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The nutritive value of foods and the significance of some household processes

Y.G. Deosthale. National Institute of Nutrition, Hyderabad, India

Abstract
Milling of food grains
Germination and malting
Cooking
References

Abstract

The nutritive value of food is modified by processing. The common household methods of food processing are milling, germination, cooking, and fermentation. Milling, in which the coarse fibrous bran or seed coat of the grain is removed, results in significant nutrient losses particularly of B-vitamins and minerals. The extent of the losses depends upon the degree of milling and the distribution pattern of nutrients in the grain. In milled products, like refined flour, pearled sorghum, and dhal, the big-availability of iron is better than the corresponding unmilled grains which is attributed to the removal of interfering substances such as phytate, tannin, and fibre. Germination or malting increases the vitamin C and folic acid content of food legumes and also degrades the anti-nutrients present in these food grains. Consequently, availability of iron in germinated grains improves significantly, especially in malted bajra and ragi. Oligos accharides, the components responsible for flatus production present in pulses, are practically completely degraded on germination and germinated pulses are likely to be less flatuspromoting than raw pulses. Cooking in boiling water or by steam pressure is yet another common household practice of food processing. Apart from making food palatable and safe, cooking inactivates practically all the anti-nutritional factors that are heat labile. Heat stable components like tannin in pulse legumes leach out in the cooking broth. Some B-vitamins like riboflavin and vitamin B-6 are also lost in the cooking of pulses. The growth-promoting properties of the foods, however, are far better than raw foods, and this is attributed not only to the destruction of anti-nutritional factors but also to better utilization of nutrients like proteins and carbohydrates. Dissemination of this knowledge of the nutritional significance of the simple methods of food processing at household level is important.

Nearly all foods, whether derived from plant or animal sources, are rarely fit for direct consumption and require some prior processing. Methods of food processing have been developed over the centuries and are adopted apparently to make the final product more attractive in flavour, appearance, taste, consistency, and so on. Besides these aspects of consumer preference, several of the methods aim at (a) making the food safe and wholesome, and (b) conserving available food for later use by increasing its shelf or storage life.

For a majority of the population, cereals, millets, pulses, and tubers in the diet are the main sources of most essential nutrients. The common household practices of processing these foods include milling, germination or sprouting, malting, fermentation, and cooking. Each of these processes qualitatively and/or quantitatively modifies the nutritive value of the food. Information on this aspect of food processing is important from both the nutritional and public health points of view. A brief review is presented here of the studies carried out at the National Institute of Nutrition on the effect of certain household practices on the nutritive value of foods.

Milling of food grains

Use of whole grain flour for the preparation of chapati from wheat, or roti from sorghum and bajra is a common household practice. On the other hand, brown or unmilled rice is rarely consumed. The most frequent form in which food legumes or pulses are consumed is as the decorticated product known as dhal. From a nutritional point of view both these practices, namely using grains with or without milling, and refining, have several advantages and disadvantages. In the process of milling food grains, the main objective is to remove the coarse fibrous bran or the seed coat. However, in all these processes the nutrient-rich parts of the grain, namely the germ and the aleurone layers, are also displaced resulting in a product poorer in nutrient content. The extent of nutrient losses - particularly of vitamins and minerals - during milling of wheat, rice, sorghum, and pulses is very large. The large variations in the nutrient losses are attributed, firstly, to the uneven distribution of these nutrients in the structural parts of the kernel and, secondly. to the degree of milling. Studies on the trace element composition of wheat flours of different extraction rates have shown that a major proportion of the phosphorus, magnesium, chromium, zinc, and manganese is located in the bran (Christensen et al. 1976; Sankara Rao and Deosthale, 1981) of the wheat kernel. Removal of the bran during milling results, therefore, in significant nutrient losses. Parboiling of the grain is a suggested remedy to minimize the milling losses of thiamin, riboflavin, and niacin in the rice grain (Kik 1957). During the parboiling process, water-soluble nutrients are driven from the outer layers into the inner layers of the grain and thus escape removal during milling. This beneficial effect of parboiling is observed also for chromium, molybdenum, and manganese (Deosthale et al. 1979). Milling losses of these nutrients are therefore 10-20 per cent lower in parboiled grain than in raw rice (see FIG. 1. Percentage of Loss of Trace Element on Milling of Raw and Parboiled Rice).

