摘要：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.
