Food preparation and processing methods on nutrient retention and accessibility in selected indigenous vegetables from East Africa

Abstract

In Africa, indigenous vegetables are a common food source and rich source of micronutrients such as carotenoids, vitamin C, calcium and iron. However, food preparation and processing operations affect contents and bioavailability of micronutrients in vegetables. Several studies have been carried out to investigate the influence of local food preparation and processing methods on iron and carotenoid accessibility of vegetables including African indigenous vegetables (AIV) such as African nightshade (Solanum scabrum), amaranth (Amaranthus blitum), slenderleaf (Crotalaria ochroleuca), sweetpotato leaves (Ipomea batata) and cowpea leaves (Vigna unguiculata). Different experiments showed that cooking vegetable mixtures from either two of these AIV together increased iron accessibility when compared to cooking one vegetable. Fried recipes had significantly higher iron content compared to boiled ones and raw vegetables. Improved recipe formulation may increase AIV acceptability and consumption and thus increase iron intake. An additional study on cooking and drying effects on carotenoid and flavonoid retention in two nightshade species (Solanum scabrum and Solanum villosum), showed that oven drying retained carotenoids while drying in direct sun and under shade cause a significant decrease in the carotenoid contents in both nightshade species. Cooking (boiling in water for 10 min) did not affect the levels of lutein and β-carotene but caused total loss of violaxanthin and neoxanthin. Cooking also reduced flavonoid content while oven drying, drying in direct sun and under shade did not affect the flavonoid content of Solanum scabrum sp. In vitro simulated gastro intestinal digestion caused a loss of approximately 80% in the carotenoid contents of the two nightshade species, and also revealed that the accessibilities of iron, carotenoids and flavonoids are improved more in the cooked leafy vegetables than in the raw, oven dried and sun dried samples. Another study investigated feasible food preparation methods to increase carotenoid retention and iron bioavailability of leafy vegetable dishes traditionally prepared by Tanzanian households. Results showed that modified traditional preparation methods with appropriate ingredients improve the retention of lutein and β-carotene; and iron accessibility in sweetpotato leaves. Specific cooking, preparation and preservation practices of AIV that are compatible with local experience can be encouraged as an alternative or adjunct to other methods of increasing the availability of micronutrients in foods.