Effect of cooking methods on levels of folates and ascorbic acid in cowpea (Vigna unguiculata L. Walp) from Nairobi, Kenya

Abstract

Folate (vitamin B9,5-methyltetrahydrofolate) and ascorbic acid (AA) (vitamin C) play a key role in human health and wellbeing. It is greatly established that AA is beneficial in preventing scurvy while folate helps in prevention of neural tube defects and congenital malformations. The main sources of these vitamins are fruits and vegetables especially green leafy vegetables including the African Indigenous Vegetables (AIVs). However, these vegetables are consumed after cooking which leads to loss of the vitamins through oxidation, thermal degradation, and leaching. The study aimed at determining the effect of different cooking methods on the retention of AA and folate in cowpea (Vigna unguiculata L. Walp) as affected by different cooking methods. Folate and AA were determined using high pressure liquid chromatography with ultra UV-visible detection, HPLC-UV after extraction of the vitamins from raw and cooked samples. Analysis of Variance (ANOVA) was used to determine difference in nutrients retention by various cooking methods. Significance was imputed at p<0.05. Raw V. unguiculata ontained 45.516±0.649 mg/100g AA and 91.736±0.586 μg/100g folate. The cooked samples of the vegetable contained folate ranging from 40.713±0.081 to 65.128±0.007 μg/100g and AA ranging from 0.719±0.063 to 24.181±0.051 mg/100g of the edible portion of the vegetable. Cooking the vegetable significantly reduced both folate and AA concentration compared to the raw vegetable samples, p<0.05. Frying raw vegetables with onions and tomatoes was found to retain significant folate and AA than boiling the vegetables, p<0.05. Addition of milk resulted in significant retention of folate and significant loss of AA than boiling, p<0.05. Both lye and sodium bicarbonate significantly reduced folate and AA concentration than boiling, with lye causing significant reduction of both vitamins than sodium bicarbonate p<0.05. It is concluded that cooking reduces folate and AA concentration in cowpea (V. unguiculata L. Walp). Additive such as lye and bicarbonate also reduce the concentration of the vitamins. Cooking cowpea leaves with addition of milk and frying with onion and tomatoes retains more folate than boiling. However, AA is significantly lost in the process than when the vegetable is just boiled.