Although the nutrient content of food grains is relatively poor after milling, the big-availability of certain nutrients appears to improve considerably. For example, the bio-availability of iron, as judged by the percentage of it present as an ionizable fraction (Narasinga Rao and Prabhavathi 1978), was found to improve significantly (table 1) by milling wheat to refined flour and decorticating chick pea to dhal (Prabhavathi and Narasinga Rao 1979). Similarly in sorghum, the ionizable iron fraction increased from 18.6 to 28.7 per cent indicating a 40 per cent rise in the availability of iron (Sankara Rao and Deosthale 1980). Complex polysaccharides of the fibrous bran, polyphenolic compounds such as tannin, and phytate are all known to limit the availability of iron (Hussein and Patwardhan 1959; Disler et al. 1975). Removal of these complex substances by milling partly explains the improved iron availability of milled grains.

TABLE 1. Soluble and Ionizable Iron Content of Some Unmilled and Milled Cereals and Pulses

Germination and malting

Soaking in water overnight followed by germination or sprouting of the grain is yet another very common household practice, especially for the processing of pulses. During germination several enzyme systems become active and bring about profound changes in the nutritive value of pulses (Subramanian et al. 1976; Ismail Noor Mohd et al. 1980). Vitamin C, which is practically absent in dry legume seeds, increases in significant amounts after germination (De and Barai 1949; Prabhavathi and Narasinga Rao 1979). Similarly Babu (1976) has found two to threefold higher values for folic acid in the germinated grains than in the raw grains of the chick-pea and ragi (table 2). On the other hand, it has been reported that anti-nutritional factors such as phytate, trypsin inhibitor, and haemaggiutinins are broken down on germination (Ready et al. 1978; Subbulakshmi et al. 1976). Phytate, which constitutes over 60 per cent of the total phosphorus in the raw grains of the Bengal gram, drops to a level of 44 per cent in the 48-hour germinated grain (table 3) with no change in total phosphorus (Prabhavathi and Narasinga Rao 1979). On malting, a process that likewise involves germination, loss of phytate is quite pronounced in both ragi and bajra (Sankara Rao and Deosthale, unpublished).

TABLE 2. Folate and Ascorbate Content of Germinated Food Grains

TABLE 3. Effect of Germination, Malting, and Baking of Bread on Phytin-P

Recent studies have shown that the most commonly consumed pulses, namely chick-pea pigeon-pea, green gram and black gram, contain a significant amount of tannin that is mostly present in the seed coat. In overnight soaking 25-50 per cent of this tannin is lost, probably through leaching (table 4). In 24 to 48-hour germination, there is an additional loss of 10 to 25 per cent (Udayasekhara Rao and Deosthale, unpublished).

TABLE 4. Effect of Soaking, Germination, and Cooking: Percentage Loss of Polyphenols of Four Indian Pulses

The beneficial effect of these changes in food grains on germination and malting are reflected in the improved big-availability of nutrients (table 5). Thus the ionizable iron content in chick-pea and wheat progressively increases on germination up to 72 hours. On removal of the seed coat, the ionizable iron is 14 per cent both in the raw and germinated grains of the chick-pea. The absence of any germination effect may be attributed to factors associated with the seed coats, which probably are progressively eliminated during germination. In ragi and bajra also, germination or malting enhances the iron availability about eight to twelve-fold (Sankara Rao and Deosthale, 1981). In the context of widespread iron-deficiency anemia in the population, and the suggested use of malted grains for the feeding of infants and children, these observations on iron availability of malted bajra and ragi are of considerable significance.

