Nutrients and antinutrients composition of raw, cooked and sun-dried sweet potato leaves
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Traditional indigenous vegetables are the most economically efficient source of micronutrients in terms of both land required and production costs per unit. Promotion of production and consumption of such micronutrient-rich foods will improve intakes, the overall diet, and health status. This study aimed to determine nutrient (iron, calcium, vitamin A and ascorbic acid) and anti-nutrient (oxalates and polyphenols) contents in raw, cooked and dried sweet potato leaves Two varieties of sweet potatoes, which were identified as commonly grown for leaves consumption were analyzed at Department of Food Technology, Sokoine University of Agriculture and at the Government Chief Chemist Laboratory Tanzania. The analysis included proximate, nutrient (ascorbic acid, carotenoids, iron and calcium) and anti-nutrient (oxalate and polyphenols) composition. The purple midrib sweet potato leaves were further analyzed for nutrient and anti-nutrient retention after cooking (with and without lemon) and open sun-drying (with and without salting). There was no significant difference (P>0.05) between the two varieties in crude protein, crude lipid and moisture content. The purple midrib sweet potato leaves had significantly (P<0.05) higher ash, crude fibre, carotenoids, calcium and iron contents while the green midrib sweet potato leaves had significantly (P<0.05) higher ascorbic acid content. The polyphenols were about 4 times higher in the purple midrib sweet potato leaves (22.16%) as compared to that of the green ones (5.28%), which had significantly higher oxalate levels (3730 mg/100g). Drying with salt and cooking with lemon reduced polyphenols significantly (p<0.05), with retention of 42% and 56% respectively; while cooking with lemon lowered significantly the oxalate levels. The traditional methods of cooking SPL with addition of lemon is advantageous because it reduces polyphenols while retaining higher levels of minerals, β carotene and vitamin C. Drying with salt results in a nutritionally and organoleptically good product, hence, drying with salt and cooking with addition of lemon is encouraged. Since the sweet potato leaves are harvested more than once before the plant is uprooted, further studies are recommended to assess whether there is variation in nutrient and anti-nutrient contents in consecutive harvests.