The thermostability, bioactive compounds and antioxidant activity of some vegetables subjected to different durations of boiling: Investigation in vitro

This research was performed in order to compare the water and acetone extracts of raw and boiled for 10, 20, 40 and 60 min Korean lotus roots (KLR) and Polish white onion (PWO) in the contents of their bioactive compounds, antioxidant activity and thermostability. It was found that polyphenols (mg GAE/g), flavanols (mu g GAE/g), flavonoids (mg CE/g), anthocyanins (mg CGE/kg) and tannins (mg CE/g) in water extract of raw lotus roots were 14.18 +/- 0.7, 8.41 +/- 0.5, 1.09 +/- 0.06, 21.3 +/- 1.2 and 7.29 +/- 0.4, and of white onion - 11.11 +/- 0.6, 6.78 +/- 0.3, 0.71 +/- 0.03, 17.00 +/- 0.9 and 1.64 +/- 0.08, respectively, and significantly higher in KLR (P < 0.05). The antioxidant activity of raw KLR water extract (139.4 +/- 6.1, 53.1 +/- 3.6 and 89.3 +/- 4.6 mu mol TE/g for DPPH, CUPRAC and ARTS, respectively) was significantly higher than in white onion (23.84 +/- 1.8, 31.9 +/- 2.1 and 38.14 +/- 2.6 for DPPH, CUPRAC and ABTS, respectively, P < 0.05). The thermostability of the water KLR extract's of polyphenols, flavanols, flavonoids, anthocyanins and tannins was high and even after 60 min of boiling remains as 40.0, 42.3, 50.5, 41.4 and 41.0%, respectively. After 60 min of boiling the most thermostable compounds were flavonoids - remaining at 50.5% in water extract of KLR. Also after 60 min of boiling the thermostability of the antioxidant activity of water extracts of KLR remained significantly high: 40.6, 42.3, 46.3 and 43.6%, according to DPPH, FRAP, ABTS and CUPRAC assays, respectively. Similar relationship was obtained with acetone extracts, but the value was lower than with the water ones. In conclusion, the contents of some bioactive compounds, the antioxidant activity and the thermostability in water and acetone extracts of KLR are significantly higher than the same indices in PWO. FTIR and fluorimetry can be used as additional markers for the characterization of bioactive compounds in vegetables. (C) 2010 Elsevier Ltd. All rights reserved.