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Thermal Stability of Ascorbic Acid and Ascorbic Acid Oxidase in African Cowpea Leaves (Vigna unguiculata) of Different Maturities

Wawire, Michael, Oey, Indrawati, Mathooko, Francis, Njoroge, Charles, Shitanda, Douglas, Hendrickx, Marc
Journal of agricultural and food chemistry 2011 v.59 no.5 pp. 1774-1783
Vigna unguiculata, ascorbic acid, cowpeas, dehydroascorbic acid, heat inactivation, heat treatment, leaves, maturity stage, oxidation, protective effect, reaction kinetics, temperature, thermal stability, vegetables
Cowpea, an African leafy vegetable (Vigna unguiculata), contains a high level of vitamin C. The leaves harvested at 4−9 weeks are highly prone to vitamin C losses during handling and processing. Therefore, the purpose of this research was to study the effect of thermal treatment on the stability of ascorbic acid oxidase (AAO), total vitamin C content (l-ascorbic acid, l-AA), and dehydroascorbic acid (DHAA) and l-AA/DHAA ratio in cowpea leaves harvested at different maturities (4, 6, and 8 weeks old). The results showed that AAO activity, total vitamin C content, and l-AA/DHAA ratio in cowpea leaves increased with increasing maturity (up to 8 weeks). Eight-week-old leaves were the best source of total vitamin C and showed a high ratio of l-AA/DHAA (4:1). Thermal inactivation of AAO followed first-order reaction kinetics. Heating at temperatures above 90 °C for short times resulted in a complete AAO inactivation, resulting in a protective effect of l-AA toward enzyme-catalyzed oxidation. Total vitamin C in young leaves (harvested at 4 and 6 weeks) was predominantly in the form of DHAA, and therefore temperature treatment at 30−90 °C for 10 min decreased the total vitamin C content, whereas total vitamin C in 8-week-old cowpea leaves was more than 80% in the form of l-AA, so that a high retention of the total vitamin C can be obtained even after heating and/or reheating (30−90 °C for 10 min) before consumption. The results indicated that the stability of total vitamin C in situ was strongly dependent on the plant maturity stage and the processing conditions applied.