13 The processing of cereal foods A. J. Alldrick, Campden and Chorleywood Food Research Association; and M. Hajsˇelová, Consultant 13.1 Introduction The domestication of different grasses, all members of the monocotyledonous family Gramineae, was a seminal event in the history of mankind. The cultiva- tion of these plants led to the generation of agricultural surpluses. These in turn enabled societies in different parts of the world to make the transition from a nomadic or semi-nomadic lifestyle to one based on communities living in per- manent settlements, forming the basis of civilisation as we now know it (dis- cussed by Roberts, 1988). The principal cereal crops grown in the world are maize, wheat, barley, rice, oats, rye and sorghum. Current estimates of world production, together with the major producing areas, are detailed in Table 13.1. It is estimated that for wheat alone, approximately 250 thousand hectares were planted world-wide during the 1998 growing season (International Grains Council, 2000). Essentially, the cereal grain comprises the embryo plant for the next gen- eration plus nutrients for its germination and initial growth. These in turn are surrounded by a protective seed coat. Kent and Evers (1994) described a ‘gen- eralised’ model for all cereal grains. In this model, the cereal grain comprises an embryo that divides into the embryonic axis (the proto-plant) surrounded by the scutellum, which provides secretory and absorptive functions during the germi- nation process. Around the embryo is the endosperm, most of which is referred to as the starchy endosperm. This consists of cells containing nutrients (in par- ticular starch) that support growth following germination. The starchy endosperm is surrounded by the aleurone layer, which is mainly comprised of protein and lipids. Enclosing the endosperm and embryo is the protective seed coat. It con- tains substantial amounts of protein as well as dietary fibre. 13.2 The nutritional significance of cereals and cereal processing Cereals are nutritionally dense. They supply carbohydrate and protein as well as a variety of micronutrients; in particular certain B vitamins, vitamin E and min- erals such as iron, in the case of wheat. In addition, cereal-based foods can also supply significant amounts of dietary fibre. Dietary studies, such as those of Gregory et al (1990) in the UK suggest that cereal-based foods contribute appro- ximately 30% of dietary energy, over 20% of protein and approximately 45% of dietary fibre to the adult diet. Additionally, cereals are an important component of animal feed. It has been estimated, using 1990 figures (Cook and Hill, 1994), that within the European Union, 81.5 million tonnes or 59% of the cereal crop was used for animal feed purposes. This compares with 36.3 million tonnes, or 27% of the crop used for direct human consumption. Cereals therefore make a major contribution to both human and animal nutrition. Although cereals are an excellent source of nutrients, they have two key limitations. 1. In terms of the amino acid composition of their proteins, cereals tend to have reduced levels of some of the indispensable amino acids, in particular lysine and threonine. While this is of significance to livestock nutrition, in human terms this only has relevance in societies where levels and diversity of protein intakes are limited. 2. Cereals have to be processed in order to maximise the bioavailability of the nutrients. The bioavailability of a nutrient can best be defined as the amount of nutrient present in a food, which is eventually absorbed from the gastro- intestinal (GI) tract of the consuming organism. Cereal processing takes two basic forms: mechanical (e.g. milling) and thermal (e.g. baking). These can be performed separately, for example in converting the grain to a meal and subsequently cooking it, or almost simultaneously, as in a 302 The nutrition handbook for food processors Table 13.1 World cereal production (1998/99 estimates) Grain Production volume Principal producing areas (by tonnage) (million tonnes) Maize 602.9 USA, China, EU, Brazil Wheat 586.6 China, EU, USA, India, CIS Rice (milled basis) 385.0 China, India, Indonesia, Barley 135.7 EU, CIS, Canada, USA Sorghum 61.9 USA, India, Nigeria, China Oats 26.5 EU, CIS, Canada, USA Rye 21.2 EU, Poland, CIS Data adapted from International Grains Council (2000) and United States Department of Agriculture (1998). puffing process. The principal nutritional benefit of processing is to increase the bioavailability of the nutrients present in the grain. Essentially this is brought about by making the cereal grain a better substrate for digestive enzymes. This is achieved at both a physical (increased surface area) and/or a chemical (more random molecular structures) level. Mechanical processes, such as milling, assist bioavailability in two ways: firstly, by breaking and often removing the outer seed coat; and secondly, by con- verting the grain to smaller particles thereby effectively increasing the surface area available to attack by digestive enzymes. While thermal processing can also in part contribute to the physical reduction of the grain, in terms of its primary nutritional benefit, the role of thermal processing is to disrupt the highly organ- ised