Food Industries Manual 015

12 Snack Foods and Breakfast Cereals

Shaping processes

INTRODUCTION

Aeration

SNACK FOODS

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Page 445

The definition of 'snack foods', which we shall follow in this chapter, is much the same as that used by the grocery trade or included in marketing surveys. The American market research company Frost and Sullivan Inc. suggest the following list:

Other processes

Extrusion

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Page 446

Nuts

Extruded products

Page 460

Page 457

Puffed products Page 458

Other products

Flaked, rolled, granulated products

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Page 454

Pacakaging Page 468

Quality aspects Page 469

Conventional snacks: Potato chips (UK 'crisps') Corn flakes and other forms of corn Savoury/extruded products Nuts Salted/savoury crackers Newer snacks: Instant hot pot snacks and cup soups Cereal bars/mixes Meat snacks Fish snacks Novel - ethnic and filled Countline confectionery items Dairy beverages (milk, yoghurt, fermented milks) Sweet baked items including cookies. Many other foods, however, are typically eaten as snacks in the other sense in which the word is commonly used. This meaning is typically defined in dictionaries as 'a hasty, casual or very light meal'. This would include the traditional midmorning, mid-afternoon or work-break snack, or the growing multi-snack meals that form the basis of a 'grazing' lifestyle in which three main meals a day can be replaced by up to seven or eight snacks taken largely 'on the hoof. No hard and fast rules can be laid down for what is and what is not a snack food. Where a particular product is frequently used as a snack food but is directly derived from a normal, nonsnack foodstuff, then it will be given only brief mention in this chapter. Further details can be found in the appropriate chapter relevant to that particular type of food. On the other hand, the

many current snack foods which have little or no 'main meal' relevance are dealt with in this chapter. BREAKFAST FOODS In the past, when hard manual labour was the lot of most people in the West, the large breakfast gained in popularity. In many countries, it became more or less standardized, for those who could afford it, as a combination of bacon, eggs (usually scrambled or fried), mushrooms, tomatoes, kidneys, fried bread and a few other odds and ends. This was frequently preceded by porridge, and followed by buttered toast and marmalade or other confiture - 'Le petit dejeuner Anglais'. This can still be encountered in many hotels, but has been substantially replaced, both in home and hotel, by a much lighter breakfast consisting basically of some form of cereal, plus frequently fruit (dried, cooked or au-naturel) or fruit juice, and tea or coffee. Breakfast cereals are produced either from individual varieties of grain or from mixtures thereof; wheat, oats, maize (corn), rye, rice and occasionally, barley, are all employed for this purpose. A range of manufacturing techniques are employed to produce a wide variety of products specifically designed to appeal to all ages and, in many cases, ready to be served at the table directly from the package. A large proportion of these breakfast cereal foods will be dealt with in this chapter; others are referred to elsewhere in this book. MANUFACTUREVG TECHNOLOGIES INTRODUCTION It can fairly be said that many of the recent major advances in food processing technology have largely emanated from the requirements of the rapidly expanding snack and breakfast foods sectors. The simple basic technologies, referred to in the text of this chapter, are not addressed separately here in any detail. However, the more complex manufacturing techniques, which are essential elements in the conversion of the various raw materials and their mixtures into the desired end products are discussed in some depth. For present purposes, these technologies, and their resulting products, have been classified as follows: shaping processes, extrusion processes, aeration processes and other miscellaneous processes.

