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 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, 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 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 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 non-starch polysaccharide (NSP) of 5.8g per 100 g, white flour has a corresponding dietary fibre content of 1.5g per 100g (Holland et al, 1991).

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