Effects of drying methods on bioactive compounds of vegetables and correlation between bioactive compounds and their antioxidants

Freeze- and heat-drying methods were applied to dry fresh vegetables (carrot, taro, tomato, red beetroot and eggplant) grown in Vietnam and then the total phenolic and flavonoid compounds extracted by alcoholic and alkaline-hydrolysis methods were evaluated to determine the effects of the drying methods on the bioactive compounds of the vegetables. Furthermore, the correlations between the content of bioactive compounds and their antioxidant capacity were also investigated in this study. The results show that phenolic and flavonoid compounds were mainly located in free form in the vegetables which was easily extracted by alcoholic solvent. A high temperature in the heat-drying method in sample preparation significantly reduced total free and bound phenolics, total free and bound flavonoids and their antioxidant capacity. The antioxidant capacity of the extracts highly correlated with free phenolic compounds (r² = 0.8936) and free flavonoid compounds (r² = 0.6682). In contrast, the antioxidant capacity of the extract did not correlate with the bound phenolic and flavonoid compounds (r² = 0.0124 and r² = 0.0854, respectively).