INTRODUCTION

SNACK FOODS

The definition of 'snack foods', which we shallfollow in this chapter, is much the same as thatused by the grocery trade or included in marketingsurveys. The American market research companyFrost and Sullivan Inc. suggest the following list:

Conventional snacks:Potato chips (UK 'crisps')Corn flakes and other forms of cornSavoury/extruded productsNutsSalted/savoury crackers

Newer snacks:Instant hot pot snacks and cup soupsCereal bars/mixesMeat snacksFish snacksNovel - ethnic and filledCountline confectionery itemsDairy beverages (milk, yoghurt, fermented

milks)Sweet baked items including cookies.

Many other foods, however, are typically eaten assnacks in the other sense in which the word iscommonly used. This meaning is typically definedin dictionaries as 'a hasty, casual or very lightmeal'. This would include the traditional mid-morning, mid-afternoon or work-break snack, orthe growing multi-snack meals that form the basisof a 'grazing' lifestyle in which three main meals aday can be replaced by up to seven or eightsnacks taken largely 'on the hoof.

No hard and fast rules can be laid down forwhat is and what is not a snack food. Where aparticular product is frequently used as a snackfood but is directly derived from a normal, non-snack foodstuff, then it will be given only briefmention in this chapter. Further details can befound in the appropriate chapter relevant to thatparticular type of food. On the other hand, the

12 Snack Foods and Breakfast Cereals

Aeration

Page 451

Shapingprocesses

Page 445

Extrusion

Page 446

Otherprocesses

Page 452

Extrudedproducts

Page 457

Nuts

Page 460

Puffedproducts

Page 458

Flaked, rolled,granulatedproducts

Page 454

Otherproducts

Page 459

Pacakaging

Page 468

Quality aspects

Page 469

many current snack foods which have little or no'main meal' relevance are dealt with in thischapter.

BREAKFAST FOODS

In the past, when hard manual labour was the lotof most people in the West, the large breakfastgained in popularity. In many countries, it becamemore or less standardized, for those who couldafford it, as a combination of bacon, eggs (usuallyscrambled or fried), mushrooms, tomatoes,kidneys, fried bread and a few other odds andends. This was frequently preceded by porridge,and followed by buttered toast and marmalade orother confiture - 'Le petit dejeuner Anglais'. Thiscan still be encountered in many hotels, but hasbeen substantially replaced, both in home andhotel, by a much lighter breakfast consisting basi-cally of some form of cereal, plus frequently fruit(dried, cooked or au-naturel) or fruit juice, andtea or coffee.

Breakfast cereals are produced either from indi-vidual varieties of grain or from mixtures thereof;wheat, oats, maize (corn), rye, rice and occasion-ally, barley, are all employed for this purpose. Arange of manufacturing techniques are employedto produce a wide variety of products specificallydesigned to appeal to all ages and, in manycases, ready to be served at the table directlyfrom the package. A large proportion of thesebreakfast cereal foods will be dealt with in thischapter; others are referred to elsewhere in thisbook.

MANUFACTUREVG TECHNOLOGIES

INTRODUCTION

It can fairly be said that many of the recent majoradvances in food processing technology havelargely emanated from the requirements of therapidly expanding snack and breakfast foodssectors. The simple basic technologies, referred toin the text of this chapter, are not addressed sepa-rately here in any detail. However, the morecomplex manufacturing techniques, which areessential elements in the conversion of the variousraw materials and their mixtures into the desiredend products are discussed in some depth. Forpresent purposes, these technologies, and theirresulting products, have been classified as follows:shaping processes, extrusion processes, aerationprocesses 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 todescribe the same operation, as for example, in theflaking or flattening by means of co-rotating rollsof partially precooked cereal grains. In contrast,the terms cannot be interchanged in the case ofnuts. Here, rolling is not normally employed;flaking, on the other hand, involves a slicingoperation as, for example, in the case of flakedalmonds.

Granulation is a very different process, normallybased on a form of deliberate partial comminutionof otherwise large pieces of the intermediate or fin-ished product. A typical example occurs in themanufacture of 'Grape Nuts', a breakfast cerealwhich, incidentally, is based neither on grapes noron nuts.

