Cooking Effects on Folate and Ascorbic Acid Levels in Selected African Indigenous Vegetables from Githurai Market, Nairobi City County, Kenya
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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 the prevention of neural tube defects and congenital malformations. The main sources of these vitamins are fruits and vegetables and 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. Main cooking methods in Kenya include boiling in unspecified amounts of water and discarding the boiling effluents leading to high nutrient loss. There is also the addition of additives such as bicarbonate of soda, lye (traditional salt), milk, cream, sesame and groundnuts paste whose effect on nutrient levels especially folate and AA levels in AIVs are yet to be explained, thus the need for investigation. The study aimed at determining the effect of different cooking methods on the retention of AA and folate in cowpea [Vigna Unguiculata (L.) Walp], saget (spider plant) [Cleome gynandra (L.)] and pumpkin leaves (Cucurbita moschata) as affected by different cooking methods. The vegetables were bought from Githurai market then sorted, prepared and cleaned using tap water then rinsed with distilled water on arrival in the laboratory. Both raw and cooked samples were analyzed. A portion of 100.000 g of the edible portion of vegetable samples including leaves and young shoots, was used in each of the cooking methods. During extraction, mortar and pestle were used to grind 10.000 g of sample then mixed with 50 mL of extraction solution containing; 20 mM KHCO3 (for vitamin B9 extraction) and 3 % Metaphosphoric acid, MPA, and 8 % acetic acid (for vitamin C extraction). The mixture was then filtered and put in a 100 mL volumetric flask and topped to the mark using the extraction solution. All samples were extracted in triplicates. Folate and AA were determined using high-pressure liquid chromatography with ultra-UV-visible detection, after extraction of the vitamins from raw and cooked samples. Analysis of Variance (ANOVA) was used to determine the difference in nutrient retention by various cooking methods. Significance was imputed at p<0.05. The AA and folate levels in raw vegetables ranged from 9.36±0.12 mg/100g to 60.28±0.32 mg/100 and 35.83±0.23 μg/100g to 258.08±0.58 μg/100g respectively. The cooked samples of the vegetables contained folate levels ranging from 15.59±0.19 μg/100g to 258.08±0.58 μg/100g. The AA mean concentration levels in cooked vegetables were found to be ranging from 1.36±0.02 mg/100g to 39.53±0.40 mg/100g. Therefore, it was determined that cooking the vegetable significantly reduced both folate and AA concentration compared to the raw vegetable samples, p<0.05. Steaming vegetables resulted in significantly higher retention of vitamins compared to other cooking methods. Significant losses of the vitamins were found in vegetables boiled in lye. Therefore, this study recommended that AIVs should be cooked by steaming which leads to higher retention of both folates and AA. The addition of lye and sodium bicarbonate should be avoided during the cooking of AIVs since they cause significant losses of vitamins. The results will be availed to relevant authorities and also used to sensitize vegetable consumers and cooks.