Abstract
The objective of this study was to investigate the effects of different microwave-assisted drying methods on the physical properties, bioactive compounds and antioxidant activity of beetroots. Beetroots were subjected to high-power microwave drying followed by low-power microwave drying (HMD+LMD), high-power microwave drying (HMD), low-power microwave drying (LMD), high-power microwave drying followed by hot air drying (HMD+HAD), hot air drying followed by low-power microwave drying (HAD+LMD), high-power microwave drying followed by vacuum drying (HMD+VD), and vacuum drying followed by low-power microwave drying (VD+LMD). The drying time, moisture content, hardness, color, microstructure, betalains, ascorbic acid, total flavonoids, 2,2′-azino-bis-(3-ethylbenzоthiazoline-6-sulfonic acid) (ABTS) radical scavenging activity and ferric reducing antioxidant power (FRAP) of beetroots were analyzed. The shortest drying time (67.0 min) was observed in HMD, while VD+LMD required the longest drying time of 308.0 min. There was no significant difference in the moisture content of dried beetroots prepared by different microwave-assisted drying methods. Beetroots dried by HMD+HAD showed the highest hardness of 1332.0 g, VD+LMD led to the most desirable color with the lowest total color change. Porous structures were found in beetroots produced by HMD+LMD, HMD and LMD. Beetroots prepared by VD+LMD displayed the highest content of betacyanin, betaxanthin and total flavonoids. While beetroots dried by HMD illustrated the highest ascorbic acid content of 272.3 mg/100 g dry weight (DW). In terms of antioxidant activity, the highest FRAP value of beetroots obtained using VD+LMD was 14.95 mg trolox equivalent (TE)/g DW. Meanwhile, beetroots dried by VD+LMD exhibited the largest ABTS radical scavenging activity (16.92 mg TE/g DW). Compared to other microwave-assisted drying methods, VD+LMD is a more promising method for drying beetroots.
Highlights
Beetroot (Beta vulgaris L.) belongs to the Chenopodiaceae family and is a root vegetable originated from Southern Europe, Eastern Europe and Northern Africa [1]
The results showed that the hardness of dried beetroots obtained by High-power microwave drying (HMD)+hot air drying (HAD), HMD+Vacuum drying (VD) was significantly higher than that of beetroots dried by HMD+Low-power microwave drying (LMD), HMD, LMD, HAD+LMD and Vacuum drying followed by low-power microwave drying (VD+LMD)
Beetroots dried by HMD+HAD and HAD+LMD demonstrated denser structures and collapse of the cell wall structure (Fig. 1, d, e), indicating certain damage to the cell structure of beetroots
Summary
Beetroot (Beta vulgaris L.) belongs to the Chenopodiaceae family and is a root vegetable originated from Southern Europe, Eastern Europe and Northern Africa [1]. Beetroots are valuable vegetables that are distributed worldwide and play a vital role in the diet of humans, provide the diet with color, phytochemicals, and nutrients. According to the data of the United States Department of Agriculture, 100 g of raw beetroots contain the following nutrients: average energy (43 kcal), water (88 g), total sugars (6.76 g), proteins (1.61 g), total dietary fiber (2.8 g), and total lipids (0.17 g) [3]. Beetroots contain a considerable amount of essential and non-essential amino acids. Beetroot is considered as a health-promoting food because it contains bioactive compounds for health promotion, such as betalains, phenolics, vitamins, minerals, carotenoids, nitrate, flavonoids, saponins, ascorbic acids, glycine, betaine and folate [4]. Beetroot is rich in betalain pigment, which has been approved as a natural colorant E-162 by European Union [5]. Betalains are commonly used for coloring a variety of foods, such as sauces, jam, jellies, ice cream, desserts, yogurt, candies and breakfast cereals
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