SHAPING PROCESSES Included in this section are descriptions of rolling, flaking and granulating processes. In certain cases, flaking and rolling can be used interchangeably to describe the same operation, as for example, in the flaking or flattening by means of co-rotating rolls of partially precooked cereal grains. In contrast, the terms cannot be interchanged in the case of nuts. Here, rolling is not normally employed; flaking, on the other hand, involves a slicing operation as, for example, in the case of flaked almonds. Granulation is a very different process, normally based on a form of deliberate partial comminution of otherwise large pieces of the intermediate or finished product. A typical example occurs in the manufacture of 'Grape Nuts', a breakfast cereal which, incidentally, is based neither on grapes nor on nuts. Flaking (rolling) of cereals Precooked cereal grains are flaked, either in whole or in part, to achieve the required final dimensions of the finished product. Thickness is normally of particular importance. The principles of this process are the same for all cereals, though the detail is necessarily different. These differences are mentioned later under PRODUCTS. The grain, or grain pieces, usually corn, wheat, oats (groats), barley (pearled) and rice, are first steamed and, with the exception of oats, usually fully cooked, with a concurrent rise in their moisture content of up to 20%. At the same time other ingredients are added if desired - sugar, salt, malt extract, honey, and so on and the temperature of the cook (99-1040C) leads to the inactivation of any enzymes which are still present in the grains. It may be necessary to hold the cooked grains for a length of time for conditioning and they may be also be partially dried as well. The hot, moist and now well plasticized grains are passed individually between a pair of large heavy steel rolls running at 250-400 rpm, heated to a specific temperature, and with a closely controlled gap between them to ensure uniformity of the eventual flake thickness. The flakes are then dried to an appropriate moisture content. A similar process may be used to flake extrudates produced from single or mixed flours or semolinas in a cooker extruder (see below under EXTRUSION PROCESSES) or pelletizer. The pieces must, of course, be appropriately conditioned

before rolling and then are treated in exactly the same manner as rolled whole cereal grains. Further processing of the flakes may entail: (i) Sugaring - accomplished by a spray of sugar plus honey or another 'non-sticky when dry' component with the sugar, (ii) Toasting in a very hot oven to cause some blistering (a form of aeration), colouring and some flavouring, (iii) Addition of flavours by incorporation in sprays or by dusting the flakes Rolling of cereals has a subdivision, which has been used for many years in the production of shreds and which still has many advantages over the use of the extruder for this purpose. Wheat is the most usual cereal to be shredded and Shredded Wheat has been a popular breakfast item for a long while. It is manufactured by a process with a stage that is unique in breakfast cereal manufacture. Washed, whole wheat is boiled in salted water for as long as is necessary to cook the contained starch thoroughly and make it fully dextrinized. At this time, the drained grains will have nearly doubled their weight. They are then allowed to cool and achieve (i) a uniformity of moisture content and (ii) a degree of starch retrogradation (i.e. of crystallization) over a lengthy period of up to a day. The wheat is then fed continuously to pairs of counterrotating rolls, one of which is grooved and the other is not. There is no clearance between the rolls and the only way that the wheat can pass between the rollers is through the grooves. 'Doctor' combs are positioned to remove the shreds so formed. A series of pairs of such rolls feed their shreds on top of each other, any necessary flavouring is added and the mass of shreds is conveyed on a belt where it is cut by deliberately blunted knives which compress and meld the sides and ends of each piece, which can be of any desired size. These now go to a hot oven (about 2550C), where they first expand somewhat before setting and drying, and are finally coloured (browned) at the end of the baking process, which is normally carried out in a typical travelling biscuit oven. The final moisture content of the product is approximately 4%. The flours or semolinas of other cereals may also be used to produce shredded products provided that appropriate modifications are made to the standard wheat process to provide fully acceptable final products. The term 'flaking', as in 'corn flakes', has been used synonymously so far with the word 'rolling'.

In fact it is not, and another form of flaking is possible and is used in a few products. Firmly structured food materials, e.g. nuts and some fruits and vegetables 'au naturel', and some cooked and solid, formed products, may be thinly sliced into flakes on what might be termed the 'spokeshave' principle. This employs a very sharp blade of some kind, set at a fixed spacing to give the desired flake thickness. Such methods of flaking usually require specialized manufacturing equipment because of the great variability in the size, shape and nature of the material to be flaked. Almond flakes are an example of product produced by this technique.

