da Silva et al., 2013 Sarrouh et al., 2009 Branco et al., 2011 Prakash et al., 2011 Kamat et al., 2013 Costanon- Rodriguez et al., 2014 Goncalves et al., 2002 Valduga et al., 2008 Freitas et al., 2014 Marquez de Silva et al., 2013 Nampoothiri and Pandey, 1996 Okano et al., 2010 Hernandez et al., 1993 Dominguez et al., 2001

Sporidiobolus salmonicolor CBS 2636 Rhodosporidium toruloides NCYC 921 Brevibacterium sp. Pleurotus eryngii Claviceps purpurea e

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Fig. 1. Schematic representation of bioconversion of sugarcane crop residue to value added products.

1.24 g/L respectively. The study revealed that there was a 2.13 fold increase in lipid content when sugarcane bagasse was used and using agricultural residues as carbon source could lead to an increase in the lipid content and reduces the cost of biofuel production. FAME obtained from S. acutus PPNK1 after trans-esterification showed fatty acid compositions of chain lengths between C16 to C18. This indicates that agricultural residues like sugarcane bagasse were suitable for the production of good quality biodiesel. Utilization of agro-residue is a promising way to reduce environmental pollution and lower cost for lipid production. 3.3. Biobutanol Butanol is a four carbon primary alcohol with a higher energy intensity and lower volatility as compared to ethanol. It can be used as fuel in current gasoline based engines with practically no changes in engine [24]. It is also an important feed stock for chemical industry since it is used for paint, solvents and plasticizers production. Butanol production from renewable source involves ABE (acetone-butanol-ethanol) fermentation of sugars derived from lignocellulosic biomass. But this method has few limitations like low productivity and low n-butanol concentration due to product inhibition. Another strategy commonly adopted for nbutanol production from lignocellulosic biomass is the ethanol chemistry route where, ethanol is used as feed stock. However production of n-butanol in the sugarcane biorefinery makes the process more economically viable by producing a biofuel more suitable for use in chemical industry. Dias et al., 2014 [25] developed a strategy for butanol production in a sugarcane biorefinery using ethanol as feed stock. In this study novel catalysts were used both in vapor and liquid-phase catalysis. Techno-economic analysis revealed that the best results were observed with n-butanol production through vapor phase catalysis. Biobutanol produced through liquid and vapor phase catalysis presents lower environmental impact. 3.4. 2, 3- butanediol 2, 3-Butanediol is used as a solvent, liquid fuel and as a precursor of many synthetic polymers, fumigant, moistening and softening agents, explosives, plasticizers, cosmetics, printing inks, medicines and resins. Methyl ethyl ketone produced by dehydration of 2, 3butanediol is used as a liquid fuel additive [26]. Several microorganisms are known to produce 2, 3-butanediol using glucose. However major cost in most biomass conversion processes appears to be the substrate cost. Exploitation of sugarcane bagasse for 2, 3butanediol seems promising. Zhao et al., 2011 [27] developed a strategy for the production of 2, 3- butanediol by simultaneous saccharification and fermentation of alkali-peracetic acid pretreated sugarcane bagasse by Klebsiella

pneumoniae CGMCC 1.9131. The yield was 0.35e0.50 g/g consumed sugar depending on the fermentation time. Production of 2, 3-butanediol by K. pneumoniae from enzymatic hydrolysis of sugarcane bagasse was reported by Song et al., 2012 [28]. The enzymatic hydrolyzate of alkali-peracetic acid and dilute acid pretreated samples were used for the production of 2, 3butanediol and the yields were 0.36 and 0.42 g/g of sugars respectively. The enzymatic hydrolyzate of alkali-peracetic acid pretreated sugarcane bagasse contains 30.54 g/L of glucose and 13.87 g/L of xylose, while the enzymatic hydrolyzate obtained from dilute acid pretreated sugarcane bagasse contains 42.59 g/L of glucose and 5.36 g/L of xylose. Since xylose is not utilized by the strain for 2, 3-butanediol production, the final concentration of 2, 3-butanediol was considerably higher for the dilute acid pretreated material (17.35 g/L) than that of alkali e peracetic acid pretreated sugarcane bagasse (14.53 g/L).

3.5. Biohydrogen Biohydrogen is considered as a future energy for its high energy content and zero CO2 emission. Hence it is a promising alternative to conventional fossil fuels. Currently majority of hydrogen is produced from fossil fuels. Lignocellulosic biomass can serve as a source for sustainable production of hydrogen. Thermophilic hydrogen conversion seems promising, since it can convert a variety of biomass based substrates into hydrogen at high yields. Plangklang et al., 2012 [29] developed a strategy for enhanced biohydrogen production from sugarcane by immobilized Clostridium butyricum TISTR 1032 on sugarcane bagasse. Immobilized cells showed approximately 1.2 times improved hydrogen production rate than free cells. The optimum conditions of hydrogen production by immobilized C. butyricum were an initial sucrose concentration of 25 g COD/L and pH was maintained at 6.5. The highest hydrogen production rate (HPR) and highest hydrogen yield (HY) were 3.5 L H2/L and 1.5 mol H2/mol hexose consumed. The study revealed that efficiency of hydrogen production from sugarcane juice by C. butyricum was enhanced by immobilization technique. The immobilized cells can tolerate harsh environmental conditions like wider range of pH and sucrose concentrations better than the free cells. The immobilized cells showed same HPR and HY for five successive cycles. Lai et al., 2014 [30] used sugarcane bagasse as a substrate for biohydrogen production using Thermoanaerbacterium aotearoense. Various process parameters affecting hydrogen production were optimized. The study revealed that dilute sulfuric acid pretreated sugarcane bagasse hydrolyzate was suitable for hydrogen production by T. aotearoense due to the presence of glucose and xylose and low level of inhibitors. Maximum hydrogen production was observed when pretreatment was carried out with 2.3% of H2SO4 for 114.2 min at 115  C. The hydrogen yield and hydrogen

