Optimization of Nutritional and Quality Properties of African Cowpea (Vigna unguiculata) Leaves and Other Selected Traditional African Vegetables from Farm to Fork

Abstract

Solanum nigrum and Cleome gynandra) are great sources of micro nutrients in the diet of many in Kenya and there is need to improve the the intake of these nutrients from these vegetables. The main objective of the thesis was to monitor the changes of nutrient and quality characteristics of the cowpea leaves at different levels of maturity, handling, thermal processing, storage up to consumption; that is, farm to fork with the aim of optimizing these nutrients‟ availability. Cowpeas were grown in a green house where the temperature, light and relative humidity were controlled. The leaves were harvested between 4-9 weeks of maturity. The analysis and quantification of ascorbic acid (AA) content was done by HPLC while the polarographic assay was used for ascorbic acid oxidase (AAO). The AAO activity, total AA content and L-ascorbic acid/dehydro ascorbic acid (L-AA/DHAA) ratio in the leaves increase with increasing maturity. Thermal inactivation of AAO follows first order reaction kinetics. The folates in the cowpea leaves (4-8 weeks of maturity) were assayed through the extraction, conversion, purification, and HPLC quantification processes. The total folate content and the folate poly-γ-glutamate chain length increases with increasing maturity of the leaves. Enzymatic conversion to short chain folate poly-γ-glutamates in crushed leaves indicates the presence of endogenous GGH enzyme. Thermal traetment affects the stability and profile of folates. The mineral content of the cowpea leaves were assayed using the inductively coupled plasma-optical emission spectrophotometry (ICP-OES). There is a general decline (>60%) in the levels of minerals during maturation of the leaves. The effect xx of thermal treatment on peroxidise (POD) inactivation in the leaves was studied in the range of 75-100°/120 mins while that of the visual green colour was in the range of 55-80°C/90 mins. The kinetics of the enzyme in situ thermal inactivation that of POD and the visual green colour shows a first-order, biphasic and fractional conversion model respectively, however they both follow an Arrhenius model of the temperature sensitivity of the reaction rates. The other ALVs were purchased from Thika municipal market, in Thika district of central Kenya and after multi stage sampling, samples were used to quantify their nutrient content using standard A.O.A.C methods. The proximate composition, total carotene and mineral content were evaluated. The molar ratio of phytates to the different minerals was also calculated. The thermal degradation of AA follows a first order reaction and the temperature dependence of k values can be described by Arrhenius relation. In conclusion, the preharvest, harvest and postharvest factors greatly affect the nutrient and quality properties of the cowpea leaves and other ALVs and the eventual availability of these nutrients to the consumer.