Flaking (rolling) of cereals

Precooked cereal grains are flaked, either in wholeor in part, to achieve the required final dimensionsof the finished product. Thickness is normally ofparticular importance. The principles of thisprocess are the same for all cereals, though thedetail is necessarily different. These differences arementioned later under PRODUCTS.

The grain, or grain pieces, usually corn, wheat,oats (groats), barley (pearled) and rice, are firststeamed and, with the exception of oats, usuallyfully cooked, with a concurrent rise in theirmoisture content of up to 20%. At the same timeother ingredients are added if desired - sugar,salt, malt extract, honey, and so on and the tem-perature of the cook (99-1040C) leads to theinactivation of any enzymes which are stillpresent in the grains. It may be necessary to holdthe cooked grains for a length of time for condi-tioning and they may be also be partially driedas well. The hot, moist and now well plasticizedgrains are passed individually between a pair oflarge heavy steel rolls running at 250-400 rpm,heated to a specific temperature, and with aclosely controlled gap between them to ensureuniformity of the eventual flake thickness. Theflakes are then dried to an appropriate moisturecontent.

A similar process may be used to flake extru-dates produced from single or mixed flours orsemolinas in a cooker extruder (see below underEXTRUSION PROCESSES) or pelletizer. The piecesmust, of course, be appropriately conditioned

before rolling and then are treated in exactly thesame manner as rolled whole cereal grains.

Further processing of the flakes may entail:

(i) Sugaring - accomplished by a spray of sugarplus honey or another 'non-sticky when dry'component with the sugar,

(ii) Toasting in a very hot oven to cause someblistering (a form of aeration), colouring andsome flavouring,

(iii) Addition of flavours by incorporation insprays or by dusting the flakes

Rolling of cereals has a subdivision, which hasbeen used for many years in the production ofshreds and which still has many advantages overthe use of the extruder for this purpose. Wheat isthe most usual cereal to be shredded and ShreddedWheat has been a popular breakfast item for along while. It is manufactured by a process with astage that is unique in breakfast cereal manufac-ture.

Washed, whole wheat is boiled in salted waterfor as long as is necessary to cook the containedstarch thoroughly and make it fully dextrinized.At this time, the drained grains will have nearlydoubled their weight. They are then allowed tocool and achieve (i) a uniformity of moisturecontent and (ii) a degree of starch retrogradation(i.e. of crystallization) over a lengthy period of upto a day. The wheat is then fed continuously topairs of counterrotating rolls, one of which isgrooved and the other is not. There is no clear-ance between the rolls and the only way that thewheat can pass between the rollers is through thegrooves. 'Doctor' combs are positioned to removethe shreds so formed.

A series of pairs of such rolls feed theirshreds on top of each other, any necessary fla-vouring is added and the mass of shreds is con-veyed on a belt where it is cut by deliberatelyblunted knives which compress and meld thesides and ends of each piece, which can be ofany desired size. These now go to a hot oven(about 2550C), where they first expand somewhatbefore setting and drying, and are finallycoloured (browned) at the end of the bakingprocess, which is normally carried out in atypical travelling biscuit oven. The final moisturecontent of the product is approximately 4%.The flours or semolinas of other cereals mayalso be used to produce shredded products pro-vided that appropriate modifications are made tothe standard wheat process to provide fullyacceptable final products.

The term 'flaking', as in 'corn flakes', has beenused synonymously so far with the word 'rolling'.

In fact it is not, and another form of flaking ispossible and is used in a few products.

Firmly structured food materials, e.g. nuts andsome fruits and vegetables 'au naturel', and somecooked and solid, formed products, may be thinlysliced into flakes on what might be termed the'spokeshave' principle. This employs a very sharpblade of some kind, set at a fixed spacing to givethe desired flake thickness. Such methods offlaking usually require specialized manufacturingequipment because of the great variability in thesize, shape and nature of the material to be flaked.Almond flakes are an example of product pro-duced by this technique.

Granulating processes

Within the breakfast foods area, there is nowadaysa frequent need for the production of granules(e.g. as in 'Grape Nuts'). There are two basicmethods of granulating which can be used, asappropriate.

The first method uses the 'pharmaceutical' styleof granulator, or the outlet die of a cooker-extruder. A fairly stiff dough of the final cookedproduct is forced through a sieve having the neces-sary aperture sizes and breaks off (or is broken orcut off) on release, falls onto a travelling oven beltand is then dried and/or toasted and brought tothe necessary final crisp state. The original doughmay or may not be aerated.

