Effect of water cooking on antioxidant capacity of carotenoid-rich vegetables in Taiwan

Carotenoid-rich green leafy vegetables including cilantro, Thai basil leaves, sweet potato leaves, and choy sum were selected to evaluate the effects of water cooking or boiling on their total carotenoid content (TCC), total phenolic content (TPC), and total antioxidant capacity (TAC). The percentage inhibition of peroxidation (%IP), Trolox equivalent antioxidant capacity (TEAC), and metal-chelating effect were used to evaluate TAC. The results indicated that TCC reached the maximum after boiling cilantro, Thai basil leaves, and sweet potato leaves for 10 minutes, 5 minutes, and 5 minutes, respectively, and choy sum remained almost unchanged after 30 minutes of boiling. Boiling cilantro and choy sum had a negative effect on their TPC, whereas there was a significant increase in TPC of Thai basil leaf and sweet potato leaf at 1 minute and 5 minutes of boiling, respectively. During water cooking, TAC of the vegetables did not demonstrate a consistent trend. However, TCC was a vital contributor to %IP, whereas TPC showed a strong association with TEAC. Our findings suggest that a boiling time of ≤5 minutes would be better for preserving or enhancing TCC and TPC as well as revealing a higher %IP, TEAC, or metal-chelating effect for the four vegetables investigated in this study.