Reduction of nitrates, oxalates and phenols in fermented solar-dried stored cowpea (Vigna Unguiculata l.) leaf vegetables

Abstract

This study was conducted to determine the effect of fermentation, solar drying and storage duration on the levels of anti-nutrients: nitrates, oxalates and phenols, in cowpea leaf vegetables. The rationale was reduction of the anti-nutrients. Reduction of nutritional stress factors in plant foods increases bioavailability of nutrients, hence improving their quality as foodstuffs. The cowpea leaves were purchased from the local markets, sorted to remove blemished leaves and foreign materials, washed in running tap water. Then, the vegetables were drained and divided into three batches of 16 kg each. One batch was heat-treated in hot water for 3 minutes and then cooled to ambient temperatures, drained and solar-dried. The second portion was acidified to a pH of 3.8, heat-treated, and solar-dried. The third portion was fermented for 21 days, heat-treated, and solar-dried. The three batches of vegetables were spread at different times on drying trays at the rate of 4 kg/m2 and dried in a solar drier to an approximate moisture content of 10%. The dried vegetables were packaged in either polyethylene bags or Kraft paper bags and stored for three months at 18oC, 22o- 26oC or 32oC. Fermentation, heat-treatment and drying of vegetables led to significant (P < 0.05) reduction in nitrates compared to fresh cowpea leaves, but the reduction in oxalates and phenols was not significant. Storage for three months led to significant (P < 0.05) reduction in nitrates in the fermented sample compared to the other samples. The acidified sample had significantly (P < 0.05) higher levels of phenols after three months of storage than the other samples. Samples stored at 18oC had higher levels of oxalates and phenols but lower levels of nitrates, compared to those stored at higher temperatures. Packaging material had no significant effect on the level of nitrates, oxalates and phenols. Data obtained in this study reveal a novel technique for the reduction of anti-nutrients in cowpea leaf vegetables, namely; fermentation followed by solar drying. The increased acceptability of these fermented-dried vegetables would help rural communities in providing better foodstuff with fewer anti-nutrients, thus alleviating micronutrient malnutrition. This novel long-term storage technology can greatly help to deal with the issue of seasonality and will increase food security, especially during the dry season.