three-dimensional structures of both the starch and protein components. This generally leads to them being better substrates for the digestive enzymes within the GI tract. In terms of foods for human use in particular, the consequences of processing on nutritional parameters have to be considered in combination with customer satisfaction. This includes both food safety and sensory aspects. However, the nutritional role of processing should not be underestimated. Developments in our understanding of nutrition and its role in health all indicate that the diversity of processing technologies can make significant contributions to the nutritional qual- ities of the grain as it is consumed. The sources of this understanding take many forms. These range from the bases of nutrient deficiency diseases such as beriberi (thiamin deficiency) through the role of diet in the aetiology of chronic diseases such as diabetes, cardiovascular disease and certain cancers, to developments in animal feed technology. A more detailed analysis of the significance of the role of technology follows in the next section. 13.3 Mechanical processing As briefly discussed above, the primary outcome of cereal processing is to increase the bioavailability of nutrients within the cereal grain to the consumer, be they human or animal. Mechanical processing can be divided into three broad categories: abrasive, reductive or a combination of the two. 13.3.1 Abrasive processes At their simplest, abrasive processes remove the outer seed coat and aleurone layers (often referred to as the bran layers) from the kernel leaving the starchy endosperm exposed and more susceptible to the effects of cooking. Two of the principal grains processed in this way are rice and barley, yielding white rice or pearled barley respectively. Production of white rice also leads to the removal of the embryo. In terms of nutrition, one of the principal challenges with abrasive and other methods, which lead to the removal of the bran layers, is the corre- sponding loss of some B-vitamins (e.g. thiamin). Kik (1943) reported losses of The processing of cereal foods 303 between 5- and 10-fold when comparing the thiamin contents of milled, polished rice with those of paddy rice. Historically, this only presented a problem in societies with a restricted food supply, leading to the vitamin deficiency disease beriberi (summarised by Bender and Bender, 1997). Despite improvements in the quality of the food supply, both in terms of quantity and diversity, sociological changes can contribute to reoccurrence of the disease, where it was once thought to have been eliminated. Kawai et al (1980) reported the reappearance of shoshin (acute) beriberi in Japanese adolescents consuming a diet made up predominantly of high carbohydrate, low nutrient density foods such as carbonated soft drinks, polished rice and ‘instant’ noodles. 13.3.2 Reductive processes Most cereals are converted to smaller particles before processing and consump- tion. At the very basic level, this can involve simply cutting the grain into large fragments, for example in the manufacture of maize grits. Alternatively, grain can be reduced to a powder form which may or may not be fractionated into differ- ent components of the kernel. This can lead to the separation of the bran, aleu- rone and embryo from the starchy endosperm. Most modern industrial flour production involves a progressive reduction process using a system of roller mills (discussed by Kent and Evers, 1994). In summary, wheat grains are adjusted to an appropriate moisture content and pass through a system whereby they are first fragmented (Break Release) and the starchy endosperm is removed from the bran. This in itself is a progressive process, involving a number of break mills. Grain particles are separated on the basis of size by a sieve process and either re-enter the break operation or pass on to the second stage of the process (Reduction). ‘Break Release’ leads to the pro- duction of two fractions, bran (seed coats) and the starchy endosperm, referred to as semolina in the UK. Particles of bran still attached to endosperm and which have not been reduced by the break system pass into the Scratch system, which effects a separation between the seed coat and the endosperm. The coarse semolina is ground to a flour of desired particle size through a further system of roller mills (between 8 and 16 grinding stages). Whereas the rollers used in the break process are fluted, those for the size reduction process are usually smooth or matt. The process not only brings about the generation of a flour with the desired particle size, but also effects a separation of the starchy endosperm from the embryo and any remaining bran. The proportion of the original wheat that is ultimately converted to flour is referred to as the extraction rate. Typical values for white flours are between 72 and 80%, between 85 and 98% for brown flours and 100% for wholemeal flour (referred to as ‘wholewheat’ or ‘Graham’ flour in the USA). 