Protein

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The protein content and quality of roots, tubers, bananas and plantains are variable; that of yam and potato is highest, being approximately 2.1 percent on a fresh weight basis. The protein contribution of these foods to the diet in developing countries, corrected by the amino-acid protein quality is, on a worldwide average, only 2.7 percent, provided mainly by potato and sweet potato. However these starchy staples do provide a much greater proportion of the protein intake in Africa (Table 4.8), ranging from 5.9 percent in East and southern Africa to a maximum of 15.9 percent in humid West Africa, supplied mainly by yam and cassava. These figures do not include the protein contribution from the leaves of crops such as cassava, sweet potato and cocoyam which are eaten as green vegetables. The amino-acid content of roots and tubers, unlike most cereals, is not complemented by that of legumes as both are limiting in respect of the sulphur amino-acids (see Table 4.9). In order to maximize their protein contribution to the diet, roots and tubers should be supplemented with a wide variety of other foods, including cereals.

To some extent the protein content of root crops is influenced by variety, cultivation practice, climate, growing season and location (Woolfe, 1987). In potato, the addition of nitrogen fertilizer increases the protein content (Eppeudorfer et al., 1979; Hoff et al., 1971) while in the case of sweet potato the protein content could vary from 2.0 to 7.5 percent depending on the cultivar and treatment. Nitrogen fertilizer increases the protein content of sweet potato, but the lysine content is decreased, while the aspartic acid and free amino-acids are increased (Yang, 1982). Also leafy growth is increased at the expense of tuber production.

In root crops the quality of the protein, in terms of the balance of essential amino-acids present, may be compared to that of standard animal proteins in beef, egg or milk (see Table 4.5). Most root crops contain a reasonable amount of lysine, though less than in legumes, but the sulphur amino-acids are limiting. For example, yam is rich in phenylalanine and threonine but limiting in the sulphur amino-acids, cystine and methionine and in tryptophan.

Protein quality may be assessed in terms of the amino-acid score but the biological utilization of protein depends also on the composition of the diet, the protein digestibility and the presence of toxins or other antinutritional factors. This is reflected in the net protein utilization (NPU) proportions of nitrogen intake that is retained or biological value (BV) of the protein, which estimates the proportion of absorbed nitrogen that is retained (Table 4.10) either by measurement of nitrogen balance, or preferably by direct studies on experimental animals. Results may also be expressed as protein efficiency ratios (PER values) where PER = gain in weight in grams divided by the protein intake in grams.

In feeding studies conducted on rats, banana proteins were utilized as well as those of maize, although their utilization was less efficient than those of yam, cocoyam and sweet potato. The protein of potato is of good nutritional quality with a relatively high lysine content, and so it can be used in developing countries to complement foods low in lysine. As shown in Table 4.10, its utilizable protein as a percentage of its calorie content is as high as that of wheat.

The protein of sweet potato is also of acceptable nutritive value, with a chemical score of 82 and sulphur amino-acids as the major limiting factors. The quality of the protein will depend on the severity of heat treatment during the processing of sweet potato products. (Walter et al., 1983). Horigone et al., (1972) reported a PER of 1.9 for a protein isolated from a sweet potato starch production factory. This value could be increased to 2.5 by the addition of lysine and methionine, indicating a deficiency of methionine and the destruction of lysine during processing. When unheated sweet potato flour was added to wheat in the diet of rats at the 30 percent level, the biological value of the diet was increased from 72 to 80 owing to the improved protein value. A similar result was obtained when sweet potato flour replaced rice (Yang, 1982). Walter and Catignani (1981) extracted a white protein isolate and a grayish-white protein concentrate (chromoplast protein) from two sweet potato varieties, "Jewel" and "Centennial" and found that they gave a very good amino-acid pattern, with lysine higher than the FAO pattern (Table 4.11). Both the isolates gave a higher gain in weight and a better PER than casein, though this was not statistically significant, indicating that some protein fractions from selected varieties of sweet potato are of very high quality (Yang, 1982).

Cassava protein is lower in total essential amino-acids than the other root crops but recently Adewusi et al. (1988) found that cassava flour used as a component in animal feeding trials was a more effective replacement for wheat than either sorghum or maize. The content of protein in yam varies between 1.3 and 3.3 percent, (Francis et al., 1975), but based on the quantity consumed by an adult in West Africa, about 0.5 to 1 kg per caput/day, it can contribute about six percent of the daily protein intake (see Table 4.8). The chemical score for yam proteins, using the FAO reference protein as standard, varied from 57 to 69 (Francis et al., 1975). The incidence of kwashiorkor has been reported to be high in yam consuming areas. This emphasizes the need to supplement a yarn-based diet with more protein-rich foods in order to support active growth in infants. Fresh cocoyam contains a high percentage of water and is a food of low energy density compared to alternative root crops. It has a protein content of about two percent (Table 4.4) with a chemical score of 70 (Table 4.5). However chemical score alone is not a satisfactory index of protein availability and efficiency in the diet. This can best be assessed by controlled feeding trials to obtain values of digestibility. Such values have been determined for many individual foods. If information is not available on the digestibility of the protein in a particular diet, the value can be estimated by using values for individual components and calculating a weighted mean according to the proportion of protein supplied by these foods. In foods of low protein content such as yam and cassava, feeding trials to determine the biological efficiency of the protein are often inconclusive. As an approximate correction, for a diet based on vegetable protein, a digestibility factor of 85 percent may be applied (WHO, 1985).

