Impact of different postharvest thermal processes on changes in antioxidant constituents, activity and nutritional compounds in sweet potato with varying flesh colour

Sweet potatoes are key to food security, nutrition and income generation in sub-Saharan Africa, and orange-fleshed sweet potato contributes towards pro-vitamin A intake. This study investigated the changes in phytonutritional components and antioxidant activity in six sweet potato cultivars with varying flesh colour in fresh form and during different postharvest thermal treatments; conventional cooking (boiled), drying in hot-air convection oven dryer (14 h at 60°C) as well as blanched and unblanched indirect solar drying (at 20-60°C for 48 h). Overall thermal processing enhanced the retention of total protein content, Zn, total phenols, epicatechin, protocatechuic acid, and 2-caffeoyl-L-tartaric acid in all six cultivars compared to the fresh form. However, vanillic acid was not detected in most of the cultivars after cooking except in ‘C5-1. Hot-air convection oven drying significantly increased the total phenolic content in all six cultivars compared to the fresh form and other drying methods, while hot-air oven dried ‘Monate’ contained the highest total phenols and protocatechuic acid. Similarly, hot-air oven dried ‘C5-1’ showed the highest epicatechin and vanillic acid content compared to the fresh form and other drying methods. Cooking increased the 2-caffeoyl-L-tartaric acid in all six cultivars, compared to the fresh form and the highest content was detected in cooked ‘Bophelo’. Blanching and solar drying improved the β-carotene content in orange cultivars ‘Beauregard’, ‘Bophelo’ and ‘C5-1’, of which ‘C5-1’ showed the highest β-carotene content. Cooking increased the antioxidant power (FRAP activity) in all six cultivars and increased the protein content and Zn content in ‘Ndou’ and ‘199062.1’ respectively. The study provided novel information on the changes in phytonutritional components of different sweet potato cultivars including the cream cultivars ‘Monate’ and ‘Ndou’ and orange cultivars ‘Bophelo’ and ‘C5-1’ bred locally in South Africa, using different processing methods.