Department of Food Science and Technology, Faculty of Agriculture, Jomo-Kenyatta University of Agriculture and Technology, P.O. Box 62000, Nairobi 00200, Kenya.
J Agric Food Chem. 2011 Mar 9;59(5):1774-83. doi: 10.1021/jf103469n. Epub 2011 Feb 10.
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.
菜豆，一种非洲叶菜（菜豆），含有高水平的维生素 C。在 4-9 周收获的叶片在处理和加工过程中极易损失维生素 C。因此，本研究旨在研究热处理对不同成熟度（4、6 和 8 周）收获的菜豆叶片中抗坏血酸氧化酶（AAO）、总维生素 C 含量（L-抗坏血酸，L-AA）、脱氢抗坏血酸（DHAA）和 L-AA/DHAA 比的稳定性的影响。结果表明，随着成熟度的增加（最高 8 周），菜豆叶片中的 AAO 活性、总维生素 C 含量和 L-AA/DHAA 比均增加。8 周龄叶片是总维生素 C 的最佳来源，且 L-AA/DHAA 比值较高（4:1）。AAO 的热失活遵循一级反应动力学。在 90°C 以上的温度下短时间加热会导致 AAO 完全失活，从而对酶促氧化起到保护作用。年轻叶片（4 周和 6 周收获）中的总维生素 C 主要以 DHAA 的形式存在，因此在 30-90°C 下处理 10 分钟会降低总维生素 C 含量，而 8 周龄菜豆叶片中的总维生素 C 有 80%以上是以 L-AA 的形式存在，因此即使在食用前进行加热和/或再加热（30-90°C 下处理 10 分钟），也能获得较高的总维生素 C 保留率。结果表明，总维生素 C 在原位的稳定性强烈依赖于植物成熟阶段和应用的加工条件。
