Comparative Study on the Effect of Hot Air and Vacuum Drying on Physiochemical Properties and antioxidant Activities of Red Dragon Fruit (Hylocereus polyrhizus) Peel

The red dragon fruit (Hylocereus polyrhizus) peel was subjected to freeze drying (FD), hot air drying (at 50 – 65°C) (HAD), and vacuum drying (at 50 – 65°C) (VD), in which physical properties (color, water activity, water absorption index-WAI, and microstructure), bioactive compounds (ascorbic acid, betacyanin, polyphenol, and flavonoid), and antioxidant activities (DPPH and metal chelating activities) were monitored. The results showed that the lightness (L*) of red dragon fruit peel (RDFP) decreased as HAD and VD were applied especially at higher drying temperatures. Whereas the water absorption index (WAI) of the RDFP was not affected by drying methods. The microstructure surfaces of FD-dried RDFP were smoother than those of samples dried by HAD and VD-dried RDFP. After the drying processes, the bioactive compounds and antioxidative properties of the RDFP powder decreased significantly. However, higher bioactive compounds and antioxidant activities in RDFP dried by VD compared to the samples dried by HAD at the same drying temperatures. The RDFP powder dried by VD at 55°C retaining high physiochemical and antioxidative properties could be utilized as a colorant and nutraceutical ingredient in several food products. Keywords: Red dragon fruit peel, Freeze drying, Vacuum drying, Hot air drying, Bioactive compounds, Antioxidant activity