13.3.3 Regulatory control of flour and its nutritional significance As indicated above, modern milling techniques not only achieve the mechanical reduction of the cereal grain, but also separate the starchy endosperm from the 304 The nutrition handbook for food processors bran and embryo. Both the bran and embryo of cereal grains contain significant quantities of essential nutrients. Even in recent times, their removal has had potentially deleterious consequences with regard to public health. In many coun- tries, therefore, the composition of flour is regulated by law. This is not only with regard to technical aspects, for example purity, but also to its nutritional com- position. In the United Kingdom these are detailed within the Bread and Flour Regulations 1998. In nutritional terms these regulations are of importance in that they specify certain nutrient contents for all flours. Flours with extraction rates of less than 100% must be supplemented with the vitamins niacin and thiamin and also with iron to make up for losses during the milling process as well as being fortified with calcium (in the form of calcium carbonate). Analysis of studies such as those by Gregory et al (1990), looking at the dietary habits of the UK adult population, shows that cereal-based foods make a significant contribution to the nation’s calcium intake. In the case of the UK this was approximately 25%. A substantial proportion (in excess of 50%) of this figure would be as a direct consequence of mandatory calcium carbonate fortification of low extraction rate flours. The sepa- ration of the bran layers from the endosperm also leads to significant reductions in the amount of dietary fibre present within the resultant flour. Thus while whole- meal flour has been reported as having a dietary fibre content expressed as non- starch polysaccharide (NSP) of 5.8 g per 100 g, white flour has a corresponding dietary fibre content of 1.5 g per 100 g (Holland et al, 1991). 13.4 Thermal processing Livestock can be fed cereals or by-products from cereal processing without any thermal processing (cooking). In contrast, cereal-based foods intended for human consumption almost inevitably undergo some form of cooking. The cooking processes can be as simple as boiling the grain or its meal in water. Alternatively, they can be complex systems involving mixing with other ingredients to form a dough, followed by mechanical processing and subsequent cooking (e.g. baking, as in the case of bread). Processed cereal products are many and diverse. This is reflected in the different technologies used and how the products are finally con- sumed. As discussed below, the technology used to make a particular product can be as nutritionally important as the ingredients themselves. In terms of nutrition, two of the most significant effects of mechanical and thermal processing concern the vitamin content and the physico-chemical structure of the complex carbohy- drates present in the finished product. 13.4.1 Vitamins A number of the vitamins associated with cereals or which are added during the manufacture of cereal-based products are thermally unstable. This is particularly true of the water-soluble vitamins (B vitamins and vitamin C). Cooking therefore leads to the destruction of a proportion of the vitamins present. The degree of The processing of cereal foods 305 destruction is dependent on the ingredient, recipe and the method of cooking of food. Nutritional databases, such as McCance and Widdowson’s The Composi- tion of Foods (Holland et al, 1991), detail typical correction factors for vitamin losses during cooking. In the case of baking, one might expect reductions of: between 15% (bread) and 25% for thiamin; 15% for riboflavin; 25% for both vitamin B 6 and pantothenate and 50% for folate values. The phenomenon of thermal destruction is particularly important with regard to those products which are marketed (in part) on the basis of their vitamin content, for example breakfast cereals. This is usually addressed by one or a com- bination of two approaches: recipe formulation, where an increased amount of vitamin is incorporated to allow for thermal destruction or spray application to the cooled intermediate or finished product. Irrespective of the method of appli- cation, appropriate QC systems must be in place. These include monitoring the rate of application of vitamin mixes into the ingredients’ mix or sprayed onto the product, as well as verification of the vitamin content in both intermediate and finished products by laboratory analysis. 13.4.2 Complex carbohydrates Cereal-based foods account for approximately 30% of the energy intake of the UK adult population (Gregory et al, 1990). This figure includes the energy con- tribution from other ingredients, for example fat in pastry products. In terms of the cereal itself, the primary energy source is complex carbohydrate. Complex carbohydrates have been defined as carbohydrate molecules that contain twenty or more monosaccharide residues (British Nutrition Foundation, 1990). In the case of cereals, interest has focused on two forms of complex carbohydrate: starch and non-starch polysaccharide (NSP). NSP can be considered to be the principal component of dietary fibre. Complex carbohydrates are believed to make a beneficial contribution towards a healthy diet. Changes in demography and public health have seen the develop- ment of longer-lived populations, increasingly suffering and/or dying from chronic diseases, in particular diabetes, cardiovascular disease and cancer. Numerous groups