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production rate (HPR) under the best conditions were 1.86 mol H2/ mol total sugar and 0.52 L/L respectively. Catabolite repression was not observed during the fermentation which would be beneficial for higher hydrogen production and shorter retention time. Comparative modeling efficiencies for biohydrogen production by C. butyricum on sugarcane molasses adopting artificial neural network and response surface modeling were evaluated by Whiteman and Kana, 2014 [31]. Parameters like concentration of molasses, pH, incubation temperature and inoculum concentration were optimized. The data obtained were used to develop models for ANN and RSM. The findings revealed that ANN has greater accuracy in modeling the relationships between the considered process inputs for fermentative hydrogen production. 3.6. Biopolymers Increase use of conventional non-biodegradable plastics leads to severe environmental as well as ecological problems. This leads to search for biodegradable plastics which can serve as an alternative to petroleum based polymers. The main competition between biodegradable plastics and petroleum based plastics is based on the cost of production. One of the main limitations for the production of biopolymer is the cost associated with the carbon source. More than 50% of the production cost is contributed by the carbon source [32]. Utilization of agro-residues or waste by product stream makes the process economically viable. Jian and Heiko, 2008 [33] utilized dilute acid pretreated sugarcane bagasse hydrolyzate for the production of polyhydroxyalkaonates (PHA) by Ralstonia eutropha. PHA biopolyesters were synthesized and accumulated 57% w/w of biomass. Only few reports are available on biopolymer production utilizing sugarcane trash as the sole carbon source. Prabisha et al., 2014 [34] evaluated the ability of Comomonas sp. isolated from a dairy effluent sample for the production of poly-3-hydroxybutyrate (PHB) using biomass hydrolyzate obtained after mild alkali pretreatment of sugarcane tops. The hydrolyzate obtained after enzymatic saccharification is devoid of major fermentation inhibitors like furfural, 5hydroxymethylfurfural, acetic acid, formic acid and propionic acid. The optimum conditions for PHB production were incubation time of 96 h, pH 7.0, reducing sugar concentration of 1.25% and KH2PO4 concentration of 1.05%. The bacterium accumulated 55.85% of PHB with a productivity of 0.195 g/l. 3.7. Enzymes Various agro-residues, especially sugarcane bagasse serve as a good substrate for the production of various industrially important microbial enzymes adopting a SSF strategy. Some of the enzymes that a produced by SSF utilizing sugarcane crop residues or byproducts of sugarcane industry include a-amylases, cellulases,

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xylanases, pectinases, protease, invertase, lipase and inulinase. Table 2 presents different enzymes produced from sugarcane crop residue. 3.7.1. a-amylase a e amylases are enzymes which convert starch to glucose and maltose or variety of malto-oligosaccharides. Amylases play a significant role in starch, detergent, beverage and textile industries. Industrial production of enzymes can be made economical by utilizing low cost substrates like agro-industrial residues in the production medium. Currently there has been an increasing effort on efficient utilization of sugarcane bagasse [2]. Sugarcane bagasse can be used as a raw fiber in solid state fermentation or as acid hydrolyzed simple sugars in submerged fermentation. Rajagopalan and Krishnan, 2008 [35] reported a-amylase production from catabolite derepressed Bacillus subtilis KCC103 utilizing sugarcane bagasse hydrolyzate (SBH). Addition of SBH (1% reducing sugar w/v) to the nutrient medium supported maximum a-amylase production (67.4 U/ml). Media engineering improved a-amylase production 2.2 fold by statistical optimization using response surface method. Existence of catabolite repression in this strain allowed production of a-amylase synthesis in B. subtilis KCC103 in the presence of simple sugars in the SBH. The study demonstrated the economical production of a-amylase using sugars derived from low cost agricultural byproduct sugarcane bagasse. 3.7.2. Cellulases The conversion of cellulose to glucose involves synergistic action of three enzymes-endo-b-1, 4-glucanases, cellobiohydrolases and b-glucosidases. Hydrolytic enzymes contribute a major cost in biofuel plants. Pereira et al., 2013 [36] evaluated Penicillium echinulatum for cellulase production using sugarcane bagasse as carbon source. Highest enzyme production (3.7 FPU/ml) and productivity (25.7 FPU/l/h) were observed with fed-batch cultivation. The study revealed that this enzyme performs better than commercial cellulase for biomass hydrolysis and can be used on site enzyme platform for bioethanol production from sugarcane lignocellulosic residue. A novel promising Trichoderma harzianum L04 strain for the production of cellulolytic enzymes using sugarcane bagasse was reported by Benoliel et al., 2013 [37]. The study revealed T. harzianum L04 to produce significant levels of cellulase when grown on sugarcane bagasse. Around 60% of sugar yield was obtained after 18 h of hydrolysis indicating the potential of cellulolytic enzymes of T. harzianum for biomass hydrolysis. 3.7.3. Xylanases Microbial production of xylanase is gaining importance due to

Table 2 Enzymes produced from sugarcane crop residues. Sugarcane residue

Product

Microorganism

Reference

Bagasse Bagasse Bagasse Bagasse Bagasse Bagasse Molasses/Bagasse Bagasse Bagasse Bagasse Bagasse

a-amylase

Bacillus subtilis KCC 103 T. harzianum L04 T. auroviride Trichoderma sp. Kluveromyces marxianus NRRL Y-7571 Bacillus licheniformis A. niger GH1 Rhizomucor pusillus/Rhizopus oryzae Rhizopus oryzae A. fumigatus A. awamori

Rajagopalan and Krishnan, 2008 Benoliel et al., 2013 Carneiro de Cunha et al., 2013 Norazlina et al., 2013 Mazutti et al., 2006 Rathakrishnan et al., 2012 Vaena et al., 2014 Cordova et al., 1998 Vaseghi et al., 2013 Naqvi et al., 2013 Baladhandayutham and Thangavelu, 2011