In the second method final product dough maybe sheeted with appropriate smooth rolls and thesheets (usually around 10 mm thick) are baked toa moisture content which makes them readilybreakable, but not overly. This is to avoid a widerange of size fragments and the generation ofexcess fines. The baked sheets are then passedbetween suitably spaced ribbed steel rollers - oralternatively between spiked rolls - to produce thenecessary granules.

There are many variants of these basic methodsto serve special needs. Aggregates, which may bemixed in colour and composition, can be producedby loosely gumming particulates together withdextrin, or another vegetable gum, drying themand breaking them up into appropriately sizedpieces.

EXTRUSION PROCESSES

Extrusion technology has grown considerablysince the 1950s. The principal developments thathave taken place include:

(i) The generation, application and control oftemperature during the total extrusionprocess;

(ii) Arising from (i), the possibility of cooking theproduct in various ways (high temperature,short time or lower temperature, longer time)during the process;

(iii) The possibility of 'puffing' or aerating theproduct by rapid evolution of steam at thepoint of extrusion at a temperature above10O0C;

(iv) The possibility of rapid, though usually onlypartial, drying of product by reason of (iii),

(v) Arising from (i), effective microbiologicalcontrol of the finished product and destruc-tion of unwanted enzyme activity;

(vi) The possibility of producing fibres as well asthe enormous variety of puffed or unpuffedpasta shapes and sizes. This enables particulartextures such as meat simulants to be pro-duced, mainly through the use of plasticizedproteins as extrudates. 'Baco-bits', whichsimulate cold, crisp fried bacon, is a typicalexample of such an extruded product.

These changes could not have taken place withoutmajor developments in extruder technology, whichhave for the most part been as follows:

(i) A change of layout from the original S-shaped format to a straight-through format.This permits better access to and control ofall sections of the extruder.

(ii) The development of a wide range of screws orworms having different pitches, with deeperor shallower flights and operating cylinderswhich may be shallow- or deeply grooved orungrooved, and of large or small diameter.This results in wide variability in the shearforces produced and the associated energyinput.

(iii) The use of jacketed barrels for steam- or hot-water heating, or cold-water or refrigerantcooling. Some designs of extruder also featurehollow screw shafts through which steam canbe passed, or electric heating. The use of wateror steam injection into the food at variousstages of the mixing and extrusion processes isalso used to control the temperature.

The basic extruder design, together with thespeed of rotation of the screw, the composi-tion and temperature of the mix, and the diedesign will all dictate the mechanical energyinput to the mix. This is partially convertedinto heat, which may then have to beremoved or augmented by the meansdescribed above.

(iv) The introduction of twin-screw, twin-cylinderdesigns.

Nowadays, the food extruder has achieved sucha high level of sophistication that it is probablyunrivalled in its range of capabilities by any otherfood-industry process. The things which can nor-mally be accomplished by most modern extrusionplants include:(i) Metering, mixing and holding the ingredients

in a manner appropriate to the desiredproduct.

(ii) Heating, and cooling if necessary, the contentsof the extruder barrel. This can be accom-plished by absorbing the energy generated inthe course of mixing, melding and otherwisemanipulating the dough, and by the furtheraddition or removal of heat as required. Thisprocess will determine the degree to which thestarch present is gelatinized.

(iii) Venting the extruder barrel, with or withoutvacuum, to control and standardize behaviourand quality of the product on final extrusion.

Increasing sophistication in extruders demandsincreasing sophistication in their feedstocks. Allthe complex control systems of such extruders canbe rendered ineffectual unless the feedstock is stan-dardized, this standardization not only encom-passing the proportional mix of items each withits own quite precise specification, but also limita-tions on the overall analysis and physical proper-ties of the final mix including of course, particlesize(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 mixthrough the extruder - too small an averageparticle size (e.g. flour) can cause jamming,whilst too large a size may cause too great aspeed of flow and resultant inadequate pro-cessing in some areas.

(ii) Some precooking of one or more ingredientsmay lead to premature plasticizing of the mixand problems arising therefrom.

(iii) In the case of expansion extrusion particu-larly, an excess of fat can abort the exercise:3% of fat or oil is the usual top limit hereand this limitation also obtains in many non-expansion extrusion situations.