have proposed that diet makes a contribution to the incidence of these diseases and dietary guidelines such as the USDA Food Guide Pyramid (United States Department of Agriculture, 1992) have recommended substantial intakes of complex carbohydrates. Processing can lead to significant differences in the effects of these dietary components on physiological function. For reasons of convenience, the effect of processing on the physiological properties of starch will be discussed here, while those for dietary fibre (including NSP) in section 13.5. At a basic level, thermal treatment of a cereal-based raw material leads to substantial changes in the physico-chemical structure of the starch within the endosperm. This expresses itself by the irreversible disruption of the starch gran- ules (gelatinisation) and, in most cases, the formation by the starch molecules present of more random structures. At a nutritional level, gelatinisation of starch 306 The nutrition handbook for food processors renders it far more susceptible to breakdown by the a-amylase enzymes present in the GI tract. What has become increasingly apparent since the late 1970s is that the situation is complex and that the combination of thermal and mechani- cal processing can lead to foods with starches of differing relative starch digestibilities. Foods containing starch that are digested at different rates can be classified in terms of their, ‘glycaemic index’ (Jenkins et al, 1981). This parameter is a mea- sure of the rate of uptake of glucose into the blood stream from a carbohydrate- rich food compared with that of a reference meal (often a glucose solution) containing the equivalent amount of carbohydrate. A glycaemic index of 100% indicates that the carbohydrate in a food is digested and absorbed as glucose into the blood stream at the same rate as the reference meal. Similarly, a value of 50% for a food indicates that the carbohydrate present is digested and absorbed into the blood stream at half the rate of that seen with the reference meal. Gen- erally speaking, white bread has a glycaemic index of approximately 100%, whole grain breads between 35 and 90% and pasta between 40 and 70% (Bj?rck and Asp, 1994). Bj?rck and Asp summarised the nutritional benefits of low glycaemic index foods as including: improved metabolic control for diabetics; lower blood lipid concentrations; improved glucose tolerance; prolonged satiety; improved performance during endurance exercise and reduced cariogenic potential. Differences in glycaemic index relate in one degree to the physical acces- sibility of the starch molecules to the intestinal a-amylase enzymes. At its simplest, this could be by virtue of using whole or crudely milled (kibbled) grains. This explanation, however, cannot apply to pasta, which although made from semolina, involves particle sizes much smaller than those associated with kibbled grains. Yokoyama et al (1994) made bread and pasta from the same type of flour and demonstrated that, under in vitro conditions, starch in white bread was digested by a-amylase approximately four times faster than starch in cooked fet- tuccine. This difference is probably attributable to the pasta manufacture process. Pasta production involves the preparation of a semolina/water dough, which is either kneaded into sheets and cut or extruded at relatively low temperatures which do not facilitate starch gelatinisation. The finished product may or may not be dried. During this process the starch granules are enrobed with protein which entraps the starch granules once they have been gelatinised during the final cooking process (Feillet, 1984). Protein enrobement of the gelatinised starch molecules restricts their accessibility to digestive enzymes in the GI tract. The underlying food technology used to prepare the food product can also modify starch structure leading to the generation of ‘resistant starch’ (RS). RS was described by Berry (1986), when he identified starch fractions that were re- sistant to a-amylase breakdown during dietary fibre determinations. It was put into a physiological context by Englyst and Cummings (1987), who used the term to describe a starch fraction which was resistant to digestion by secreted a-amylases within the GI tract, and which passed into the large intestine undigested. Three types of RS were originally identified (Englyst et al, 1992): The processing of cereal foods 307 ? RS 1 Physically inaccessible starch, found in partly milled grains or seeds. ? RS 2 Resistant starch granules, starches of a particular crystalline form that are refractory to breakdown by a-amylases. ? RS 3 Retrograded starch, in other words gelatinised starch which has recrystallised to an indigestible form. Brown et al (1995) extended this list to include: ? RS 4 Chemically modified starches (e.g. esters and ethers). Of particular interest to the cereal foods industry is RS 3 (retrograded starch). This is often formed during the production of cereal-based foods. The amount produced is in part determined by the product and the processes used in its manufacture. For example, breakfast cereals produced by methods such as flaking tend to have higher amounts of RS compared to products made by alternative technologies. The quantity of retrograded starch produced is not only determined by the processes but also by how much starch is present in the form of amylose. The higher the amylose content, the greater the amount of retrograded starch pro- duced (Brown et al, 1995). At a physiological level, resistant starch, in particu- lar RS 3 , is interesting, since it appears to behave in many ways like dietary fibre (reviewed by Annison and Topping, 1994) and a proportion of it can be detected as such in the AOAC dietary-fibre assay. 