Modified traditional cowpea leaf vegetable preparation methods for enhancing carotenoid retention and iron bioavailability

Consumption of vegetables is the most sustainable way of reducing and controlling micronutrient deficiencies in resource-poor communities. However, not much has been documented about standard recipes that are of high nutritional quality in terms of minerals and vitamins. The aim of this study was to modify traditional cowpea leaf vegetable preparation methods to enhance carotenoid retention and iron bioavailability. Modification principles included reducing cooking time, addition of yoghurt, addition of oil and use of oven drying. Other objectives were to measure nutrient composition of two varieties of cowpea leaf (Dakawa and Ex-Iseke) found in rural Tanzania and to compare carotenoid retention and iron bioavailability of cowpea leaves with those of selected leafy vegetables (cowpea, pumpkin, amaranth, kangkong and sweet potato). Carotenoids were analyzed using High Performance Liquid Chromatography and iron bioavailability by in vitro method. The two cowpea leaf varieties were found to be good sources of carotenoids 44.43±0.03mg (Dakawa) and 41.54±0.01mg (Ex-Iseke), vitamin C 86±0.02mg and 94±0.00mg, calcium 165±0.03mg and 142±0.01mg, phenols 575±0.02mg and 558±0.01mg, flavonoids 604.47±0.03mg and 723.36±0.02mg per 100g edible portion and antioxidants 2596±0.01μmoleTE and 2416±0.01μmoleTE respectively. However, very high amounts of anti-nutrient oxalate 418±0.00mg and 348±0.03mg were observed. Traditional cowpea leaf dish cooked with oil, onion, tomatoes and coconut milk for 30 minutes had significantly (P<0.05) the highest beta-carotene (40.83%±7.00) and lutein (34.60%±3.30) retention compared to other traditional recipes. The highest iron bioavailability (10.04%±0.49) was observed in traditional recipe which involved boiling fresh cowpea leaves for 15 minutes. Modifying traditional preparation methods did not significantly improve carotenoid retention and iron bioavailability (p>0.05). All selected vegetables had promising carotenoid retention with lutein and beta-carotene having more than 50% retention. Iron bioavailability increased with cooking except for sweet potato leaves. Based on the results, it is concluded that not all principles used for modifying traditional vegetable recipes for enhanced carotenoid retention and iron bioavailability works for every vegetable