(iv) The overall efficiency (in terms of throughputand energy usage) of the extrusion processcan frequently be improved by the addition ofGMS (glyceryl monostearate) to the mix. Theoptimal quantity (frequently of the order of1.5%) has to be determined experimentallyfor each specific mix).

Types of extruder

Piston or pump extruders

The simplest form of extrusion is exemplified bythe syringe and the spinneret. Here liquids, viscousor otherwise, are extruded through a narroworifice by pressure, either from a piston in acylinder or from a pump. In both cases the pres-sure employed is quite low and no significantamount of work of any kind is applied. The com-position or formulation of the liquid has beenachieved before it enters the cylinder of theextruder and is unchanged during the operation.

The extrusion of foods started, and still con-tinues, in the context of the butcher's shop. Pre-minced, spiced and generally formulated sausage-meat paste is caused by a piston to extrudethrough a nozzle located in the end of a cylinder.This carries a circlet of prepared tubular sausageskin, so that the minced meat, as it extrudes, isprovided with appropriate edible containment.Modern large scale sausage making is carried outon the same principle but uses a straightforwardcontinuous screw extruder, which is usually alliedwith a manufactured rather than a natural skin.Miniature, cocktail or snack sausages, usuallyabout 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) processingmaize grits. It is capable of producing a modestrange of variously shaped expanded cookedsnacks from low moisture feedstocks. It is a highshear, high pressure, self-heating and self-cookingextruder; the high shear generates a lot of heat ina short length of time. Inlet moisture content istypically 11% (range 9-17%); outlet moisturecontent 5%; temperature of mix immediatelybefore extrusion 175-18O0C; about 0.10% of theinput energy to the screw drive motor is convertedinto heat absorbed by the product.

The products require little drying after extrusionand can readily be sprayed with sugar syrup orother flavouring additives and toasted if desiredbefore consumption.

Pasta press

Most forms of extrusion employed in the prepara-tion of snack foods stem from the pasta press (orextruder) which is itself also used in the manufac-ture of the first stages of some modern snackfoods.

The feed materials for ordinary pasta (spaghetti,macaroni, tagliatelle, etc.) are very simple - wheat

semolina and water. The wheat used in the manu-facture of good quality pasta is almost exclusivelydurum or amber durum, the semolina from whicheventually gives a product that has a translucentappearance. If significant quantities of other wheatsemolinas are used the appearance and eatingquality of the product are inferior; the presence ofnon-durum semolina must be declared on theproduct label.

The pasta press is a single-screw extruder withthree major zones: (i) mixing and conditioning, (ii)plasticizing and (iii) extrusion. In early pasta man-ufacturing machines these three zones were posi-tioned for compactness one above the other,giving an inverted S flow. Modern pasta pressesare designed on a straight-through format.

(i) The first zone receives a metered supply ofthe durum semolina and water with anoverall moisture content of about 22%. Thismixture is propelled along a trough bybroken screw flights which also act as beatersor mixers.

(ii) The semolina, now appropriately hydrated,drops into the entry of the closed cylinderplasticizer, which incorporates a propellingscrew furnished with appropriate flights andwalls to ensure adequate shear and moulding.As a result, the semolina reaches the finalpre-extrusion state as a uniform, relativelyhigh viscosity plastic dough. The moisturecontent remains about 22%; about 0.02% ofthe input energy to the screw drive motor isconverted into heat absorbed by the product.Most pasta machines have cold-water coolingjackets to remove the heat generated by thisintensive work. Before the final stage, theextruder is vented, frequently with a vacuumoutlet, to ensure the absence of air bubbles inthe dough, which would interfere with thetranslucency of the final product and itssurface integrity.

(ii) The final stage is a very low shear building upof pressure in the cylinder at the end of whichare the nozzles constituting the die fromwhich the extrudate appears. The shape of thenozzles governs the shape of the finishedpasta, from simple rods of spaghetti, strips oftagliatelli or tubes of macaroni to the more orless complicated shapes of various short-cutpastas. The temperature of the paste at thisstage is about 550C, well below the boilingpoint of water; there is therefore no puffingeffect as the pressure is brought down toatmospheric.

The paste is then cut off to an appropriate length.