TABLE 5. Ionizable Iron in Germinated and Malted Food Grains

a. Seed coat removed.

Germination also modifies the carbohydrate component of the food grains (Subramanian et al. 1976; Lebaneiah and Luh 1981). It is well known that consumption of pulses leads to flatulence. Inclusion of legumes in the diet at levels that provide 20-25 per cent of total calories results in a manifold rise in the amount of gas produced in the intestines (Steggerda and Dimmick 1966). Studies in humans fed different pulses have shown that chick-pea is more gas-forming than other pulses (Narayana Rao et al. 1973). One of the factors associated with flatulence is the high concentration in pulses of certain oligosaccharides of the raffinose family. Because of the absence of suitable digestive enzymes these sugars are not utilized by man (Gitzelmann and Auricchio 1965). The unabsorbed sugars are acted upon by the micro-flora of the large intestine resulting in gas production (Calloway et al. 1966; Richards et al. 1968). Studies on common pulses like the chick-pea, green gram, black gram, and red gram (Udayasekhara Rao and Belavady 1978) have indicated a continuous fall in oligo-sugar content on germination (Figure 2). In grains germinated for twenty-four hours the oligo-sugar levels are at 50 per cent of their initial value, and by 48-72 hours at less than 25 to 15 per cent. These observations suggest that sprouted pulses are likely to be less flatus-producing.

Cooking

Food is cooked by heat treatment in several ways, which include boiling in water, pressure steaming, roasting, frying, and baking. Cooking is important to make the food safe by killing contaminating bacteria, and also to inactivate several heat-labile anti-nutritional factors present in many foods. Among the heat-stable factors in food legumes, tannin appears to leach out into the broth during cooking in water and thus the level in the grain is reduced to 30 per cent of the initial value (Udayasekhara Rao and Deosthale, unpublished). Cooking also results in the loss of vitamins. Studies reported by Raghunath and Belavady (1979a) have shown that cooking loss of riboflavin varies from 13 to 35 per cent in the four pulses (table 6). On the other hand cooking losses of vitamin B6 in these pulses were relatively low (13 per cent) and constant. However the availability of food iron is not altered by the cooking of rice and dhal, or by baking a phulka from whole wheat or refined flour (table 7). In baking bread, the rise in ionizable iron content is attributed to the prior fermentation of the ingredients (Prabhavathi and Narasinga Rao 1979).

TABLE 6. Cooking Losses of Riboflavin and Total B6 Content of Dhals

TABLE 7. Effect of Cooking on Ionizable Iron

Cooking of food improves the biological value of its protein. Udayasekhara Rao and Belavady (1979) have shown that the growth performance of rats was better on cooked than on uncooked or raw diets based on different pulse and tuber proteins (table 8). This effect was observed to be particularly pronounced in diets based on the winged bean and its tuber. Animals fed raw winged bean or its tuber failed to grow and showed a significant loss in body weight, and those fed the tubers even died.

TABLE 8. Growth Performance of Rats Fed Raw and Cooked Diets Based on Different Sources of Proteins

Improved growth responses to cooked diets were attributed to the destruction of the anti-nutritional factors present in these foods (Raghunath and Belavady 1979b). The utilization of protein is also influenced by the starches isolated from different sources, as shown by Nageswara Rao and Narasinga Rao (1978). These studies further showed (see TABLE 9. Net Protein Utilization (NPU) of Casein Diets containing Starches from Different Sources) that cooking of the diet significantly improved the net protein utilization of casein when the diets contained legume and potato starches.

Conclusion

The practical significance of simple household methods of food processing is obvious to those working in the field of food science and nutrition. There is a great need, of course, to make the consumer aware of the nutritional dimensions of such food processing, which has been discussed in this paper. Education and extension personnel, who are juxtaposed in the interface between the consumer and the food scientist, have a significant role to play.

References

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