Granulating processes

Within the breakfast foods area, there is nowadays a frequent need for the production of granules (e.g. as in 'Grape Nuts'). There are two basic methods of granulating which can be used, as appropriate. The first method uses the 'pharmaceutical' style of granulator, or the outlet die of a cookerextruder. A fairly stiff dough of the final cooked product is forced through a sieve having the necessary aperture sizes and breaks off (or is broken or cut off) on release, falls onto a travelling oven belt and is then dried and/or toasted and brought to the necessary final crisp state. The original dough may or may not be aerated. In the second method final product dough may be sheeted with appropriate smooth rolls and the sheets (usually around 10 mm thick) are baked to a moisture content which makes them readily breakable, but not overly. This is to avoid a wide range of size fragments and the generation of excess fines. The baked sheets are then passed between suitably spaced ribbed steel rollers - or alternatively between spiked rolls - to produce the necessary granules. There are many variants of these basic methods to serve special needs. Aggregates, which may be mixed in colour and composition, can be produced by loosely gumming particulates together with dextrin, or another vegetable gum, drying them and breaking them up into appropriately sized pieces. EXTRUSION PROCESSES Extrusion technology has grown considerably since the 1950s. The principal developments that have taken place include:

(i) The generation, application and control of temperature during the total extrusion process; (ii) Arising from (i), the possibility of cooking the product in various ways (high temperature, short time or lower temperature, longer time) during the process; (iii) The possibility of 'puffing' or aerating the product by rapid evolution of steam at the point of extrusion at a temperature above 10O0C; (iv) The possibility of rapid, though usually only partial, drying of product by reason of (iii), (v) Arising from (i), effective microbiological control of the finished product and destruction of unwanted enzyme activity; (vi) The possibility of producing fibres as well as the enormous variety of puffed or unpuffed pasta shapes and sizes. This enables particular textures such as meat simulants to be produced, mainly through the use of plasticized proteins as extrudates. 'Baco-bits', which simulate cold, crisp fried bacon, is a typical example of such an extruded product. These changes could not have taken place without major developments in extruder technology, which have for the most part been as follows: (i) A change of layout from the original Sshaped format to a straight-through format. This permits better access to and control of all sections of the extruder. (ii) The development of a wide range of screws or worms having different pitches, with deeper or shallower flights and operating cylinders which may be shallow- or deeply grooved or ungrooved, and of large or small diameter. This results in wide variability in the shear forces produced and the associated energy input. (iii) The use of jacketed barrels for steam- or hotwater heating, or cold-water or refrigerant cooling. Some designs of extruder also feature hollow screw shafts through which steam can be passed, or electric heating. The use of water or steam injection into the food at various stages of the mixing and extrusion processes is also used to control the temperature. The basic extruder design, together with the speed of rotation of the screw, the composition and temperature of the mix, and the die design will all dictate the mechanical energy input to the mix. This is partially converted into heat, which may then have to be removed or augmented by the means described above.

(iv) The introduction of twin-screw, twin-cylinder designs. Nowadays, the food extruder has achieved such a high level of sophistication that it is probably unrivalled in its range of capabilities by any other food-industry process. The things which can normally be accomplished by most modern extrusion plants include: (i) Metering, mixing and holding the ingredients in a manner appropriate to the desired product. (ii) Heating, and cooling if necessary, the contents of the extruder barrel. This can be accomplished by absorbing the energy generated in the course of mixing, melding and otherwise manipulating the dough, and by the further addition or removal of heat as required. This process will determine the degree to which the starch present is gelatinized. (iii) Venting the extruder barrel, with or without vacuum, to control and standardize behaviour and quality of the product on final extrusion. Increasing sophistication in extruders demands increasing sophistication in their feedstocks. All the complex control systems of such extruders can be rendered ineffectual unless the feedstock is standardized, this standardization not only encompassing the proportional mix of items each with its own quite precise specification, but also limitations on the overall analysis and physical properties of the final mix including of course, particle size(s). The reasons for this necessity are fourfold: (i) The particle size of the ingredients must be such as to allow the appropriate flow of mix through the extruder - too small an average particle size (e.g. flour) can cause jamming, whilst too large a size may cause too great a speed of flow and resultant inadequate processing in some areas. (ii) Some precooking of one or more ingredients may lead to premature plasticizing of the mix and problems arising therefrom. (iii) In the case of expansion extrusion particularly, an excess of fat can abort the exercise: 3% of fat or oil is the usual top limit here and this limitation also obtains in many nonexpansion extrusion situations. (iv) The overall efficiency (in terms of throughput and energy usage) of the extrusion process can frequently be improved by the addition of GMS (glyceryl monostearate) to the mix. The optimal quantity (frequently of the order of 1.5%) has to be determined experimentally for each specific mix).

