Effects of blanching time-temperature combinations and solar-drying on the nutritional and microbial quality of indigenous leafy vegetables in Kenya

Abstract

The abundance of indigenous leafy vegetables (ILVs) during rainy seasons and accompanying poor storage systems leads to high post-harvest losses (PHL). These gluts in the rainy seasons are usually followed by scarcity in the dry seasons. Currently some limited preservation is employed involving boiling of ILVs and open sun drying, a practice that is inappropriate and done under unhygienic conditions. To reduce the huge losses, a cheaper, hygienic and locally adaptable preservation method for ILVs is required. Solar drying could be a useful dehydration option after blanching as applied in exotic vegetables, but there have been no studies on suitable combinations of blanching and solar drying protocols for preservation of ILVs. This study was conducted to determine the effects of controlled blanching time/temperature combinations followed by solar-drying on the nutritional and microbial quality of selected ILVs with a view to finding a suitable preservation technique. Three common ILVs in Kenya, spiderplant (Cleome gynandra), slenderleaf (Crotalaria ochroleuca) and cowpeas (Vigna unguiculata) were used in this study. The ILVs were grown at Egerton University’s horticulture teaching and research station and harvested after 4 weeks. Two blanching conditions (80oC/10 min and90oC/5 min) followed by solar-drying were tested. Blanching of ILVsat 100oC/30 min andopen sun-drying was used as a control, while conventional oven drying of the ILVs was used as a standard for comparison. Analyses on fresh ILVs, blanched and dried ILVs and ILVs stored for three months, were done to determine nutrient content and microbial loads. Generally, the greatest nutrient losses were observed in ILVs blanched at 100oC for 30 min and sun-dried, while most nutrients were retained in ILVs blanched at 80oC/10 min. The total viable counts (TVC) were lowest (log10 5.3-log10 5.6 cfu/g) in solar dried ILVs blanched at 90oC/5 min. Storage of the solar-dried ILVs for three months at 18oC led to 13-16% moisture loss, 42-44% ascorbic acid loss, 7-10% β-carotene content loss and, minor losses in iron (1.2-1.6%) and calcium (1.2-1.5%). The storage period had no significant effect on the crude protein and fibre composition of the blanched and dried ILVs. The TVC in solar-dried ILVs increased from log10 5.9 to log10 6.3 cfu/g during storage period. There were minor increases in yeasts and mould counts (log10 1.2 to log10 1.4 cfu/g) and in coliform counts (log10 1.3 to log10 1.5 cfu/g) for the solar-dried ILVs. This research indicates that blanching at 80oC/10 min followed by solar drying is a potential preservation technique for ILVs in Kenya and is therefore recommended for use by farmers to preserve ILVs.