Effect of Pretreatments and drying on Nutrient Content of Orange Fleshed Sweet Potato Tubers and Cowpea leaves used in Maswa District, Tanzania

Abstract

Rural communities in Tanzania are increasing suffering from food shortage and malnutrition (Vitamin A deficiency, Protein energy malnutrition and Iron deficiency Anaemia). A household survey was conducted to determine the baseline situation in Maswa district, regarding production, processing and storage of orange fleshed sweet potato tubers and cowpea leaves. Chemical analysis was carried out on four sweet potato varieties (Jewel, Karoti dar, Kabode and Ejumula) possessing different intensities of orange-flesh colour and cowpea leaves to establish levels of nutrients in fresh and processed products using standard methods of analysis. Beta carotene was determined by spectrophotometric method while mineral content was determined using Atomic Absorption Spectrophotometric method. Both cowpea leaves and sweet potatoes were subjected to pretreatments including blanching and boiling. The baseline survey indicated that, respondents have limited knowledge regarding nutritional excellence and blanching of orange fleshed sweet potato tubers and cowpea leaves. Michembe is more preferred than matobolwa dried sweet potato. Fresh samples of sweet potatoes and cowpea leaves had significantly low proximate composition results (protein, fat, fiber and carbohydrate) and mineral content compared to dried samples due to the fact that during drying food loses a significant amount of moisture resulting into concentrating other nutrients. Moisture content of fresh cowpea leaves was 89.54g/100g and dried cowpea leaves were below 11% moisture. Solar dried samples indicated higher fat content compared to sun dried. Fibre and protein contents of blanched samples were significantly higher than that of cooked samples. Ash content of cooked samples was significantly higher than the blanched samples, regardless of the drying method used. There was a three-fold reduction in β-carotene content when fresh samples were dried. Boiling have a different effect on sweet potatoes compared to cowpea leaves, boiling results into more retention of beta carotene than blanching in sweet potatoes while in cowpea leaves blanching results into more retention than boiling. Results further showed that fresh dried had significantly low β-carotene content and low retention on storage compared to boiled and blanched chips and blanched cowpea leaves retained more beta carotene after six months storage at room temperature. Therefore, blanching should be introduced to rural communities during processing of green vegetables and orange fleshed sweet potato tubers to protect nutrients loss. Education on processing and nutritional excellence of orange fleshed sweet potato tubers and cowpea leaves should also be provided to assist reduction of incidences of community malnutrition in Maswa district and Tanzania in general.