Types of extruder

Piston or pump extruders The simplest form of extrusion is exemplified by the syringe and the spinneret. Here liquids, viscous or otherwise, are extruded through a narrow orifice by pressure, either from a piston in a cylinder or from a pump. In both cases the pressure employed is quite low and no significant amount of work of any kind is applied. The composition or formulation of the liquid has been achieved before it enters the cylinder of the extruder and is unchanged during the operation. The extrusion of foods started, and still continues, in the context of the butcher's shop. Preminced, spiced and generally formulated sausagemeat paste is caused by a piston to extrude through a nozzle located in the end of a cylinder. This carries a circlet of prepared tubular sausage skin, so that the minced meat, as it extrudes, is provided with appropriate edible containment. Modern large scale sausage making is carried out on the same principle but uses a straightforward continuous screw extruder, which is usually allied with a manufactured rather than a natural skin. Miniature, cocktail or snack sausages, usually about 4 cm long, are made by the same method. Collet extruder The Collet extruder was designed in the 1940s for (and now is really only useful for) processing maize grits. It is capable of producing a modest range of variously shaped expanded cooked snacks from low moisture feedstocks. It is a high shear, high pressure, self-heating and self-cooking extruder; the high shear generates a lot of heat in a short length of time. Inlet moisture content is typically 11% (range 9-17%); outlet moisture content 5%; temperature of mix immediately before extrusion 175-18O0C; about 0.10% of the input energy to the screw drive motor is converted into heat absorbed by the product. The products require little drying after extrusion and can readily be sprayed with sugar syrup or other flavouring additives and toasted if desired before consumption. Pasta press

Most forms of extrusion employed in the preparation of snack foods stem from the pasta press (or extruder) which is itself also used in the manufacture of the first stages of some modern snack foods. The feed materials for ordinary pasta (spaghetti, macaroni, tagliatelle, etc.) are very simple - wheat

semolina and water. The wheat used in the manufacture of good quality pasta is almost exclusively durum or amber durum, the semolina from which eventually gives a product that has a translucent appearance. If significant quantities of other wheat semolinas are used the appearance and eating quality of the product are inferior; the presence of non-durum semolina must be declared on the product label. The pasta press is a single-screw extruder with three major zones: (i) mixing and conditioning, (ii) plasticizing and (iii) extrusion. In early pasta manufacturing machines these three zones were positioned for compactness one above the other, giving an inverted S flow. Modern pasta presses are designed on a straight-through format. (i) The first zone receives a metered supply of the durum semolina and water with an overall moisture content of about 22%. This mixture is propelled along a trough by broken screw flights which also act as beaters or mixers. (ii) The semolina, now appropriately hydrated, drops into the entry of the closed cylinder plasticizer, which incorporates a propelling screw furnished with appropriate flights and walls to ensure adequate shear and moulding. As a result, the semolina reaches the final pre-extrusion state as a uniform, relatively high viscosity plastic dough. The moisture content remains about 22%; about 0.02% of the input energy to the screw drive motor is converted into heat absorbed by the product. Most pasta machines have cold-water cooling jackets to remove the heat generated by this intensive work. Before the final stage, the extruder is vented, frequently with a vacuum outlet, to ensure the absence of air bubbles in the dough, which would interfere with the translucency of the final product and its surface integrity. (ii) The final stage is a very low shear building up of pressure in the cylinder at the end of which are the nozzles constituting the die from which the extrudate appears. The shape of the nozzles governs the shape of the finished pasta, from simple rods of spaghetti, strips of tagliatelli or tubes of macaroni to the more or less complicated shapes of various short-cut pastas. The temperature of the paste at this stage is about 550C, well below the boiling point of water; there is therefore no puffing effect as the pressure is brought down to atmospheric. The paste is then cut off to an appropriate length.

Drying completes the manufacturing operation. (Other aspects of pasta manufacture are dealt with in Chapter 5.)

