Determination of phytate and oxalate in selected recipes of underutilized local vegetables from the Lake Victoria Basin, Kenya

Underutilized local vegetables (ULVs) in Vihiga County within the Lake Victoria Basin that could provide micronutrients to fight hidden hunger contain phytate and oxalate that reduce bioavailability. These can be reduced through appropriate traditional food processing techniques adopted by households. The aim of this study was to determine the content of phytate and oxalate in formulated ULV recipes. This was an experimental design with eleven selected ULVs: spider plant (C. gynandra), pumpkin leaves (C. moschata), cowpea (V. unguiculata), amaranth (A. blitum), jute mallow (C. olitorius), sweet potatoes leaves (I. batatas), African nightshade (S. nigrum), cassava leaves (M. esculenta), Slender leaf (C. ochroleuca), vine spinach (B. alba) and kales (B. carinata). Eleven single vegetables and five vegetable combinations were blanched and divided into two lots. One lot was boiled and fermented for 48 h and the other was not fermented. The non-fermented were subjected to three treatments; cooked by boiling in water, cooked by boiling with the addition of cow's milk and lye and cooked by sautéing. Phytate and oxalate content in the obtained recipes were determined using HPLC. Independent t-test was used to compare the mean content of phytate and oxalate between fermented recipes and non-fermented recipes. One-way ANOVA was used to compare the mean content of phytate and oxalate between different methods of cooking. The phytate content in fermented ULVs ranged from 27.5-70.67 mg 100 g-1 fresh weight (FW) and 40.0-83.44 mg 100 g-1 FW for non-fermented ones. The oxalate content ranged from 2.62-10.17 mg 100 g-1 FW for non-fermented and 1.54-20.36 mg 100 g-1 FW for fermented. Mean levels of phytate in ULVs were higher than those of oxalate in the same vegetables. The mean phytate and oxalate levels were lower in most fermented vegetables compared to non-fermented ones (p<0.05). However, there was no significant difference in the mean phytate and oxalate levels between all fermented and all non-fermented ULVs (p>0.05) suggesting that degradation of phytate and oxalate by fermentation depends on the type of vegetable since different ULVs responded differently to fermentation. Cooking methods and microbial fermentation differently affected the levels of phytate and oxalate; in some ULVs a decrease was observed while in others there was no significant change. It is concluded that heat during cooking reduced phytate and oxalate levels in most ULVs and that microbial fermentation reduced phytate and oxalate levels in most ULVs but the degree of degradation depends on the type of vegetable sampled since different samples have a varied sample matrix. It is suggested that additives like onions and tomatoes during sautéing and milk and lye during boiling may have introduced some phytate and oxalate to the ULV recipes.