Varietal Changes in Proximate Composition and the Effect of Processing on the Ascorbic Acid Content of Some Nigerian Vegetables

摘要

The proximate composition of some varieties of commonly eaten vegetables in Nigeria was determined and some processing and storage methods evaluated using ascorbic acid as an index. The local variety of A. viridis contained a higher level of ash, crude fibre, ascorbic acid and lipid content compared to the exotic green variety while the green C. argentea vegetable has a statistically higher content of crude protein, ash, ascorbic acid and less of crude fibre and total carbohydrate compared to the purple variety to justify the introduction of the exotic green variety of C. argentea. Blanching Vernonia amygdalina and Struchium spargonophora in hot water reduced the ascorbic acid by 12-13% while maceration to remove the bitter taste reduced it by 96%; increased the tannin content in Struchium (14%) but increased the oxalate level by 25% in Vernonia. There was a 27% decrease in the ascorbic acid content of fresh Amaranthus and Struchium over the 288 h storage period in the freezer compared to a loss of 47 - 67% for the blanched samples. The sun and oven dried materials lost all their ascorbic acid content during the storage period at room temperature with 88 - 97% loss within the first 48 h.