TABLE 4.9 - Essential amino-acids of plantain, cassava, sweet potato, cocoyam and yam compared with cowpea

Source: FAO, 1970.

TABLE 4.10 - Utilizable protein In some staple foods (percentage of calories)

Source: Payne, 1969.

TABLE 4.11 Comparison of essential amino-acid patterns for chromoplast and white protein In Jewel and Centennial sweet potato roots to the FAO reference protein

¹g amino-acid/16 g N

Source: Walter and Catignani, 1981.

Human dietary tests have been carried out using root crops to test the efficiency of the root crop protein to maintain good health in the absence of other protein sources. Most of this work has been done on potato and is well documented by Woolfe (1987). The classical work of Rose and Cooper (1907) indicated that young women could be maintained in nitrogen balance for seven days on a diet in which potato supplied 0.096 g N/kg body weight. This has been confirmed more recently in experiments in which a potato protein level of 0.0545 g/kg body weight was found to maintain nitrogen balance in healthy college students, compared to a value of 0.0505 g/kg body weight obtained for egg.

Lopez de Romana et al. (1981) in Peru reported that potato can be used successfully to supply up to 80 percent of the daily requirement of protein and SO to 75 percent of the energy of infants and young children if the remaining energy and nitrogen is provided by a non-bulky, easily digestible food. Acceptability, digestibility, tolerance and growth of children were analysed. Excellent acceptability and tolerance were found for a diet providing about 50 percent of the energy from potato with casein added to make up to 80 percent of the total dietary energy from protein. Raising the level of potato to provide 75 percent of the dietary energy tends towards poor acceptability and tolerance near the last week of the three-month study mainly because of the bulk and the poor digestibility of the carbohydrates.

When the British settled on the remote South Pacific Island of Tristan da Cunha in 1876, it was reported in 1909 that the population had increased and were very healthy on a potato-based diet, consuming about 3-4 lb of potatoes per day (Kahn 1985). Even in an affluent country such as the United Kingdom, potato contributed about 3.4 percent of the total household protein intake according to the National Food Survey Committee (1983), compared to 1.3 percent for fruit, 4.6 percent for egg, 4.8 percent for fish, 5.8 percent for cheese, 5.7 percent for beef, 9.8 percent for white bread and 14.6 percent for milk.

In dietary tests adult Yami tribesmen were given a diet based on sweet potato supplemented with fish and vegetables, designed to supply 0.63 g protein/kg body weight/day. They did not show any physical abnormality after two months, but appeared to tire more easily after a more prolonged period on this diet. As a result of the high dietary fibre content the faecal volume of the test subjects was very high, an average of 800 g on a wet weight basis per day. This diet, contrary to expectation, did not generally reduce the serum cholesterol and total lipids, as did some other vegetables, though a particular sweet potato variety did significantly reduce these factors (Yang, 1982).

However, when seven teenage boys were placed on two similar diets based on sweet potato, supplying 0.67 g protein and 0.71 g protein kg body weight respectively, they exhibited a negative nitrogen balance and their plasma urea nitrogen decreased from 8-11 mg to 2-3 mg per 100 ml. Their plasma free amino-acid pattern also showed some abnormalities, with the branched chain amino-acids, valine, isoleucine and leucine values decreasing, indicating some degree of protein depletion (Huang, 1982). This finding confirms that sweet potato protein alone cannot meet adequately the nutritional requirements of a growing child, but appears to be more promising in the case of adults. In an attempt to improve the diets of the people of Taiwan, Yang (1982) found that when 13 percent of sweet potato was substituted equicalorically for rice in the Taiwanese diet, the nitrogen balance was improved to complementarily of the proteins. The same replacement was found to prolong the longevity of tested male and female rats. Thus, if it can be produced at a competitive price, sweet potato can provide a supplementary staple for rice, wheat flour and other cereals.

Food containing about 5 percent of total energy provided by utilizable, balanced protein can sustain health if it can be eaten in sufficient quantities to meet energy requirements. It is therefore important to review the factors affecting the protein content of root crops. If varieties with a high protein content and good carbohydrate digestibility could be developed these could be used in the formulation and production of supplementary weaning foods. Experimental production of weaning foods containing potato has been reported by Abrahamsson (1978). Breeding programmes for improved protein, vitamin or mineral content in food crops should also include consumer preference studies, to ensure acceptance of the improved varieties at producer level.