LIVEBIN

High pressure shaper

SCREW FEEDER

This is a single screw machine used for snack foods which are cooked with or without expansion after leaving the extruder. Inlet moisture content 25%; outlet moisture content 25%; temperature of product immediately before extrusion 9O0C; 0.03% of the input energy to the extruder motor is converted to heat absorbed by the product. The products may be part dried and may be expanded in a fryer or an expansion chamber.

DOUBLE CONDITIONING CYLDfDER

VACUUM

VENTED HEAD

EXTRUDER DIE

Low shear cooker-extruder

This is used mainly for making semi-moist pet foods but could also be used for similar intermediate moisture human foods. The high moisture content of such products limits the amount of energy that can be derived from the screw operation and this necessitates jacket heating. Inlet moisture content 28%; outlet moisture content 25%; temperature of the mix immediately before extrusion 12O0C; 0.02% of the input energy to the extruder drive motor is converted into heat in the product.

Figure 12.1 Cutaway view of twin-screw extruder (from Booth, 1990).

or expanded (aerated) form. Products from a twincylinder machine may be of two colours and flavours, in sandwich form, or of filled round bar shape. The expanded product may be further dried, and can be fat- and flavour-sprayed and dusted. Among the snack foods made possible by this type of extruder are the so-called flat breads, the expanded-wall tubes or rods of cooked breakfast cereals and snacks, and expansion-extruded products of the type resembling wafer biscuits. Half products for further processing into snack foods may also be produced. Figures 12.1-12.4 depict typical exterior and

High shear cooker-extruder

This is the most modern development and is extremely versatile. High shear cooker-extruders can produce products of widely varying size, shape, colour, degree of cooking, and so on, in either solid

FEED ZONE

FEED SCREWS

KNEADING ZONE

COOKING ZONE

KNEADING SCREWS

SHEARLOCKS

Figure 12.2 Extruder components (from Booth, 1990).

VENT ZONE

CUT FLIGHT SCREWS

FORMING ZONE

CONE SCREWS

FEED ZONE

KNEADING ZONE

(Raw Material and Surface Moisture)

(Dough Like Mass)

FINAL COOKING ZONE

i!

r£5SffSSSr

DIRECTION OF FLOW

FEEDING SCREW

KNEADING SCREW

FINAL COOKING SCREW

Figure 12.3 Extruder configuration (from Booth, 1990).

Figure 12.4 Die types: (a) single die, (b) double die, (c) triple die, (d) single die with spacer (from Booth, 1990).

interior layouts of modern extruders. Preliminary mixing and any necessary preconditioning of the feedstocks are not shown: more exhaustive details may be found in the references listed under Further Reading at the end of this chapter. The machines are normally of twin-screw design. There is frequently the possibility of interchanging parts, particularly screws, cylinders and dies of different geometry. This enables a wide

range of raw materials to be processed, with a wide range of moisture contents. Typical operating conditions are: inlet moisture content 15%; outlet moisture content 8%; temperature of product immediately before extrusion 12O0C; 0.11% of the input energy to the extruder drive motor is converted to heat absorbed by the product. Pre-die temperatures are considerably in excess

of 10O0C5 so the extrudate is normally therefore fully cooked and any starch present is gelatinized. The pre-die temperatures may be varied almost at will and the amounts of expansion and moisture flash-off can be controlled through the effective use of change parts and modern control systems.

(i) (ii)

(iii)

AERATION PROCESSES Aeration consists of changing a non-porous liquid, plastic or solid food into a porous spongy-textured or honeycomb one. The vacuoles may range in size from the microscopic to the relatively large, such as those found in bread, and may be filled with air, carbon dioxide, nitrogen, or occasionally other gases such as nitrous oxide. Aeration can be reversed in some cases by liquid absorption (e.g. trifle). The means by which aeration is accomplished are quite varied, as is necessary to cope with individual materials and circumstances. They may, however, be classified into various categories such as expansion extrusion; gas injection (e.g. air into ice-cream mix); flash frying of moist material; rapid high temperature baking; the use of yeast fermentation; the use of vacuum together usually with some heat and dehydration; the use of the very hot plate; whipping of slurries, batters and creams; the use of baking powder and other raising agents. The efficacy of the method used depends on the variable characteristics of the material it is desired to aerate. These characteristics include surface tension, fat content and dispersion, whether starch is present and if so, in what form, and the presence of gums, pentosans, albumens and similar natural or synthetic materials. Many of the foods we eat are aerated - most of these would be very much less attractive to the palate if they were not - and the marketing advantages of an expanded product are, or should be, obvious for the most part. Quite a large proportion of snack and breakfast foods are aerated to a greater or lesser degree though the expanded state is frequently unrecognized by the consumer. For example, the fact that icecream is normally half product and half air is not usually appreciated but it has several advantages to the consumer as well as to the producer. Aeration can provide better eating qualities, such as lightness and crispness, greater bulk, a greater surface area in relation to weight and better facilities for flavouring. The summarize, the methods currently used are as follows:

(iv)

(v) (vi) (vii) (viii) (ix) (x)

Fermentation, in which aeration is provided by yeast growth and the evolution of CO2 The use of raising agents, comprising baking powders and ammonium bicarbonate or 'stuff, which also depend on CO2 production 'Puffing', in which product containing superheated moisture is subjected to a sudden release of pressure. This is analogous in many ways to (iv) but is normally only applicable to whole cereal grains Expansion extrusion, by which superheated product is caused to emerge under pressure from a die and the moisture in it immediately vaporizes Frying in very hot fat Whipping to entrap air Vacuum-oven drying Air injection (e.g. into ice-creams) or CO2 injection (Oakes bread process) Nitrous oxide (concurrently used as a propellant) Special cases - hot plate or microwave oven to cook and expand cereal grains.

The relatively new processes of ohmic heating and ultra-high pressure treatment may in due course also offer opportunity for aeration. The essence of satisfactory aeration is that the product should retain the cellular structure it gains, and this depends on the structural qualities of the ingredients in the food, the way in which they have been affected by the process overall, and the presence or absence of modifying ingredients such as oil or fat. The 'fixing' of the aeration is all important. All these methods have to be regarded as 'horses for courses' - the formulation or required product frequently narrows choice to one or, at most, two alternatives. This arises because of two major factors (i) the presence of too much fat in the product may exclude many potential alternatives, and (ii) the degree of cooking of the product, and where and how it happens, is frequently a deciding factor. In this context, it should be mentioned in passing that the use of cold extrusion in the case of a product which cannot be expansion extruded can frequently be employed as a prelude to aeration by another method. Aids to aeration, such as appropriate additives to, or the pretreatment of, formulations are widely used. Additives such as guar, carob or xanthan gums are frequently effective, as are the addition

of ingredients rich in pentosans, and changes to the nature of the starch included in the formulation.

These types of oven are referred to later in relation to specific kinds of product. Frying

OTHER PROCESSES Baking and drying

The oven is one of the most ancient of all food manufacturing facilities. However, the technology has developed considerably over the years. It was originally used for the production of bread, the cooking of meats and for the drying and/or smoking of foods. It now it has many other specialized functions, which include expansion, colouring, flavour development, sterilizing, enzyme destruction and conditioning, to mention but a few. These varied functions have made necessary the development of multifunctional equipment. Many of these types of oven are concerned with the manufacture of snack foods and breakfast foods; their general features are described briefly below. (i) The heating of ovens is almost always carried out using gas or oil as fuel. Special radiant heat generators and so-called microwave units are also occasionally employed. By appropriate use of these techniques including combinations thereof and of proper control methods, various temperature zones can be created and the requisite heating of the product from without and within may be accomplished. So temperatures can be arranged for immediate rapid heating or an initial slow bake, and for such requirements as toasting, browning or drying. Steam may also be injected into the oven to reduce moisture loss. (ii) Ovens can be static; the brick oven is still used for some purposes. However, most are usually mechanized and are loaded and unloaded by a mechanically driven steel belt which can have a longitudinal or spiral path. If the product is loaded on trays, then a tower oven with mechanical uplift and descent may be used. (iii) The vacuum oven, usually used for low temperature dehydration, is a special case but is now engineered so as to be available for continuous throughput operation. (iv) The drying oven, usually heated by forced air circulation, is available in many forms and is widely used in breakfast and snack food production.

