Research of physicochemical properties and antioxidant activity of beetroots as affected by vacuum microwave drying conditions

Abstract

The object of research is the beetroots, dried by vacuum microwave drying at different conditions. Physicochemical properties and antioxidant activity of beetroots were studied using vacuum microwave drying at different microwave power (500, 1000, and 1500 W), vacuum degree (–0.05, –0.07, and –0.09 MPa) and sample thickness (2, 4, and 6 mm). A colorimeter was used to evaluate the color quality of beetroots. Colorimetric methods were used to determinate contents of betalain, ascorbic acid and total flavonoid, and antioxidant activity (ferric reducing antioxidant power assay) of beetroots. Results showed that the drying time decreased with increasing microwave power and vacuum degree, while increased significantly with the increase of sample thickness. The lightness (L*) of dried beetroots was higher than that of fresh beetroots. The values of redness (a*) increased with the increase of vacuum degree. The values of yellowness (b*) increased with the growth of vacuum degree and microwave power, while reduced as the sample thickness added. The total color difference (∆E) of dried beetroots reduced with increasing vacuum degree, and displayed the lowest value (5.95) at a vacuum degree of –0.09 MPa as compared to fresh beetroots. The content of betacyanin, betaxanthin and ascorbic acid displayed a declining tendency with the growth of microwave power, while increased with the increase of vacuum degree. And the total flavonoid content of beetroots illustrated a decreasing tendency with the increase of vacuum degree, microwave power and sample thickness. The ferric reducing antioxidant power (FRAP) of dried beetroots decreased significantly with the increase of microwave power, and showed the highest value (14.70 mg trolox equivalents/g) at a microwave power of 500 W. The most favorable conditions for vacuum microwave drying of beetroots were microwave power of 500 W, vacuum degree of –0.09 MPa and sample thickness of 2 mm. It leads to better physicochemical properties of bioactive compounds and higher antioxidant activity of dried beetroots. The dried beetroots can be used as functional foods and value-added food products.

Optimization and Testing of the Technological Parameters for the Microwave Vacuum Drying of Mulberry Harvests

“…When the drying temperature is relatively high, the Vc was significantly destroyed. This is consistent with the research results of Silvana M et al [40], where the Vc was damaged more seriously if the microwave power was higher because excessive microwave power can destroy the cell structure and release bioactive compounds to the external environment [41]. Hence, the thermal degradation of the Vc content at 400 W of microwave power was higher than that at 350 W. The Vc content in the mulberry increased first and then decreased with increases in the microwave temperature and power, which is consistent with research results of Deepa [42].…”