Study on the effect of intermittent microwave drying conditions on the bioactive compounds and antioxidant capacity of beetroots

Abstract

The object of this research was the beetroots prepared by intermittent microwave drying at different conditions. The paper aimed to investigate the influence of intermittent microwave drying conditions (power density, microwave gap ratio and slice thickness) on the bioactive compounds and antioxidant capacity of beetroots. A microwave drying system SAM-255 (CEM Corporation, USA) was used to intermittent microwave drying of fresh beetroots. The effect of different power densities (1.0, 1.5, 2.0, and 2.5 W/g), microwave gap ratios (1, 2, 3, and 4) and slice thicknesses (2, 4, 6, and 8 mm) on the bioactive compounds and antioxidant capacity of beetroots were investigated. Colorimetric methods were used to determine contents of betalains, total phenolic and total flavonoid, and antioxidant capacity of dried beetroots. The ascorbic acid content was determined using 2,6-dichloroindophenol titration method. Results showed that power density, microwave gap ratio and slice thickness significantly affected the drying time, bioactive compounds and antioxidant capacity of beetroots. The drying time decreased with the increasing of power density, while increased significantly with the growth of slice thickness and microwave gap ratio. The shortest drying time (35.4±2.6 min) of beetroots was occurred at microwave gap ratio of 2. The content of betacyanins was found to be the highest in the dried beetroots with thickness of 2 mm. The beetroots with slice thickness of 2 and 4 mm showed the highest betacyanins content. Moreover, the highest content of ascorbic acid (240.00±2.32 mg/100 g) and total flavonoid (14.52±0.06 mg rutin equivalent (RE)/g) was appeared at power density of 2.0 W/g, while the content of total phenolic to be highest (12.54±0.13 mg gallic acid equivalent (GAE)/g) at slice thickness of 6 mm. For the antioxidant capacity of dried beetroots, the 1,1-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity reached to the highest value of 6.43±0.03 mg trolox equivalent (TE)/g at power density of 2.5 W/g. While the highest values of ferric-reducing antioxidant power (FRAP) (15.47±0.10 mg TE/g) and 2,2′-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) radical scavenging activity (25.31±0.30 mg TE/g) at microwave gap ratio of 2. It was found that ABTS radical scavenging ability and FRAP were related to the presence of reductions including betalains, ascorbic acid, and total flavonoid in beetroots. The most effective condition for intermittent microwave drying of beetroots were microwave gap ratio of 2, power density of 2.0 W/g, and slice thickness of 4 mm, leads to a better preservation of bioactive compounds and high antioxidant capacity.