13.5 Developing nutritionally-enhanced cereal-based foods 13.5.1 Current status Reference has already been made to the role that diet plays in the incidence of chronic disease (section 13.4.2). One of the major challenges to face the modern food industry is the need to develop products that can contribute to the customers’ desire for a healthy diet and the additional benefits such a diet provides. Alldrick (1998, 2001) has previously discussed the general principles underlying the design and marketing of these types of products both in general terms and with specific regard to cereal products. A number of foods designed to meet the consumers’ aspiration of improving their health through diet might be described as ‘Functional Foods’. Ichikawa (1994) defined these products as: ‘Processed foods containing ingredients that aid specific bodily functions in addition to being nutritious.’ A key point to remem- ber about functional foods is that they are not medicines. Thus while health claims (e.g. ‘as part of a low fat diet our product may help to reduce blood cholesterol concentrations’) might be permitted, medical claims, in other words claims to the effect that the food could cure, prevent or alleviate a disease, are generally prohibited. Over the last few years, an increasing number of cereal-based products have come onto the functional-foods market. Some have been marketed on the basis of elevated vitamin or mineral content. What has been more novel is the devel- 308 The nutrition handbook for food processors opment of products containing pharmacologically active compounds. One group of compounds that has attracted interest is the phyto-oestrogens. These are plant compounds which bear a structural similarity to the female sex hormone oestra- diol and fall into three broad chemical categories: isoflavones, coumestans and lignans. They are thought to have beneficial health effects with regard to cardio- vascular disease, certain cancers and the menopause (Bingham et al, 1998). A number of baked products, for example breads containing soya, linseed and/or flax and with high contents of phyto-oestrogens, have been released (Dalais et al, 1998; Payne, 2000). These have sometimes been referred to as ‘Sheila’ breads, reflecting their Australian origin. Arguably, however, the key growth area has been fibre-enriched products. These products may be enriched with cereal-based dietary fibre or fibre derived from other plants, for example psyllium. As discussed in more detail elsewhere in this book, dietary fibre has been shown, or is thought to be, beneficial in man- aging the risk of a number of diseases. From classical times (British Nutrition Foundation, 1990) the beneficial effects of cereal fibre and in particular insolu- ble fibre on colonic function have been known. More recent work, such as the meta-studies of Ripsin et al (1992) have demonstrated that soluble fibres may have beneficial blood cholesterol lowering effects, particularly when eaten as part of a reduced fat diet. 13.5.2 Design considerations Within the context of this chapter, a fundamental aspect that must be addressed in the design, development and manufacture of any nutritionally enhanced cereal food, is that of functionality. Here the term functionality not only applies to achieving the desired physiological consequences of eating the food in question, but also to the way ingredients behave during processing, together with their effects on product attributes. In developing this type of product, therefore, two questions need to be addressed. ? Can the product deliver the desired amount of the physiologically active ingredient and, where necessary, the desired physiological consequences, without adverse side effects? ? What effect does the physiologically active ingredient have on the tech- nological functionality of other ingredients in the recipe with regard to giving an acceptable product? These questions can be best examined looking at specific examples using one particular physiologically active dietary component. Here the component dis- cussed is dietary fibre. Physiological functionality The two questions raised above can be intimately linked, and in unexpected ways. For example, the introduction by the animal feed industry of ‘High Temperature Short Time’ (HTST) processing (e.g. use of expanders), particularly for poultry The processing of cereal foods 309 feed, in the early 1990s led to a reduction in feed efficiencies. This was related to an increase in the proportion of soluble dietary fibre in the feed and conse- quent increased viscosity of the animal’s digesta (Sundberg et al, 1995). In the previous decade, it had been demonstrated that processing cereals in equipment such as expanders led to a solubilisation of the fibre component (Ralet et al, 1990). The degree to which this occurred was