Nutritional Quality and Anti-Oxidant Properties of Minimally Processed African Indigenous Leafy Vegetables

Abstract

African indigenous vegetables (AIVs) are excellent sources of β-carotene, vitamin C, iron as well as minerals, fiber and bioactive compounds. In the recent past, AIVs have gained commercial importance as a result of increased awareness of their nutritional and health benefits and are now produced in both formal and informal marketing channels. One of the challenges in production, marketing and consumption of AIVs is that they are highly perishable and there is inadequate capacity for their storage in fresh state. This is because most storage techniques require low temperatures, which are nonexistent for AIVs in Kenya. Minimal processing can enable AIVs produced in far flung locations to be stabilized and transported to the markets in urban areas. However, this can affect the color, texture, flavor, and nutritional quality of AIVs. This study aimed at; examining the influence of harvest maturity and minimal processing techniques on the nutritional, phytochemical and anti-oxidant capacity in stinging nettle, amaranth and black nightshade. The results indicated significant differences between treatments and stages of maturity. Results further show that the highest contents of β-carotene in fresh state, at young stage was 47.82±1.32 mg/100g in amaranth and mature stage was 71.22±0.87 mg/100g in black night shade. For vitamin C, the highest content was 142.06 mg/100g±2.71 in stinging nettle at young stage while amaranth had the highest content of vitamin C at mature stage as 193.52±3.04 mg/100g. The highest phenol content in fresh state was in black night shade at 1.09±0.14 g/100g and 1.29±0.12 g/100g at young stage and mature stage respectively. Among the processed, the highest content of vitamin C was detected in Freeze-Dried Unsliced Unblanched black nightshade at both young and mature stage as 86.64±4.37mg/100g and 111.14±2.41 mg/100g respectively. For β-carotene, the highest content was observed in Freeze-Dried Unsliced Blanched in amaranth as 30.24±1.35mg/100g at young stage and mature stage had 57.12±1.23mg/100g in black nightshade. This study also aimed at 2; examining the retention of β-carotene and vitamin C content of blanched and unblanched vegetable amaranth, African nightshade and stinging nettle, subjected to Ziploc® and Xtend® bag packaging under freezing and room temperature conditions. Both mature and young fresh amaranth leaves had the highest β-carotene content at 75.08±1.3 mg/100g and 44.53±3.1 respectively. African Nightshade had the lowest β-carotene content of 45.05±1.9 and 37.12±3.0 mg/100g respectively. Stinging nettle exhibited the highest Vitamin C content at 197.75±6.9 and 149.7±6.1 mg/100g at both the young and mature stages respectively. Blanching had a significant difference (P<0.05) on both β- carotene and vitamin C retention as the contents in blanched samples were lower by about 10-15% as compared to fresh leaves samples. Under Ziploc® bag packaging in frozen temperatures, both young and mature amaranth leaves were able to retain a significantly higher (at least 50%) β-carotene content by week 4, with the highest retention being observed in unblanched samples. Blanching of young African nightshade and stinging nettle leaves led to least retention of β- carotene by week 4 (~30%). However, in all the AIVs, the retention of Vitamin C was lower with less than 50% of the initial Vitamin C content retained by week 4. The AIVs were able to keep their marketability for 5-7 days in Xtend® bag packaging under room conditions. However, nutrient loss was quite high with fresh amaranth losing over 80% of its initial β-carotene and vitamin C content. This study indicates that modified atmosphere packaging in Ziploc® bag and Xtend® bags, stage of maturity and blanching, had significant effect on the on the stability of β-carotene and vitamin C of the AIVs. The results of this study can be useful to AIV farmers as it can help in extending the shelf life of the vegetables while maintaining their nutritional quality and improving safety thus preventing post-harvest losses.