Two forms of frying are possible: (i) that carried out in a pan with a very shallow layer of fat, where the material being fried is frequently turned over so that the whole of its surface is eventually heated by the hot fat as well as by conduction within itself; and (ii) total immersion or deep frying, where the material either floats in or is completely covered by the fat. The 'shallow layer' method of frying is only very infrequently used in food production on the large scale and so only frying by immersion will be considered here. A subdivision of deep frying is termed 'flash frying'. This is widely used in snack food production when a very short term, high temperature cook is needed. Frying has a number of functions, which are by no means, all related to cooking in a medium in which heat transfer is much more rapid than in an oven. The things which are customarily expected to be achieved include: (i) Total cooking or final cooking of snack or breakfast food items to the eventual condition required. (ii) Reduction of moisture content and also of any excess fat content of the material in question (e.g. as in pork scratchings). (iii) Expansion of moisture-containing materials and the 'fixing' of the expansion by protein denaturation, drying of contained dextrinized starch, etc. (iv) 'Crisping' and colouring the product as necessary (e.g. potato crisps). (v) Overall improvement in rigidity, strength and mouthfeel of the product. The quality of the oil used for frying is dealt with in Chapter 8 but it is pertinent to mention here that the flash, fire and smoke points need to be taken into account in view of the necessary temperatures required. Also, the stability of the oil and its nutritional qualities (preferably high in monounsaturates) have to be considered because, inevitably, small amounts of residual fat are usually left on the product. This can frequently be minimized by centrifugation or otherwise, but is usually sufficient to allow flavouring mixes or powders in sufficient quantity to adhere firmly to the product after frying. The relatively low bulk density and large surface area of many products help in this matter. Oils used for frying frequently contain antioxidant stabilizers, both for the sake

of maximizing the shelf life of the final product and also extending the working life of the frying oil itself. Coating Many snack foods and breakfast foods are coated with a variety of materials, such as sugar, honey, natural or synthetic flavour concentrates, cheese powder, batters and savoury mix powders, in order to make them attractive on the palate and give them an individuality. For example, cereal flakes and expanded cereals may be sprayed with sugar solution and dried off after each application. Up to 50% of the final product may be sugar. Methods used for coating are very varied. Some examples are as follows: (i) The coating pan, as used for pharmaceutical tablets and products such as almonds, where a pear-shaped open pan angled at about 45% is slowly revolved. This results in its contents being tumbled, while sprays of, for example, sugar solution are applied alternately with jets of hot or warm drying air until the desired level of application is reached. Such coatings may be 'polished' by a final dusting of icing sugar, talc or similar powder. (ii) The products travel on a belt which is vibrated at intervals and dusted with appropriate powders; some of the powder adheres and the rest is recirculated. The product needs to be somewhat adhesive for this method to be effective. (iii) The products may be dipped into or sprayed with a batter, which is then converted into a 'shell' coating by heat and drying. The batter may be smooth cream or particulate in nature and may, in some cases, be aerated itself. (iv) Products such as a fruit and/or cereal bar or biscuit, may be coated in an 'enrober', through which it is usually carried on a wire mesh belt and deluged from above and below with tempered liquid chocolate or a substitute. The excess drains off and the product then enters a cooling tunnel where the couverture sets. (v) A 'coated' product can be produced in a twin-screw extrusion operation. A central main core of product is surrounded by an annular extruded 'jacket' which may be of very different composition and style from the core. The Japanese-made Rheon machine, based on a similar approach, will produce multicoated products such as analogues of Scotch eggs.