Cellulase Xylanase Xylanase Inulinase Protease Invertase Lipase Lipase Lipase Pectinase

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its wide industrial and biotechnological applications. Xylanases are widely used for bleaching paper and pulp to reduce the usage of chlorine. It is also used for brewing, baking, fruit and vegetable processing and as feed additives in broiler and animal diets. Xylan is the major component of hemicellulose and its complete degradation takes place by the synergetic action of xylanolytic enzymes like endoxylanase, b-xylosidase and accessory enzymes like a-arabinofuranosidase, acetylesterase and a-glucuronidase. Production cost is the major factor limiting its use indicating the need for low cost production systems. Cane molasses an important residue of the sugar industry serves as cost effective carbon source for the production of various industrially important enzymes. Production of xylanase by filamentous fungi using sugarcane and sugarcane bagasse as substrate was reported by da Cunha et al., 2013 [38]. Fungal species isolated from various parts of sugarcane were evaluated for xylanase production using sugarcane bagasse as sole carbon source. Trichoderma auroviride showed highest xylanase production (2037 U). The optimum conditions of xylanase production were an incubation temperature of 35  C, 150 rpm stirring intensity and incubation time of 120 h. Xylanase production by Trichoderma sp. by SSF using sugarcane bagasse was carried out by Norazlina et al., 2013 [39]. Highest xylanase activity (380 U/g) were observed with 5.6 g of sugarcane bagasse, 1% of sucrose, incubation temperature at 50  C, incubation time of 6 days and moisture content of 70% (v/w). The study revealed that xylanase can be produced by Trichoderma using sugarcane bagasse as substrate which is cheap and available throughout the year. 3.7.4. Inulinase Inulinase are important enzymes used for the production of high fructose syrups from inulin. It catalyzes the hydrolysis of inulin into fructose and fructo-oligosaccharides which are widely used as food additives. Inulinase based hydrolysis of inulin can yield products with 95% of fructose. Mazutti et al., 2006 [40] optimized conditions for inulinase production by Kluveromyces marxianus NRRL Y-7571 using sugarcane bagasse as substrate for solid state fermentation. Maximum inulinase yield and productivity were 390 U/g and 3.34 U/g/h and this is the highest reported value. Sugarcane bagasse seems to present a great nutritional potential for growth of K. marxianus NRRL Y-7571 and production of inulinase. 3.7.5. Proteases Enzymes that hydrolyze peptide bonds are called proteases. It is an important industrial enzyme which finds application in detergent, leather, food, pharmaceutical and for bioremediation processes. They regulate various metabolic processes like blood coagulation, fibrinolysis, complement activation, phagocytosis and blood pressure control. Rathakrishnan et al., 2012 [41] developed a strategy for protease production using sugarcane bagasse under SSF using Bacillus licheniformis. Various process parameters affecting protease production were optimized by adopting Plackett- Burman design. The results indicate that sugarcane bagasse serves as a best source for the production of protease. Under optimized conditions 146.28 U/gds of protease activity was observed. 3.7.6. Invertases Invertases are enzymes which catalyzes the hydrolysis of sucrose into glucose and fructose. This enzyme is very important in food industry for the production of artificial sweetener. This enzyme has fructosyltransferase activity which is important for the synthesis of short chain fructo-oligosaccharide compounds. This improves intestinal microflora and prevents cardiovascular disease, colon cancer and osteoporosis [42]. Utilization of sugarcane molasses and bagasse was evaluated for the production of fungal

invertase in solid state fermentation using Aspergillus niger GH1 was reported by Veana et al., 2014 [43]. The by-products of sugar industry molasses and bagasse were employed as substrates for invertase production. Fermentation with A. niger GH1 yielded 5231 U/L of invertase. Utilization of sugar industry by-products for invertase production by A. niger GH1 seems promising since it lower the enzyme production cost as well as the enzyme yield is high since the enzymatic yield is higher than those reported by other A. niger strains under SSF using dilute acid treated bagasse hydrolyzate. 3.7.7. Lipases Lipase hydrolyzes fats into fatty acids and glycerol at the waterelipid interface and can reverse the reaction in non-aqueous media. It finds application in different industries like food, pharmaceutical, cosmetics, oleo-chemicals, fuel and detergents. The use of solid state fermentation for the production of thermo-stable lipases is an interesting alternative to the valorization of bagasse and olive oil cake. Lipase production by SSF using olive oil cake and sugarcane bagasse by Rhizomucor pusillus and Rhizopus rhizopodiformis was reported by Cordova et al., 1998 [44]. The maximum lipase activity for R. pusillus and R. rhizopodiformis were 1.73 U/ml and 0.97 U/ml respectively. The study revealed that when sugarcane bagasse and olive oil cake were mixed in equal proportion, the lipase activity increased to 43.04 U/ml and 10.83 U/ml respectively. The synergetic effect of olive oil cake added to bagasse has been confirmed. Vaseghi et al., 2013 [45] evaluated production of active lipase by Rhizopus oryzae from sugarcane bagasse in a tray fermenter. A tray reactor was designed for the extracellular enzyme production. The results indicate that the newly constructed tray bioreactor had the potential to produce lipases with high activity. Addition of olive oil resulted in a 1.6 fold increase in lipase activity. Maximum activity observed under optimized conditions is 215 U/gds. Lipase production using different pretreated sugarcane bagasse hydrolyzate supplemented with mineral salts was evaluated for the production of lipase by Aspergillus fumigatus [46]. Maximum yield of 40 U/ml was observed for 0.6 N NH4OH pretreated sugarcane bagasse medium supplemented with mineral salts in comparison to other acid and alkali pretreated bagasse hydrolyzate. Sugarcane bagasse serves as a cheap source for the production of lipase. 3.7.8. Pectinases Pectinases catalyze the hydrolysis of pectin. It finds extensive applications in food and beverage industries. It is used for clarification of fruit juice, pulp and paper industry, coffee and tea fermentation, waste management, protoplast isolation, retting of flax and vegetable fibers and haze removal from wines. SSF using agro-industrial residues is an attractive option since it presents higher productivity, lower capital and operating costs and easier downstream processing compared to submerged fermentation. Baladhandayutham and Thangavelu, 2011 [47] reported pectinase production by Aspergillus awamori using sugarcane bagasse and rice bran as substrate. Maximum pectinase production (103.33 U/ ml) was observed when 85% of rice bran and 15% of sugarcane bagasse were used and incubation temperature of 35  C. 3.7.9. Laccases Laccases are multi-copper containing enzymes which catalyze the oxidation of a wide variety of aromatic compounds with concomitant reduction of oxygen to water. Potential applications of laccase include biopulping and biobleaching, industrial dye degradation, improving the digestibility of lignocellulosic biomass, delignification, removal of phenolic compounds and toxic pollutants. The use of inexpensive agro-residues for the economic