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

High pressure shaper

This is a single screw machine used for snackfoods which are cooked with or without expansionafter leaving the extruder. Inlet moisture content25%; outlet moisture content 25%; temperature ofproduct immediately before extrusion 9O0C; 0.03%of the input energy to the extruder motor is con-verted to heat absorbed by the product. The pro-ducts may be part dried and may be expanded ina fryer or an expansion chamber.

Low shear cooker-extruder

This is used mainly for making semi-moist petfoods but could also be used for similar inter-mediate moisture human foods. The high moisturecontent of such products limits the amount ofenergy that can be derived from the screw opera-tion and this necessitates jacket heating. Inletmoisture content 28%; outlet moisture content25%; temperature of the mix immediately beforeextrusion 12O0C; 0.02% of the input energy to theextruder drive motor is converted into heat in theproduct.

High shear cooker-extruder

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

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

or expanded (aerated) form. Products from a twin-cylinder machine may be of two colours and fla-vours, in sandwich form, or of filled round barshape. The expanded product may be further dried,and can be fat- and flavour-sprayed and dusted.

Among the snack foods made possible by thistype of extruder are the so-called flat breads, theexpanded-wall tubes or rods of cooked breakfastcereals and snacks, and expansion-extruded pro-ducts of the type resembling wafer biscuits. Halfproducts for further processing into snack foodsmay also be produced.

Figures 12.1-12.4 depict typical exterior and

Figure 12.2 Extruder components (from Booth, 1990).

SHEARLOCKS

CONESCREWS

CUT FLIGHTSCREWS

KNEADINGSCREWS

FEEDSCREWS

FEEDZONE

KNEADINGZONE

COOKINGZONE

VENTZONE

FORMINGZONE

LIVEBIN

SCREWFEEDER

DOUBLE CONDITIONINGCYLDfDER

VACUUM

VENTEDHEAD

EXTRUDERDIE

FEEDZONE

(Raw Material andSurface Moisture)

KNEADINGZONE

(Dough Like Mass)

FINAL COOKINGZONE

i!r£5SffSSSr

DIRECTION OF FLOW

FEEDINGSCREW

KNEADINGSCREW

FINALCOOKING 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. Preliminarymixing and any necessary preconditioning of thefeedstocks are not shown: more exhaustive detailsmay be found in the references listed underFurther Reading at the end of this chapter.

The machines are normally of twin-screwdesign. There is frequently the possibility of inter-changing parts, particularly screws, cylinders anddies of different geometry. This enables a wide

range of raw materials to be processed, with awide range of moisture contents. Typical oper-ating conditions are: inlet moisture content 15%;outlet moisture content 8%; temperature ofproduct immediately before extrusion 12O0C;0.11% of the input energy to the extruder drivemotor is converted to heat absorbed by theproduct.

Pre-die temperatures are considerably in excess

of 10O0C5 so the extrudate is normally thereforefully cooked and any starch present is gelati-nized. The pre-die temperatures may be variedalmost at will and the amounts of expansionand moisture flash-off can be controlled throughthe effective use of change parts and moderncontrol systems.

AERATION PROCESSES

Aeration consists of changing a non-porous liquid,plastic or solid food into a porous spongy-texturedor honeycomb one. The vacuoles may range insize from the microscopic to the relatively large,such as those found in bread, and may be filledwith air, carbon dioxide, nitrogen, or occasionallyother gases such as nitrous oxide. Aeration can bereversed in some cases by liquid absorption (e.g.trifle).

The means by which aeration is accomplishedare quite varied, as is necessary to cope with indi-vidual materials and circumstances. They may,however, be classified into various categories suchas expansion extrusion; gas injection (e.g. air intoice-cream mix); flash frying of moist material;rapid high temperature baking; the use of yeastfermentation; the use of vacuum together usuallywith some heat and dehydration; the use of thevery hot plate; whipping of slurries, batters andcreams; the use of baking powder and otherraising agents. The efficacy of the method useddepends on the variable characteristics of thematerial it is desired to aerate. These characteris-tics include surface tension, fat content and disper-sion, whether starch is present and if so, in whatform, and the presence of gums, pentosans, albu-mens and similar natural or synthetic materials.