dependent on the energy input. The problem was overcome eventually by the addition of appropriate dietary-fibre degrading enzymes (Sundberg et al, 1995). Although extreme, the above example does highlight the need to consider other physiological effects that may impact on the consumer. This can be in the form of unpleasant side effects, such as, in the case of dietary fibre, large bowel complaints such as flatulence (Bolin and Stanton, 1998) or impaired eating qual- ities in certain subgroups of the population, such as the elderly (Laurin et al, 1994). A simple moral can be derived from these examples. Failing to have a detailed understanding of both the links between the effects of food processing on the physiologically functional ingredient and all of the roles played by that ingredient on the consumer’s physiology and/or sensory expectations can be prejudicial to the product’s commercial success. Technological functionality In addition to the needs of the customer, any new product design process must also take into account the need for the product to be compatible with existing manufacturing technology. This is particularly true for high dietary fibre prod- ucts. One such example is the manufacture of wholemeal (100% extraction) flour bread using ‘no-time’ dough making processes (e.g. Chorleywood Bread Making Process). Studies performed at the Flour Milling and Baking Research Associa- tion (Collins and Hook, 1991) have demonstrated that production of a loaf with those characteristics preferred by the consumer is dependent on a number of parameters. Particle size distribution within the flour was important not only for baking quality but also for the overall appearance of the crust and crumb. Addition of dried gluten protein was found to be a suitable method for improv- ing hedonic parameters including loaf volume and crumb-score. Increasing the content of a physiologically desirable component within an existing product can have consequences with regard to the behaviour of inter- mediate products and/or the appearance of the finished product. Work with bread containing elevated amounts of dietary fibre has shown that the addition of extra dietary fibre could lead to changes in dough rheology and handling characteris- tics as well as product appearance and texture (Pomeranz et al, 1977; Laing et al, 1990). 13.5.3 Marketing In marketing any cereal product with enhanced physiological benefits, it will be necessary to ensure that the product contains sufficient active ingredients to meet 310 The nutrition handbook for food processors local regulatory requirements. However, it will also be important to ensure that the food is physiologically functional and has acceptable sensorial properties. Any advertising campaign for the product should be, ‘legal, honest and truthful’ (Advertising Standards Authority, 1999). Enthusiasm for the product should not be allowed to diminish the accuracy of advertisements. The functional foods area is a growing market. Young (1999) valued the European market at USD 1.24 billion in 1997, with bakery and cereal products commanding a 9% share. Consequently, functional foods also attract attention from consumer protection groups. Winkler (1998) reported that during the period 1994–1997, the UK Advertising Standards Authority upheld 21 complaints concerning advertise- ments for functional foods. 13.6 Conclusions Cereals have been, and continue to be, a staple foodstuff. What has changed and what is expected to change further is the type of food products they are presented as. For example, in the United Kingdom bread consumption has halved during the last 50 years, while breakfast cereal consumption has tripled (Griffiths, 1999). These changes have been brought about, in no small part, by the tremendous socio-economic changes experienced in the Western world, which have led to the consumerist society. The demands of this society plus the consequences of demo- graphic changes, together with the identification of the significant role that diet plays in the incidences of a wide number of chronic diseases have become key motivators in the design of new food products. Cereal foods are well placed to meet these challenges. This can take many forms, including capitalising on exist- ing nutritional attributes (e.g. insoluble fibre in wholemeal flour products) to new functional foods. One example of functional high fibre foods is a range of prod- ucts (e.g. pasta, breakfast cereals and snack bars) supplemented with psyllium fibre. Psyllium fibre has been demonstrated to be capable of reducing blood cho- lesterol concentrations (Roberts et al, 1994). Developing such foods is not nec- essarily a guarantee for commercial success, persuasive and ethical marketing is also required. There has been a number of well published cases (e.g. Buss, 2000), where cereal-based functional foods have failed by virtue of inadequate market research and poor sales strategies rather than by a defective product per se. 13.7 Sources of further information and advice bender d a and bender a e (1997), Nutrition, A Reference Handbook, Oxford, Oxford University Press kent n l and evers a d (1994), Kent’s Technology of Cereals (4 ed.), Oxford, Elsevier mccleary b v and prosky l (eds.) 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