Nutritional studies on the bioavailability of iron from cereals and pulses

Abstract

Nutritional studies to evaluate the bioavailability of iron was conducted among three selected cereals i.e., rice, wheat and ragi and six pulses i.e., Bengal gram, green gram, black gram, horse gram, cowpea and soya bean. The results of the study indicated that among cereals though the total iron was maximum for ragi but the absolute available iron was less mainly due to its high fibre and tannin content. Its availability was considerably improved by germinating ragi for 48 hours when cereals were processed, refined wheat flour had the maximum iron availability owing to the reduction of fibre and phytic acid content on refining, while ragi had the least iron availability. Among pulses the total and absolute available iron was maximum for bengal gram and soya bean and least for black gram which was found to be high in phytin phosphorus and tannin content. The total and absolute available iron increased on dehulling pulses except in the case of soyabean. This increase in iron was mainly due to the fact that most of the tannins reside in the seed coat of pulses which are removed on dehulling. A further increase in the iron content was observed on germinating the pulses due to the break down of antinutritional factors during germination. A slight increase in the absolute available iron of fermented and unfermented idli batter was observed which became more available on cooking by steaming. On germinating and roasting ragi almost a four to six fold increase in the absolute available iron was observed while in pulse the absolute available iron almost doubled on roasting bengal gram while a ten to fifteen fold increase was observed in black gram. A further increase in iron was observed on roasting the pulses after dehulling. Similarly, on frying cereals and pulses only a slight increase in total and absolute available iron was observed which considerably increased when pulses were fried after dehulling. The total iron and absolute available iron was maximum for pressure cooked rice rather than rice boiled by excess water method. Similar results were observed when pulses were boiled after soaking due to the leaching of appreciable quantities of tannins. A further increase in iron content was observed on pressure cooking the soaked pulses. The effect of different cooking vessels on the total and absolute available iron, observed by different cooking and processing methods revealed that the total iron was maximum for cereals and pulses cooked in iron vessels mainly due to the contamination of iron from the iron cook ware. The absolute available iron was maximum for cereals and pulses cooked in inert vessels like glass, mud and tufflon coated vessels and it was least in the case of aluminium vessels which react with food constituents forming complex compounds which makes the iron unavailable. Phytin phosphorus, an inhibitor of iron absorption was noted to reduce on dehulling and germinating of cereals and pulses thus increasing the absolute available iron. Among pulses, whole black gram contained the maximum amount of phytin phosphorus and tannins while green gram contained the least amount of phytin phosphorus. It was observed that the tannin content in pulses greatly reduced on dehulling pulses as the tannins are in the seed coat, consequently increasing the iron availability. When ragi was germinated the tannin content reduced increasing the total, ionisable and absolute available iron. On analysing the fibre content of cereals it was observed that whole wheat flour had the maximum amount of crude fibre which reduced to a minimum on refining thus increasing the absolute available iron. Similar results were observed when ragi was germinated where the fibre content reduced with increasing hours of germination up to 48 hours. When pulses were dehulled, fibre was seen to reduce greatly simultaneously increasing the iron availability. On germinating pulses, similar results were observed up to 48 hours of germination indicating that 48 hours of germination is adequate for maximum reduction in fibre content. Pulses do not contain ascorbic acid other than when they are germinated. Thus with increase in germination time the ascorbic acid is seen to increase simultaneously increasing the total and absolute available iron. Among the pulses analysed bengal gram was found to have the highest ascorbic acid content on germination. Iron absorption studies from typical Kerala diets revealed that diet III containing heme iron had the highest ionisable iron while all the diets were high in iron absorption inhibitors. Modification of the local diets by supplementing or substituting foods containing iron absorption enhancers or by processing methods increased the absolute available iron from the diets. Thus by substituting or supplementing locally available, low cost foods, high in bioavailable iron to rice based diets would enhance the absolute available iron of our diets.