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production of laccase seems promising. Singh et al., 2010 [48] evaluated laccase production by Aspergillus heteromorph using distillery spent wash and lignocellulosic biomass. The study showed that lignocellulosic biomass enhanced laccase production. Anaerobically treated distillery spent wash and lignocellulosic biomasses like sugarcane bagasse are cheap and easily available and serve as an ecofriendly and economic strategy for the production of laccase. 3.8. Composites Natural fibers serve as emerging alternatives to glass-reinforced composites. Few advantages of natural fiber composite are low cost, light weight, renewable and biodegradable. Other environmental advantages include lower greenhouse gas emissions and enhanced energy recovery [49]. Alkali treatment of bagasse was carried out to increase the adhesion between the fiber and the resin matrix and the mechanical properties of the composite samples to different environmental treatments were carried out by Acharya et al., 2011 [49]. A chemical pretreatment was carried out to overcome the drawbacks associated with natural fiber-reinforced composites like high moisture absorption, poor wettability and poor adhesion. The study revealed that sugarcane bagasse serves as a good raw material for the production of composite by suitably bonding with resin for a value added product. da Silva et al., 2013 [50] developed a strategy for value addition of lignin extracted from sugarcane bagasse by organosolv pulping by reacting with glutaraldehyde. The organosolv ligninglutaraldehyde resin was used to prepare a composite reinforced with sugarcane bagasse fibers. The study revealed that the use of phenolic materials originating from renewable resources for various industrial applications could contribute to an increase in the profitability of bio-refineries where lignin is generated as byproduct. 3.9. Organic acids Fermentative production of organic acids is a promising approach for obtaining organic acids from renewable carbon source. Organic acids constitute the key group among the building block chemicals which can be produced by microbial processes. Biotechnological processes are favorable from a chemical as well as economic point of view. Table 3 shows different organic acids produced from sugarcane crop residue. Some of the organic acids that are produced using sugarcane crop residue or using byproducts of sugarcane industry include itaconic acid, succinic acid, citric acid, lactic acid, butyric acid and propionic acid.

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3.9.1. Itaconic acid The wide spread use of itaconic acid in synthetic resins, synthetic fibers, surfactants, rubbers, plastics and oil additives have resulted in an increased demand for this product [51]. It also provides possibilities for selective enzymatic transformations to create useful poly-functional building blocks. Many researchers have attempted to replace the expensive carbon source used for itaconic acid production with cheaper alternative substrates. Nubel and Ratajak, 1960 [52] reported improved yield of itaconic acid by replacing refined glucose with less expensive carbohydrate source like sugarcane and sugar beet molasses. They developed a strategy for conversion of inexpensive carbohydrates to itaconic acid in high yield by submerged aerobic fermentation. The molasses were pretreated with ion exchange resins, ferrocyanide, bentonite and lime for the removal of impurities like heavy metals or alkaline earth substances from the molasses. The molasses medium was inoculated with Aspergillus terreus and A. itaconicus, which are capable of producing itaconic acid by submerged fermentation of carbohydrates. Cane molasses medium (1800 ml) was diluted to 18% w/v of sugar and mixed with beet molasses medium (200 ml) was diluted to 18% w/v of sugar and inoculated with a spore suspension of A. terreus and incubated at 35e40  C. Fermentation was carried out until the itaconic acid concentration reached 5 g/100 ml. The fermented broth is filtered and concentrated to crystallize the product. Sugarcane bagasse was utilized for the production of itaconic acid from A. niger, A. oryzae, A. flavus and Penicillium sp. in solid state fermentation by Paranthaman et al., 2014 [53]. Among the different fungi screened, A. niger produced highest itaconic acid (8.24 mg/kg) in SSF. The study revealed the suitability of sugarcane bagasse powder for the fermentative production of itaconic acid.

3.9.2. Succinic acid Succinic acid is a dicarboxylic acid and is produced by plants, animals and microorganisms. It finds wide applications in industries involved in producing food, green solvents, biodegradable plastics and ingredients used for the stimulation of plant growth [54]. Succinic acid is mostly used as surfactant, additive, foaming agent and detergent. It is also used as ion chelator which prevents corrosion and pitting in the metal industry and also as antimicrobial and flavoring agent and also as an additive in the production of vitamins, antibiotics and amino acids [55]. The cost of succinic acid production is affected by its productivity, raw material cost, and yield as well as product recovery system. Hence exploiting the potential of cheaper and surplus available lignocellulosic biomass as carbon source will makes the process more economical. Borges and Pereira, 2011 [56] developed a strategy for succinic acid production from sugarcane bagasse hemicellulose hydrolyzate

Table 3 Organic acids produced from sugarcane crop residue. Sugarcane residue

Product

Microorganism

Reference

Molasses Bagasse Bagasse Bagasse Bagasse Bagasse/Vinasse Bagasse Molasses Bagasse Bagasse Molasses Bagasse Bagasse

