Microwave Drying of Pitaya (Hylocereus) Peel and the Effects Compared with Hot-Air and Freeze-drying

Abstract

Abstract. Microwave drying (MD) of pitaya peel was performed at 75, 225, 375, 525, and 750 W. The drying effects on the effective moisture diffusivity (De), total polyphenol content (TPC), betalain content, DPPH scavenging ability, and rehydration abilities of pitaya peel were examined. The data were compared with hot-air drying (HD) at 100°C for 4 h and freeze-drying (FD) for 24 h. The results showed that the MD kinetics of pitaya peel fit Fick’s law and the Arrhenius equation with an activation energy (Ea) of 34.08 W g-1. The De values of MD, which ranged from 2.35E-07 m2 s-1 (at 75 W) to 5.56E-06 m2 s-1 (at 750 W), indicated that the effectiveness of MD was appreciably higher than that of HD (2.44E-08 m2 s-1) and FD (4.43E-09 m2 s-1). Although MD resulted in decreased betacyanin (9.31 mg L-1 extract) and betaxanthin (6.58 mg L-1 extract) contents, the DPPH inhibition of pitaya peel remained at 75% due to an increase in TPC (115 mg GAE per 100 FW). MD of pitaya peel can also provide higher rehydration ability than HD and was comparable to FD. This study concludes that MD of pitaya peel was highly effective while maintaining high levels of TPC and DPPH inhibition. Keywords: Arrhenius equation, Effective moisture diffusivity, Fick’s law, Microwave drying, Pitaya peel.