Postharvest UV-C treatment for extending shelf life and improving nutritional quality of African indigenous leafy vegetables

Abstract

Currently, consumer eating habits have shifted to an increasing demand for high quality, safe and healthy food products worldwide. In many African countries, specifically African indigenous leafy vegetables (AIVs) gained importance in this respect contributing to human diet by providing minerals, proteins, vitamins and health-promoting antioxidant compounds. Moreover, these vegetables have an immense potential in creating job opportunities in rural as well as peri-urban areas. However, AIVs tend to suffer severe quantitative and qualitative postharvest losses because of their high perishability. UV-C has been mainly applied in sanitation and food safety for its germicidal effect but also has an impact on preventing nutritional losses. To address this, studies were conducted to evaluate the effect of postharvest application of hormic UV-C dosages on bioactive plant compounds of two AIVs, i.e. African nightshade (Solanum scabrum Mill.) cv. Olevolosi and vegetable amaranth (Amaranthus cruentus L.) cv. Madiira. Eight weeks after planting, the leaves were harvested and treated with UV-C (254nm) at either 1.7kJm−2 or 3.4kJm−2 while untreated leaves served as control. The leaves were kept for 4 and 14d at 20°C (65% RH) and 5°C (85% RH), respectively. The quality parameters studied were fresh weight loss, mineral elements (N, P, K, Ca, Mg, Fe, and Zn), protein, and structural carbohydrates determining dietary fibre content and microbial counts. In addition, antioxidative, health promoting plant compounds, i.e. carotenoid, and chlorophyll contents were evaluated. The results showed that fresh weight loss of both AIVs was significantly reduced with application of lower UV-C dosage (1.7kJm−2). Mineral elements and proteins were variedly affected with a general decline in the initial stages followed by an increase compared to the untreated leaves. Hemicellulose and cellulose was significantly increased in vegetable amaranth and lignin content was significantly increased in African nightshade following UV-C treatment. Chlorophyll and carotenoid contents declined within 2–4d during storage, depending on storage conditions; but thereafter increased again significantly compared to the control. Aerobic mesophyllic and yeast counts were significantly reduced by UV-C treatment, while mould counts were not affected. The findings demonstrate the potential of using hormic UV-C for maintaining the nutritional quality of AIVs during their supply chain as an easy to apply and effective tool, hence contributing to improved food accessibility and food safety in Sub-Saharan areas such as Kenya.