Itaconic acid Itaconic acid Succinic acid Succinic acid Citric acid Citric acid Citric acid Lactic acid Lactic acid Butyric acid Propionic acid Propionic acid Propionic acid

A. terreus/A. itaconicus A. niger/A.oryzae/A. flavus/Penicillium Actinobacillus succinogenes Actinobacillus succinogenes A. niger DS1 A.niger A. niger e Bacillus C. tyrobutyricum Propionibacterium freudenreichii CCTCC M207015 Propionibacterium freudenreichii CCTCC M207015 Propionibacterium acidipropionii

Nubel and Ratajak, 1960 Paranthaman et al., 2014 Borges and Pereira, 2011 Xi et al., 2013 Kumar et al., 2003 Oliveira et al., 2012 Amenaghawon et al., 2013 Lunelli et al., 2010 Peng et al., 2014 Wei et al., 2013 Feng et al., 2011 Chen et al., 2012 Zhu et al., 2012

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by Actinobacillus succinogenes. The study revealed that supplementation of NaHCO3, MgSO4 and yeast extract played a significant role in succinic acid production. The conversion yield of succinic acid from sugarcane bagasse hydrolyzate was relatively high and produced 22.5 g/l. Production of succinic acid using A. succinogenes by ultrasound pretreatment and hydrolysis of sugarcane bagasse was reported by Xi et al., 2013 [57]. Sugarcane bagasse hemicellulose hydrolyzate was used as the carbon and nitrogen source for green and economical production of succinic acid. Ultrasound assisted dilute acid hydrolysis of sugarcane bagasse serves as a time saving and economical method for hydrolyzing sugarcane bagasse. The nondetoxified hydrolyzate produced 23.7 g/l of succinic acid with a yield of 79% and productivity of 0.99 g/l/h. 3.9.3. Citric acid Citric acid finds application in various industries. It is used as an anti-oxidising, flavoring, preserving, chelating and buffering agent in the food, beverages, pharmaceutical and cosmetic industries [58]. Traditionally it is produced by submerged fermentation using A. niger. The increase in demand of citric acid leads to search for more economical means for its production. Recently studies have been carried out by several researchers for the production of citric acid using agricultural residues. SSF process for the production of citric acid by A. niger DS1 using sugarcane bagasse as a carrier and sucrose or molasses based medium as a moistening agent was reported by Kumar et al., 2003 [59]. Sugarcane bagasse serves as a good carrier since it did not show agglomeration after moistening with the medium and helps in better heat and mass transfer during fermentation and higher product yield. The citric acid yield from sucrose, clarified and nonclarified molasses medium were 69.6, 64.5 and 62.4% respectively after nine days of incubation. The decrease in citric acid yield when non-clarified molasses were used is due to inhibition by metal ions. Though metal ions supported growth of the fungus it has a negative impact on citric acid yield. Oliveira et al., 2012 [60] developed a strategy for the production of citric acid using sugarcane bagasse with vinasse by A. niger. The fermentation was carried out in a packed bed reactor with sugarcane bagasse impregnated suspension of A. niger and vinasse with 80% moisture, incubation temperature of 25  C, aeration flow rate of 0.4L/min of water saturated air and incubation time for 6 days. The citric acid yield under these conditions was 1.45 g of total acid/g of dry bagasse/day. The study represents an alternative to conventional submerged processes for obtaining bio-products from A. niger. Amenaghawon et al., 2013 [61] carried out modeling and optimization of citric acid production from solid state fermentation of sugarcane bagasse using A. niger. Various process parameters affecting citric acid production like media pH, substrate loading and incubation time were optimized by adopting response surface methodology. The optimal fermentation conditions were media pH of 2.0, incubation time of 6 days and substrate loading, 80 g/L. Under optimized conditions the citric acid produced was 18.63 g/L. Yadigary et al., 2013 [62] optimized conditions for citric acid production from sugarcane bagasse by adopting Taguchi design. The study revealed that sugarcane bagasse serves as a cost effective substrate for the production of citric acid. The residue left out after extraction of citric acid and destroying the microbes can be used as an animal feed since SSF decreases the concentration of antinutritional factors in bagasse. 3.9.4. Lactic acid Lactic acid is used in the pharmaceutical, chemical, cosmetic and food industries as well as for biodegradable polymer and green

