Effects of Various Cooking Techniques on the Vitamin C Levels in Cowpea Vegetables (Vigna unguiculata)

Background: Vitamin C, also known as ascorbic acid, is a water-soluble vitamin required for several functions in the human body. Its chemical formula is C6H8O6. Vitamin C plays an important role in forming collagen, a protein that gives structure to bones, muscles, and blood vessels. Vitamin C is a water-soluble vitamin widely distributed in nature, mostly rich in fresh fruits and green leafy vegetables. The amount of Vitamin C in a given food depends on soil condition, climate in their area of growth, storage condition after harvest, and methods of preparation. A significant amount of Vitamin C in vegetables is lost during cooking, and there is still limited information on the impact of different cooking methods on the Vitamin C content in leafy vegetables such as cowpea (Vigna unguiculata).

Aim: This study aimed to determine the concentration of Vitamin C in cowpea vegetables when cooked using different cooking methods.

Methodology: Vitamin C concentration in a solution of cowpea vegetable cooked by different cooking methods (boiling, steaming and microwave methods) was determined by a redox titration using iodine in the presence of a starch indicator. This was conducted at Busi tema University, Faculty of Science and Education, Biology Laboratory in Uganda. The sample was titrated with 0.005 molar iodine solutions until the endpoint was reached (i.e., the point at which the stable traces of blue-black colour are formed due to the formation of iodine-starch complex) and the volume of the iodine solution (standard solution) used in the titration was recorded.

Results: The study shows that cooking cowpea vegetables using a microwave yields the highest concentration of vitamin C (154.8 mg/L), followed by steaming (142.9 mg/L) and boiling yielding the lowest concentration of vitamin C (130.4 mg/L).

Conclusion: The concentration of vitamin C obtained by the three methods of cooking cowpea vegetables has shown that the microwaving method of cooking yields the highest concentration of vitamin C followed by the steaming method and finally boiling. Further studies should be done to explain the variation of vitamin C concentration in cowpea vegetable when cooked using the above cooking methods. Similar studies should be conducted on other leafy vegetables which are potential sources of Vitamin C.