Many of the foods we eat are aerated - mostof these would be very much less attractive tothe palate if they were not - and the marketingadvantages of an expanded product are, orshould be, obvious for the most part. Quite alarge proportion of snack and breakfast foodsare aerated to a greater or lesser degree thoughthe expanded state is frequently unrecognized bythe consumer. For example, the fact that ice-cream is normally half product and half air isnot usually appreciated but it has several advan-tages to the consumer as well as to the producer.Aeration can provide better eating qualities, suchas lightness and crispness, greater bulk, a greatersurface area in relation to weight and better facil-ities for flavouring.

The summarize, the methods currently used areas follows:

(i) Fermentation, in which aeration is providedby yeast growth and the evolution of CO2

(ii) The use of raising agents, comprising bakingpowders and ammonium bicarbonate or'stuff, which also depend on CO2 produc-tion

(iii) 'Puffing', in which product containing super-heated moisture is subjected to a suddenrelease of pressure. This is analogous inmany ways to (iv) but is normally onlyapplicable to whole cereal grains

(iv) Expansion extrusion, by which superheatedproduct is caused to emerge under pressurefrom a die and the moisture in it immedi-ately vaporizes

(v) Frying in very hot fat(vi) Whipping to entrap air(vii) Vacuum-oven drying(viii) Air injection (e.g. into ice-creams) or CO2

injection (Oakes bread process)(ix) Nitrous oxide (concurrently used as a pro-

pellant)(x) Special cases - hot plate or microwave oven

to cook and expand cereal grains.

The relatively new processes of ohmic heating andultra-high pressure treatment may in due coursealso offer opportunity for aeration.

The essence of satisfactory aeration is that theproduct should retain the cellular structure itgains, and this depends on the structural qualitiesof the ingredients in the food, the way in whichthey have been affected by the process overall, andthe presence or absence of modifying ingredientssuch as oil or fat. The 'fixing' of the aeration is allimportant.

All these methods have to be regarded as'horses for courses' - the formulation or requiredproduct frequently narrows choice to one or, atmost, two alternatives. This arises because of twomajor factors

(i) the presence of too much fat in the productmay exclude many potential alternatives, and

(ii) the degree of cooking of the product, andwhere and how it happens, is frequently adeciding factor. In this context, it should bementioned in passing that the use of coldextrusion in the case of a product whichcannot be expansion extruded can frequentlybe employed as a prelude to aeration byanother method.

Aids to aeration, such as appropriate additivesto, or the pretreatment of, formulations are widelyused. Additives such as guar, carob or xanthangums are frequently effective, as are the addition

of ingredients rich in pentosans, and changes tothe nature of the starch included in the formula-tion.

OTHER PROCESSES

Baking and drying

The oven is one of the most ancient of all foodmanufacturing facilities. However, the technologyhas developed considerably over the years. It wasoriginally used for the production of bread, thecooking of meats and for the drying and/orsmoking of foods. It now it has many other spe-cialized functions, which include expansion, col-ouring, flavour development, sterilizing, enzymedestruction and conditioning, to mention but afew. These varied functions have made necessarythe development of multifunctional equipment.Many of these types of oven are concerned withthe manufacture of snack foods and breakfastfoods; their general features are described brieflybelow.

(i) The heating of ovens is almost always carriedout using gas or oil as fuel. Special radiantheat generators and so-called microwave unitsare also occasionally employed. By appro-priate use of these techniques including com-binations thereof and of proper controlmethods, various temperature zones can becreated and the requisite heating of theproduct from without and within may beaccomplished. So temperatures can bearranged for immediate rapid heating or aninitial slow bake, and for such requirementsas toasting, browning or drying. Steam mayalso be injected into the oven to reducemoisture loss.

(ii) Ovens can be static; the brick oven is stillused for some purposes. However, most areusually mechanized and are loaded andunloaded by a mechanically driven steel beltwhich can have a longitudinal or spiral path.If the product is loaded on trays, then atower oven with mechanical uplift anddescent may be used.

(iii) The vacuum oven, usually used for low tem-perature dehydration, is a special case but isnow engineered so as to be available for con-tinuous throughput operation.

(iv) The drying oven, usually heated by forced aircirculation, is available in many forms and iswidely used in breakfast and snack food pro-duction.

These types of oven are referred to later in rela-tion to specific kinds of product.

