13 The processing of cereal foods A J. Alldrick, Campden and Chorleywood Food Research Association; and M. Hajselova, Consultant 13.1 ntroduction 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 tur enabled societies in different parts of the world to make the transition from 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 ussed 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 utellum, 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 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 cultivation 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 permanent settlements, forming the basis of civilisation as we now know it (discussed 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 generation plus nutrients for its germination and initial growth. These in turn are surrounded by a protective seed coat. Kent and Evers (1994) described a ‘generalised’ 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 germination process. Around the embryo is the endosperm, most of which is referred to as the starchy endosperm. This consists of cells containing nutrients (in particular 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 contains substantial amounts of protein as well as dietary fibre
302 The nutrition handbook for food processors Table 13.1 World cereal production(1998/99 estimates) Grain Production volume (million tonnes) Principal producing areas(by tonnage Maize USA, China, EU, Brazil China, EU, USA, India, CIS Rice(milled basis) 385.0 China, India, Indonesia, 135.7 EU, CIS, Canada, USA 61.9 USA, India, Nigeria, China EU, CIS, Canada, USA Rye 21.2 EU. Poland. CIS Data adapted from Intermational Grains Council (2000)and United States Department of Agriculture 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%o 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%o 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 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 erms 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. Q Cereal processing takes two basic forms: mechanical(e. g. milling) and thermal 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
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 minerals 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 approximately 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 gastrointestinal (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)
The processing of cereal foods 303 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 t 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 nd 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
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 converting 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 organised 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 qualities 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 corresponding loss of some B-vitamins (e.g. thiamin). Kik (1943) reported losses of The processing of cereal foods 303
304 The nutrition handbook for food processors 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 andinstant 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 d bryo from the starchy Most modern industrial four 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%0, between 85 and 98% for brown flours and 100% for wholemeal flour(referred to as'wholewheat or Grahamflour 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
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 consumption. 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 different components of the kernel. This can lead to the separation of the bran, aleurone 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 production 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
The processing of cereal foods 305 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 fours. Flours with extraction rates of less than 100%o 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 nations calcium intake. In the case of the UK this was approximately 25%0 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. 8g per 100g, 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
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 countries, 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 composition. 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 separation 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 wholemeal flour has been reported as having a dietary fibre content expressed as nonstarch 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 consumed. 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 carbohydrates 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