(vi) Glazing, or the coating of ice on frozen snack products (e.g. prawns) resulting from the application of a cold water spray has the function of preventing dehydration and avoiding toughening and a general deterioration of appearance and mouthfeel. Mixing This is a process which is often incorporated within another process as a necessary ancillary (e.g. as in extrusion) but is also recognized as a process in its own right. When employed in this context, it may also incorporate functions other than mixing, such as aeration, emulsification, dough formation and comminution. Consequently, a wide range of mixing equipment has been developed and is available commercially. Some of the principal 'mixing' functions are as described below. Mixing of solids Particle size defines the necessary style of mixer for powders, the ribbon mixer in a trough is one possibility, as is a sealed drum with baffles, rotating on the axis of the drum or end-over-end. Such machines are also suitable for mixing materials of particle size averaging that of granulated sugar. For mixing solids of disparate but larger size (e.g. muesli ingredients) a rotating twin conical 'V type of mixer is suitable. Mixing of solids and liquids Here the required state of the mixture defines the type of plant to be used. For the production of suspensions or solutions, the high speed propeller agitator within an open tank may be adequate. If some degree of comminution is necessary, or lump formation is a problem, then the Silverson type of mixer is needed. Aeration is a feature of some mixers (e.g. in the Chorleywood bread process). If the final product is a dough, the sort of mixer required becomes a more massive Z-blade or similar machine, or the type of integral doughforming mixer incorporated into extruders. Mixing of non-miscible liquids Here the aim, aided by stabilizers, is to produce an emulsion or cream from, for example, an oilwater mixture. A high speed propellant mixing screw may be adequate here, otherwise a homogenizer may be necessary.

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Mixing ofmiscible liquids This hardly needs mention, merely stirring with paddles or a recirculating pump is usually adequate so long as sufficient time is taken, particularly when the liquids have different specific gravities.

cutter used for this purpose consists of a hollow perforated drum, rotating with a doctor knife close to its outside surface. Groats within the drum project though the holes and are cut, usually into three pieces, which then constitute 'pinhead' oatmeal. These are the bare bones of what is quite a sophisticated operation. Slicing of nuts, e.g. almonds, also requires specialized equipment.

Masa production A wide range of snacks manufactured and used in USA, Mexico, Central America, and also now in the UK, require a special form of maize as a starting point - masa flour or masa meal. Masa is produced by treating whole grains of maize with lye (lime, caustic soda or KOH) which renders the pericarp easily removable. The temperature of the treatment also partially cooks the maize. Subsequently, the alkali is washed out of the grain, the pericarp removed and the masa can then be used immediately for tortilla or taco manufacture. However, it is mainly dried and ground to provide masa flour. Among other uses, this can be used as a basis for making corn flakes. Sieving The mouthfeel of products markedly affects their acceptability. It is therefore necessary to control the perceived granularity, roughness or whatever criterion may be applied to measure this factor. To standardize on the optimum range of particle size, whether as a powder, or as suspended particles in a liquid, it is necessary to sieve many products. Fines go to rework, oversize go back to recomminution.

Chopping and slicing

Here, to a large extent, the processes used are specifically related to individual raw materials or products. There is, however, an overriding necessity, which holds in almost all cases, to keep the knife blades sharp. Blunt knives give rise to fines and improperly cut material and a corresponding financial loss. Chopping is the easier process - the ubiquitous bowl chopper, properly programmed, can deal with most requirements. Slicing, however, frequently has to be carried out by specially tailored equipment Such a case is that of cut groats, which constitute the highest volume oat product for human consumption, forming as they do the basis of porridge, oat cakes, cereal bars, and so on. The

Chilling and freezing

Quite apart from their preservation function, these processes, particularly freezing, are integral parts of the structuring and presentation of some snack foods. The obvious examples are ice-cream and many analogues. The aeration and the physical nature (hard or soft) of ice-cream cannot be accomplished without ancillary freezing.

Brining and curing

Salt is the primary agent in both these processes, which are quite similar. Other agents may be added to the brine for curing purposes: nitrates and/or nitrites, for example, and special flavouring materials. Brining, which is mostly carried out on meat and fish, results in two essential effects, namely dehydration by osmosis and preservation. Additional effects are on flavour and colour. Brining and curing may be carried out by straightforward immersion, by 'tumbled' immersion to speed up penetration of the cure into hams and by vacuum and pressure methods.

PRODUCTS FLAKED, ROLLED AND GRANULATED BREAKFAST PRODUCTS The line dividing breakfast food products and snack foods is not at all precise - corn flakes, for example, are now being widely advertised as being suitable for any daytime use. Consequently, this section cannot be unambiguously divided into the two areas. A start will, however, be made on what is largely accepted as being the breakfast food area, viz. cereal products. These may, however, be at times combined with items sourced from potatoes and other vegetable (and largely starchy) items and small quantities of other additives.