Frying

Two forms of frying are possible: (i) that carriedout in a pan with a very shallow layer of fat,where the material being fried is frequently turnedover so that the whole of its surface is eventuallyheated by the hot fat as well as by conductionwithin itself; and (ii) total immersion or deepfrying, where the material either floats in or iscompletely covered by the fat. The 'shallow layer'method of frying is only very infrequently used infood production on the large scale and so onlyfrying by immersion will be considered here. Asubdivision of deep frying is termed 'flash frying'.This is widely used in snack food productionwhen a very short term, high temperature cook isneeded.

Frying has a number of functions, which are byno means, all related to cooking in a medium inwhich heat transfer is much more rapid than in anoven. The things which are customarily expectedto be achieved include:

(i) Total cooking or final cooking of snack orbreakfast food items to the eventual conditionrequired.

(ii) Reduction of moisture content and also ofany excess fat content of the material in ques-tion (e.g. as in pork scratchings).

(iii) Expansion of moisture-containing materialsand the 'fixing' of the expansion by proteindenaturation, drying of contained dextrinizedstarch, etc.

(iv) 'Crisping' and colouring the product as neces-sary (e.g. potato crisps).

(v) Overall improvement in rigidity, strength andmouthfeel of the product.

The quality of the oil used for frying is dealtwith in Chapter 8 but it is pertinent to mentionhere that the flash, fire and smoke points need tobe taken into account in view of the necessarytemperatures required. Also, the stability of the oiland its nutritional qualities (preferably high inmonounsaturates) have to be considered because,inevitably, small amounts of residual fat areusually left on the product. This can frequently beminimized by centrifugation or otherwise, but isusually sufficient to allow flavouring mixes orpowders in sufficient quantity to adhere firmly tothe product after frying. The relatively low bulkdensity and large surface area of many productshelp in this matter. Oils used for frying frequentlycontain antioxidant stabilizers, both for the sake

of maximizing the shelf life of the final productand also extending the working life of the fryingoil itself.

Coating

Many snack foods and breakfast foods are coatedwith a variety of materials, such as sugar, honey,natural or synthetic flavour concentrates, cheesepowder, batters and savoury mix powders, inorder to make them attractive on the palate andgive them an individuality. For example, cerealflakes and expanded cereals may be sprayed withsugar solution and dried off after each application.Up to 50% of the final product may be sugar.

Methods used for coating are very varied. Someexamples are as follows:

(i) The coating pan, as used for pharmaceuticaltablets and products such as almonds, wherea pear-shaped open pan angled at about 45%is slowly revolved. This results in its contentsbeing tumbled, while sprays of, for example,sugar solution are applied alternately with jetsof hot or warm drying air until the desiredlevel of application is reached. Such coatingsmay be 'polished' by a final dusting of icingsugar, talc or similar powder.

(ii) The products travel on a belt which isvibrated at intervals and dusted with appro-priate powders; some of the powder adheresand the rest is recirculated. The product needsto be somewhat adhesive for this method tobe effective.

(iii) The products may be dipped into or sprayedwith a batter, which is then converted into a'shell' coating by heat and drying. The battermay be smooth cream or particulate in natureand may, in some cases, be aerated itself.

(iv) Products such as a fruit and/or cereal bar orbiscuit, may be coated in an 'enrober',through which it is usually carried on a wiremesh belt and deluged from above and belowwith tempered liquid chocolate or a substi-tute. The excess drains off and the productthen enters a cooling tunnel where the couver-ture sets.

(v) A 'coated' product can be produced in atwin-screw extrusion operation. A centralmain core of product is surrounded by anannular extruded 'jacket' which may be ofvery different composition and style from thecore. The Japanese-made Rheon machine,based on a similar approach, will producemulticoated products such as analogues ofScotch eggs.

(vi) Glazing, or the coating of ice on frozen snackproducts (e.g. prawns) resulting from theapplication of a cold water spray has thefunction of preventing dehydration andavoiding toughening and a general deteriora-tion of appearance and mouthfeel.

Mixing

This is a process which is often incorporatedwithin another process as a necessary ancillary(e.g. as in extrusion) but is also recognized as aprocess in its own right. When employed in thiscontext, it may also incorporate functions otherthan mixing, such as aeration, emulsification,dough formation and comminution. Consequently,a wide range of mixing equipment has been devel-oped and is available commercially. Some of theprincipal 'mixing' functions are as describedbelow.