solvent production. It can be produced by chemical synthesis or by fermentation. The fermentative production of lactic acid has received a lot of interest in the present scenario since it offers an alternative to environmental pollution caused by petrochemical industry and the limited supply of petrochemical resources [63]. Currently lactic acid consumption has been increased a lot due to its role as a monomer in the production of biodegradable polymer, poly lactic acid (PLA). Use of refined materials for the production of lactic acid increases the costs for production even though the cost for product purification should be significantly reduced. Several attempts were going on for the economical production of lactic acid. Utilization of cellulosic materials seems promising since they are cheap, abundant and renewable. Lunelli et al., 2010 [64] reported fermentative production of lactic acid using sucrose obtained from sugarcane molasses. Fermentation was carried out at pH 5.0, incubation temperature of 34  C, 200 rpm and sucrose concentration of 12 g/L. The yield of lactic acid obtained from diluted sugarcane molasses fermentation was 0.83 g/g. Peng et al., 2014 [65] developed an efficient open fermentative production of polymer grade lactic acid from sugarcane bagasse hydrolyzate by thermotolerant Bacillus strain P38. In this study the lactic acid reached a concentration of 185 g/L with a volumetric productivity of 1.93 g/L/h by using sugarcane bagasse hydrolyzate as the sole carbon source along with cotton seed meal as cheap nitrogen source. This is the highest reported lactic acid production using lignocellulosic source. The high tolerance of Bacillus strain P38 to the toxicity of fermentation inhibitors indicate that this strain can be used for the development of an efficient and economical process for lactic acid from various lignocellulosic biomasses. 3.9.5. Butyric acid Butyric acid finds applications in chemical, food and beverage, cosmetic, plastic and textile fiber industries. Its applications as bioactive and therapeutic agents in nutraceutical market in growing rapidly [66]. Currently butyric acid is produced by petroleum based oxo-synthesis of butyraldelyde from propylene. Due to rising oil price, the production of butyric acid by anaerobic fermentation form natural resources has become attractive [67]. A fermentation process for the production of butyric acid from sugarcane bagasse hydrolyzate by Clostridium tyrobutyricum immobilized in a fibrous bed reactor was reported by Wei et al., 2013 [67]. The acid pretreated and enzymatically saccharified sugarcane bagasse was used as carbon source without any detoxification. The butyric acid yield and productivity were 0.48 g/g and 0.51 g/L/h respectively. This is the first report demonstrating the feasibility of butyric acid production from sugarcane bagasse hydrolyzate. 3.9.6. Propionic acid Propionic acid is an important short chain fatty acid with many applications. It finds applications in industries like cellulose plastic, herbicides, perfumes and food. The traditional route of propionic acid production is by the oxidation of propane or propionaldehyde. Currently, the traditional petrochemical route faces more challenges due to limited supply of petroleum. In this scenario, the production of propionic acid from renewable sources seems promising. Considering the cost-efficiency of microbial fermentation, the exploitation of low cost, renewable carbon sources have a positive impact. The utilization of cheap and surplus available sugarcane bagasse as a renewable source for the production of propionic acid will reduce the cost considerably. Green and economic production of propionic acid by Propionibacterium freudenreichii CCTCC M207015 in plant fibrous bed (PFB)

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reactor was reported by Feng et al., 2011 [68]. Propionic acid production from molasses was studied in PFB reactor. With nontreated molasses yielded 12.69 g/L of propionic acid where as PFB fermentation yielded 41.22 g/L of propionic acid. When fed-batch fermentation was performed with hydrolyzed molasses in PFB yielded 91.89 g/L of propionic acid after an incubation time of 254 h. The study revealed that low cost molasses can be utilized for the green and economical production of propionic acid by P. freudenreichii. Chen et al., 2012 [69] evaluated propionic acid production in a plant fibrous-bed bioreactor (PFB) with immobilized P. freudenreichii CCTCC M207015. Sugarcane bagasse was applied to the PFB as immobilizing material. The highest propionic acid concentration obtained was 136.23 g/L which is 1.4 times higher than the highest concentration previously reported (97.0 g/L). Compared with free cell fermentation the fluxes of propionic acid synthesis and the pentose phosphate pathway in PFB fermentation were increased by 84.65% and 227.62% respectively. The results suggest that PFB is a simple and effective method for the high concentration production of propionic acid. Improving the productivity of propionic acid with fibrous bed bioreactor (FBB) e immobilized cells of an adapted acid-tolerant Propionibacterium acidipropionici was carried out by Zhu et al., 2012 [70]. A propionic acid concentration of 51.2 g/L with a high productivity of 0.71 g/L/h was achieved via fed-batch fermentation in FBB system. The productivity was increased by supplementation of sugarcane bagasse hydrolyzate gave 58.8 g/L of propionic acid. The results revealed the potential of sugarcane bagasse as a substrate for the economic production of propionic acid at industrial scale. 3.9.7. Gluconic acid Gluconic acid is a dehydrogenation product of D-glucose which finds application in food, feed, pharmaceutical, textile, cement and chemical industries. The process of gluconic acid production can be made more economic by utilization of agro-industrial residues as substrates for SSF. Singh et al., 2003 [71] developed a strategy for gluconic acid production by A. niger in SSF, SmF, SF (surface fermentation) and SmSF (semi solid state fermentation). The study revealed that overproduction of gluconic acid was observed under SSF conditions using sugarcane bagasse as substrate. 3.10. Xylitol Xylitol is a polyol naturally found in various fruits and vegetables and possess a high sweetening power which finds application in food and pharmaceutical industries. Being a sugar substitute it is used in dietary foods for insulin deficiency patients. Currently the large scale production is typically carried out by a chemical process of D-xylose hydrogenation [72]. Waste utilization for xylitol production seems promising. Hence development and optimization of methods for obtaining xylose from lignocellulosic biomass and conversion to xylitol seems promising. Production of xylitol using hydrolyzate obtained after dilute acid pretreatment of sugarcane bagasse was reported by Sarrouh et al., 2009 [73]. The study revealed that post hydrolysis of the dilute acid pretreated sugarcane bagasse hydrolyzate resulted in an increase in xylose release in the hemicellulose fraction. The advantage of using post-treated hydrolyzate is that it requires less concentration of sugars resulting in a lower concentration of fermentation inhibitors and there was an increase in high xylose to xylitol conversion efficiency (0.7 g xylitol/g xylose) and volumetric productivity compared to the usage of original hemicellulosic hydrolyzate (0.65 g xylitol/g xylose). The post-hydrolysis stage resulted in an increase of xylose concentration from 18.4 g/L to 23.5 g/L. Hence