Lipids

All the root crops exhibit a very low lipid content. These are mainly structural lipids of the cell membrane which enhance cellular integrity, offer resistance to bruising and help to reduce enzymic browning (Mondy and Mueller, 1977) and are of limited nutritional importance. The content ranges from 0.12 percent in banana to about 2.7 percent in sweet potato. The lipid may probably contribute to the palatability of the root crops. Most of the lipid consists of equal amounts of unsaturated fatty acids, linoleic and linolenic acids and the saturated fatty acids, stearic acid and palmitic acid. In dehydrated products such as dehydrated potato or instant potato, the high percentage of unsaturated fatty acids in the lipid fraction may accelerate rancidity and auto-oxidation, thereby producing off-flavours and odour. The low fat content of plantain, coupled with its high starch content, makes it an ideal food for geriatric patients. Banana is the only raw fruit permitted for people suffering from gastric ulcer, and is also recommended for infantile diarrhea. Banana is also used as a source of carbohydrate in coeliac disease and in the relief of colitis.

Vitamins

Since roots and tubers are very low in lipid they are not in themselves rich sources of fat-soluble vitamins. However, provitamin A is present as the pigment beta-carotene in the leaves of root crops, some of which are edible. Most roots and tubers contain only negligible amounts of beta carotenes with the exception of selected varieties of sweet potato. Deep coloured varieties are richer in carotenes than white cultivars. In the orange variety "Goldrush", the pigment is made up of about 90 percent beta carotene and in "Centennial" the corresponding figure is 88 percent. This is one of the nutritional advantages of sweet potato because sufficient and regular ingestion of sweet potato leaves, together with the tubers of high beta-carotene varieties can meet the consumer's daily requirement of vitamin A, and hence prevent the dreadful disease of xerophthalmia, which is responsible for nutritional blindness in many sub-Saharan countries and in Asia. The dessert type of sweet potato is even higher in beta-carotene and it has been estimated that an intake of 13 g/day will be sufficient to meet the vitamin A requirement. Similarly some varieties of yam are highly coloured, especially D. cayenensis, called yellow yam. The colour of yellow yam is also because of carotenoids, consisting mainly of beta-carotene in quantities of 0.14-1.4 mg per 100 g (Murtin and Ruberté, 1972) and other carotenoids which have no nutritional significance (Martin et al., 1974b). Some Pacific Island varieties of yam contain up to 6 mg per 100 g (Coursey, 1967) of carotene; cocoyam also has a generous amount. Other sources of beta-carotene include the deep orange varieties of banana. The concentration, however, decreases from 1.04 mg per 100 g when green (unripe) to 0.66 mg when ripe (Asenjo and Porrata, 1956). Plantain contains very little beta-carotene.

Potato has no vitamin A activity. There is some report of the occurrence of some vitamin E, up to 4 mg per 100 g in sweet potato.

Vitamin C occurs in appreciable amounts in several root crops. The level may be reduced during cooking unless skins and cooking water are utilized. Root crops, if correctly prepared, can make a significant contribution to the vitamin C content of the diet. Banana contains about 10-25 mg of vitamin C per 100 g, though figures as high as 50 mg have been quoted in some varieties. The quantity is the same whether it is ripe or unripe. Yam contains 6-10 mg of vitamin C per 100 g and up to 21 mg in some cases. The vitamin C content of potato is very similar to those of sweet potato, cassava and plantain, but the concentration varies with the species, location, crop year, maturity at harvest, soil, nitrogen and phosphate fertilizers (Augustin et al., 1975). One hundred grams of potato boiled with the skin is sufficient to provide about 80 percent of the vitamin C requirement of a child and 50 percent of that for an adult. According to the 1983 Nutritional Food Survey Committee, potato was a principal source of vitamin C in British diets, providing 19.4 percent of the total requirement. McCay et al. (1975) estimated that in the United States of America potato provided as much vitamin C (20 percent) as did fruits (18 percent).

Most of the root crops contain small amounts of the vitamin B group, sufficient to supplement normal dietary sources. The B-group of vitamins acts as a co-factor in enzyme systems involved in the oxidation of food and the production of energy. These vitamins are found mainly in cereals, milk and milk products, meat and green vegetables, including the leaves of roots and tubers. For every 1 000 kcal of carbohydrate ingested about 0.4 mg of vitamin B. (thiamine) is needed for proper digestion. Sweet potato contains about double this required amount of vitamin B. (0.8-1.0 mg/1 000 kcals). Villareal (1982) has estimated that a hectare of land planted with sweet potato will provide about eight times as much vitamin B1 (thiamin) and 11 times as much vitamin B2 (riboflavin) as a hectare planted with rice (see table 4. 12). Similarly it has been estimated by the Nutrition Food Survey Committee (1983) that in the United Kingdom potato supplied 8.7 percent of the riboflavin, 10.6 percent of the niacin (vitamin B3), 12 percent of the folic acid, 28 percent of the pyridoxine (vitamin B6) and 11 percent of the panthothenic acid (Finglas and Faulks, 1985).

TABLE 4.12 Number of persons a hectare of crop can support per day In terms of different nutrients

Source: Villareal, 1970

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