The effect of processing techniques on the microbiological and nutritional qualities of the leafy vegetables Vigna unguiculata and Moringa oleifera grown in South Africa

Abstract

Cowpea (Vigna unguiculata) and moringa (Moringa oleifera) are nutritious and medicinal vegetables, but could also harbor harmful microbial contaminants. The main aim of the project was to determine the effect of each processing techniques on the microbiology, proximate nutrients and shelf life of these vegetables to produce nutritious, tasty, safe and long lasting vegetable products. The processing techniques used were: washing, blanching, and drying. Leaf samples were collected at each stage of processing and were analysed for total viable count, coliform count, yeast and mould count and nutritional content. Microbial plate analysis showed the presence, particularly on cowpea leaves, of yeasts and bacteria such as Pseudomonas, Klebsiella, Staphylococci, Streptococci, and enterobacter including enteropathogens such as Salmonella spp., Shigella dysenteriae and E coli. The presence of E.coli on the leaves was also confirmed using polymerase chain reaction-amplified ribosomal DNA analysis. The most effective processing technique which reduced microbial load to below SABS standards while retaining nutritional quality was the washing of the leaves twice with tap water followed by steam tunnel blanching at 94oC for 12 minutes. Oven drying the leaves at 60oC gave satisfactory and extended shelf life results. Proximate analysis comparison of the two leaf types showed that on average moringa leaves contained more ash (2.37 vs 1.1 g), protein (6,9 vs 3,6 g), fat (0,41 vs 0.2 g) and energy (305,1 vs 70KJ) but less dietary fibre (0,9 vs 7,5 g) than cowpea leaves. No significant differences were noted in these values following washing and steam blanching. These results indicate that washing of these leaves is effective as to reducing microbial load and maintaining proximate values in the short term (up to 4 days) but that oven drying is effective for longer-term storage.