title: Thermal stability of ascorbic acid and ascorbic acid oxidase in african cowpea leaves ( Vigna unguiculata ) of different maturities.
abstract: 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.
authors.affiliation: Department of Food Science and Technology, Faculty of Agriculture, Jomo-Kenyatta University of Agriculture and Technology, P.O. Box 62000, Nairobi 00200, Kenya. wawiremm@hotmail.com
authors.forename: Michael
authors.initials: M
authors.lastname: Wawire
authors.forename: Indrawati
authors.initials: I
authors.lastname: Oey
authors.forename: Francis
authors.initials: F
authors.lastname: Mathooko
authors.forename: Charles
authors.initials: C
authors.lastname: Njoroge
authors.forename: Douglas
authors.initials: D
authors.lastname: Shitanda
authors.forename: Marc
authors.initials: M
authors.lastname: Hendrickx
mesh_terms: D001204:Ascorbate Oxidase / Q000737:chemistry* / Q000378:metabolism; D001205:Ascorbic Acid / Q000032:analysis / Q000737:chemistry*; D003683:Dehydroascorbic Acid / Q000032:analysis; D004355:Drug Stability; D004795:Enzyme Stability; D007887:Fabaceae / Q000737:chemistry* / Q000201:enzymology; D006358:Hot Temperature; D007700:Kinetics; D018515:Plant Leaves / Q000737:chemistry* / Q000201:enzymology
chemical_list: D001204:Ascorbate Oxidase; D001205:Ascorbic Acid; D003683:Dehydroascorbic Acid