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there is no need for concentration of the hydrolyzate and resulted in lower fermentation inhibitors like phenolic compounds and acetic acid which in turn have a positive impact by increasing productivity by 13% and xylitol yield by 7%. Branco et al., 2011 [74] developed a strategy for enzymatic production of xylitol using sugarcane bagasse hydrolyzate with glucose dehydrogenase (GDH) system for NADPH in situ generation as well as to verify technical feasibility and potential of enzymatic production of xylitol as an alternative to traditional production processes. Enzymatic strategy is a new alternative for conventional microbiological process which can achieve 100% conversion. The high conversion rate is due to direct transformation of xylose to xylitol which cannot be achieved in conventional fermentative process. The enzymatic strategy involves the direct reduction of xylose to xylitol by the enzyme xylose reductase assisted by the coenzyme reduced form of nicotinamide adenine dinucleotide phosphate (NADPH). The study revealed that 40% v/v concentration of sugarcane bagasse hemicellulosic hydrolyzate (SCBHH) does not interfere with xylitol production but when high content of SCBHH e 80% and 100% v/v, showed a negative impact on xylitol production. Fine tuning of the various process variables affecting xylitol production will improve the yield. Prakash et al., 2011 [75] exploited the potential of microbial production of xylitol from sugarcane bagasse hemicellulose using free and immobilized cells of Debaryomyces hansenii. The efficiency for free and immobilized cells was compared for xylitol production in batch culture at 40  C. The maximum xylitol yield and volumetric productivity produced by free cells were 0.69 g/g and 0.28 g/L/h respectively after detoxification with activated charcoal and ion exchange resins. The maximum xylitol yield and productivity of calcium alginate immobilized cells of D. hansenii were 0.82/g and 0.46 g/L/h respectively. As compared to free cells, immobilized cells produce xylitol more efficiently and can be reused for six cycles without any apparent loss in their fermentation capability. Coupled production of biodiesel, xylitol and xylanase from sugarcane bagasse in a biorefinery concept using fungi was reported by Kamat et al., 2013 [76]. Dilute acid pretreated sugarcane bagasse hydrolyzate was utilized for the production of xylitol by Williopsis saturnus resulted in a yield 0.51 g/g of xylose consumed after 72 h of incubation. Co-cultures for simultaneous production of ethanol and xylitol under continuous multistep versus fed-batch production modes using Candida tropicalis IEC5-ITV and S. cerevisiae ITV01-RD in a simulated medium of sugarcane bagasse hydrolyzate was reported by Castanon- Rodriguez et al., 2014 [77]. The study explores the biotechnological production of ethanol and xylitol by two wild type yeasts as a strategy for biorefinery. The best condition was observed for simultaneous culture was S. cerevisiae co-culture and C. tropicalis sequential cultivation at 24 h. The xylitol productivity and yield at simultaneous culture condition were 0.10 g/L/h and 0.31 g/g respectively. For fed-batch culture the xylitol productivity and yield were the same. The results suggest that the co-culture of these wild type yeasts has the potential for fermenting lignocellulosic substrates to simultaneously produce xylitol and ethanol using continuous cultures. This is a good strategy to make complete use of lignocellulosic residues while obtaining simultaneously two value added products. 3.11. Chelating agents Chelating agents are used for removing heavy metals from industrial effluents. A good chelating agent contains functional groups with high electronic density like carbonyl, amines, thiols, hydroxyls and aromatic rings. Since the lignocellulosic biomass component, lignin contains several of these groups can be used as a

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chelating material. Enzymatic systems serve as a promising strategy for oxidation of lignin. Polyphenoloxidases (PPO) oxidizes lignin producing cresols or quinone structures increasing the number of chelating groups in the lignin. Goncalves et al., 2002 [78] developed a strategy for the production of chelating agents through the enzymatic oxidation of acetosolv sugarcane bagasse lignin. Oxidation of lignin obtained from acetosolv pulping of sugarcane bagasse was performed by polyethylene glycol to increase the number of carbonyl and hydroxyl groups in lignin for improving the chelating capacity. The study revealed that the chelating capacity of lignin oxidized with PPO showed a 73% increase in chelating property when compared to original lignin and is due to incorporation of vinyl hydroxyl groups. Chelating property increases with increase of molecular weight of lignin and is due to increase of polar groups in the lignin and cause an increase in hydrodynamic volume which in turn resulted in an increase in molecular weight.

effluents in a large pH range, which is very useful in industrial processes. 3.14. Amino acids Amino acids find wide range of applications as food additives, feed supplement and therapeutic agents. L-glutamic acid is a nonessential acidic amino acid. It is an important neurotransmitter and plays an important role in neural activation. Monosodium glutamate, the sodium salt of glutamic acid is widely used as flavor enhancer. Nampoothiri and Pandey, 1996 [84] reported L-glutamic acid production by solid state fermentation by Brevibacterium sp. using sugarcane bagasse as substrate. The media was moistened to 85e90% level with mineral salt solution containing glucose, urea and vitamins. The maximum glutamic acid yield was 80 mg/g dry substrate. This is the first report on cultivation of Brevibacterium sp. in solid cultures for the production of glutamic acid.

3.12. Carotenoids 3.15. Animal feed Carotenoids are natural pigments responsible for coloring foods and have important biological activities. It finds application in pharmaceutical, chemical, food and feed industries. Biotechnological route for carotenoid is currently limited by the high cost of production. However the cost can be minimized by using high pigment producing strains cultured in cheap industrial byproducts or agro-residues as nutrient source [79]. Bio-production of carotenoids by Sporidiobolus salmonicolor CBS 2636 using pretreated agro-industrial substrate was reported by Valduga et al., 2008 [80]. Fermentation was carried out with 10 g/L of sugarcane molasses, 5 g/L of corn steep liquor, 5 g/L of yeast hydrolyzate, agitation at 180 rpm and initial pH of 4.0 produced a total carotenoid content of 541.5 mg/L. Freitas et al., 2014 [81] evaluated low-cost carbon sources for carotenoid production by Rhodosporidium toruloides NCYC 921. The yeast carotenoid productivity in sugarcane molasses was 3.85 mg/L/ h. Flow cytometry analysis revealed that most of the yeast cells grown on sugarcane molasses displayed permeabilised cytoplasmic membranes. 3.13. Modified catalysts The utilization of lignocellulosic materials as supports for the adsorption of metallic cat-ions has received much attention due to their low cost. Several research groups have developed adsorption materials based on lignocellulosic matrices as solid supports with good chemical affinity for metallic ions [82]. Modified sugarcane bagasse can efficiently adsorb metallic cat-ions present in water bodies and effluent, making positive impact from an economical and environmental point of view [83]. A novel use for modified sugarcane bagasse containing adsorbed Co2þ and Cr3þ ions as heterogeneous catalysts for the autooxidation of monoterpenes were evaluated by Marquez da Silva et al., 2013 [82]. They developed a process that uses agricultural byproducts like sugarcane bagasse modified with organic ligands like succinic anhydride and EDTA dianhydride and used for the removal of Co2þ and Cr3þ ions from single metal aqueous solutions. These adsorbent materials containing adsorbed Co2þ and Cr3þ as heterogeneous catalysts for the chemical transformation of natural terpenic substrates were evaluated. The study revealed that these materials serve as promising catalysts for the oxidation of monoterpenes. This is the first report in which lignocellulosic adsorbents are applied in a catalytic oxidation process. The catalysts can be reused for three cycles without any loss of activity. The adsorption studies also demonstrated the potential of these adsorbents to treat