Mixing of solids

Particle size defines the necessary style of mixer -for powders, the ribbon mixer in a trough is onepossibility, 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 mate-rials of particle size averaging that of granulatedsugar. For mixing solids of disparate but largersize (e.g. muesli ingredients) a rotating twinconical 'V type of mixer is suitable.

Mixing of solids and liquids

Here the required state of the mixture defines thetype of plant to be used. For the production ofsuspensions or solutions, the high speed propelleragitator within an open tank may be adequate. Ifsome degree of comminution is necessary, or lumpformation is a problem, then the Silverson type ofmixer is needed. Aeration is a feature of somemixers (e.g. in the Chorleywood bread process). Ifthe final product is a dough, the sort of mixerrequired becomes a more massive Z-blade orsimilar machine, or the type of integral dough-forming mixer incorporated into extruders.

Mixing of non-miscible liquids

Here the aim, aided by stabilizers, is to producean emulsion or cream from, for example, an oil-water mixture. A high speed propellant mixingscrew may be adequate here, otherwise a homoge-nizer may be necessary.

Mixing ofmiscible liquids

This hardly needs mention, merely stirring withpaddles or a recirculating pump is usually ade-quate so long as sufficient time is taken, particu-larly when the liquids have different specificgravities.

Masa production

A wide range of snacks manufactured and used inUSA, Mexico, Central America, and also now inthe UK, require a special form of maize as astarting point - masa flour or masa meal. Masa isproduced by treating whole grains of maize withlye (lime, caustic soda or KOH) which renders thepericarp easily removable. The temperature of thetreatment also partially cooks the maize. Subse-quently, the alkali is washed out of the grain, thepericarp removed and the masa can then be usedimmediately for tortilla or taco manufacture.However, it is mainly dried and ground to providemasa flour. Among other uses, this can be used asa basis for making corn flakes.

Sieving

The mouthfeel of products markedly affects theiracceptability. It is therefore necessary to controlthe perceived granularity, roughness or whatevercriterion may be applied to measure this factor.To standardize on the optimum range of particlesize, whether as a powder, or as suspended parti-cles in a liquid, it is necessary to sieve many pro-ducts. Fines go to rework, oversize go back torecomminution.

Chopping and slicing

Here, to a large extent, the processes used are spe-cifically related to individual raw materials or pro-ducts. There is, however, an overriding necessity,which holds in almost all cases, to keep the knifeblades sharp. Blunt knives give rise to fines andimproperly cut material and a correspondingfinancial loss.

Chopping is the easier process - the ubiquitousbowl chopper, properly programmed, can dealwith most requirements. Slicing, however, fre-quently has to be carried out by specially tailoredequipment Such a case is that of cut groats, whichconstitute the highest volume oat product forhuman consumption, forming as they do the basisof porridge, oat cakes, cereal bars, and so on. The

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

Chilling and freezing

Quite apart from their preservation function, theseprocesses, particularly freezing, are integral partsof the structuring and presentation of some snackfoods. The obvious examples are ice-cream andmany analogues. The aeration and the physicalnature (hard or soft) of ice-cream cannot beaccomplished without ancillary freezing.

Brining and curing

Salt is the primary agent in both these processes,which are quite similar. Other agents may beadded to the brine for curing purposes: nitratesand/or nitrites, for example, and special flavouringmaterials.

Brining, which is mostly carried out on meatand fish, results in two essential effects, namelydehydration by osmosis and preservation. Addi-tional effects are on flavour and colour.

Brining and curing may be carried out bystraightforward immersion, by 'tumbled' immer-sion to speed up penetration of the cure into hamsand by vacuum and pressure methods.

PRODUCTS

FLAKED, ROLLED AND GRANULATEDBREAKFAST PRODUCTS

The line dividing breakfast food products andsnack foods is not at all precise - corn flakes, forexample, are now being widely advertised asbeing suitable for any daytime use. Consequently,this section cannot be unambiguously dividedinto the two areas. A start will, however, bemade on what is largely accepted as being thebreakfast food area, viz. cereal products. Thesemay, however, be at times combined with itemssourced from potatoes and other vegetable (andlargely starchy) items and small quantities ofother additives.

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