One of the major causes of poor livestock productivity in tropical regions of the world is due to inadequate nutrition. This is due to shortage of feed as well as high cost of feed constituents. Exploiting the surplus available sugarcane bagasse for animal feed production seems promising. Sugarcane bagasse has commonly been used for the production of protein enriched animal feed. Treating of sugarcane bagasse with fungus like Pleurotus would remove lignin from the bagasse and improves the nutritive value. Okano et al., 2010 [85] cultivated Pleurotus eryngii on sugarcane bagasse to enhance the digestibility of bagasse. The study revealed that cultivating P. eryngii on bagasse completely removed lignin after incubation for 95 days and there is no difference in in vitro organic matter digestibility (IVOMD), in vitro gas production (IVGP) and in vitro NDFom digestibility (IVNDFom D). After 95 days of biological treatment with P. eryngii the spent bagasse substrate could be used as feed for ruminants. 3.16. Ergot alkaloids Ergot alkaloids are mycotoxins produced by several species of Claviceps. There are four main groups of ergot alkaloids e clavines, lysergic acids, lysergic acid amides and ergopeptides. The demand for ergot alkaloids and their derivatives has increased in recent years due to applications in the treatment of various diseases. Hernandez et al., 1993 [86] used impregnated sugarcane pith bagasse to grow a fungal culture for the production of ergot alkaloids. Sixteen different combinations of liquid nutrient medium were used for impregnating bagasse for the production of ergot alkaloids by Claviceps purpurea. The study revealed that it is possible to achieve tailor made spectra of ergot alkaloids by changing the liquid nutrient media composition used for impregnation. This opens a new avenue of achieving tailor made spectra of ergot alkaloids at an economical cost. 3.17. Antibiotics Antibiotics are one of the best groups of the secondary metabolites synthesized by microorganisms which are active against other microorganisms. Due to its importance in human health care, demand for antibiotic is increasing worldwide. Several efforts have been made to decrease its production cost by process optimization using agricultural residues. Utilization of agro-industrial waste products as substrate has opened the potential to reduce production costs up to 60% by reducing the cost of raw material during

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fermentation [87]. Antibiotic production using SSF requires low energy, less investment cost, higher productivity and ecofriendly than SmF. An advanced SSF system in which a liquid medium adsorbed on an inert sugarcane bagasse support has been applied for antibiotic production by Dominguez et al., 2001 [88]. The main components of the medium are bagasse, nutrients and water. The study revealed that bagasse content strongly controls penicillin production in the SSF system. The bagasse content of the solid medium affects physiology and particular idiophase. The higher bagasse content facilitates water and nutrient transport in the solid medium. Hence, decreasing the bagasse content in the solid medium reduces the growth rate to a more adequate level for Penicillin production. 3.18. Plant growth hormone- gibberellic acid Gibberellic acid is an important fungal secondary metabolite and is a plant growth stimulant widely used in agriculture. Currently gibberellic acids were produced by SmF and the cost is very high due to extremely low yield and expensive downstream processing. SSF production of gibberellic acid has attracted a great deal of attention. Tomasini et al., 1997 [89] evaluated gibberellic acid production by Gibberella fujikuroi in SSF system using different agroresidues. The study revealed that this phytohormone can be produced effectively by SSF on sugarcane bagasse and cassava flour. 4. Conclusion and future perspectives Bioconversion of crop residues is an ecofriendly biotechnological application for sustainable development. Sugarcane crop residues and by-products from sugar industries like bagasse, molasses and vinasse offers great opportunities for interesting product outlet including the production of a wide variety of value added products. Utilization of these residues for alternative energy sources and high value products could improve the sustainability of the bioenergy chain and reduce the negative environmental impacts related to inappropriate disposal. Several R and D activities are going on in this direction to develop an economically as well as ecofriendly strategy for the sustainable production of bioenergy and other value added products. The by-products generated from agroindustrial processing of sugarcane serves as an efficient carbon source for the production of various value added products of commercial interest, most of this is still in infancy and scaling up to pilot scale is a necessity. Targeting on one product is not economically viable, therefore alternative strategies for targeting production of some value addition like production of low volume high value products like amino acids seems promising and make the process economically viable. Though several pretreatment strategies were available none of them can be used as a standard method for the pretreatment of sugarcane crop residues or by products of sugar industry. Several key factors like technical, economic and environmental considerations to be taken into account before selecting a technology for bioconversion. Based on the targeted product, the best pretreatment method has to be selected. Sometimes development of an integrated approach seems promising and fine tuning of the process will make it economically viable. Lot of opportunities are possible for the fundamental R and D for successful exploitation of the full biomass potential by fine tuning technological development and performance improvements to achieve economically feasible and environmentally sustainable yields of desired products. Acknowledgments One

of

the

authors

Raveendran

Sindhu

acknowledges

213

Department of Biotechnology for financial support under DBT BioCARe scheme. Raveendran Sindhu and Parameswaran Binod
de
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