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Analysis and modeling of pitaya slices in a heat pump drying system

Abstract The objective of this article was to investigate the drying kinetics, effective moisture diffusivity, and quality of pitaya in the heat pump drying process. The experiment was conducted at a drying temperature of 50–70°C, slice thickness of 8–10 mm, and a relative humidity of 10–30%. The results showed that the heat pump drying of pitaya was a deceleration process. The drying temperature has the greatest influence on the drying rate. The drying time decreased by 28.57% with the drying temperature increased from 50 to 70°C, while that increased the least by 12% with the slice thickness dropped from 10 to 8 mm. Six drying models were analyzed comparatively based on experimental data, and calculations indicated that the Avhad and Marchetti model could better describe the moisture migration law during the heat pump drying process of pitaya. The optimal drying kinetics model was established to predict the change of moisture content under different drying conditions, and the average error of the model compared with the experimental values was 5.56%. In addition, the effective moisture diffusivity of pitaya ranged from 6.4167 × 10−10 to 9.8156 × 10−10 m2/s, and the drying temperature had a remarkable influence on the effective moisture diffusivity while the slice thickness had the least. According to the Arrhenius equation, the drying activation energy of pitaya under the experimental conditions was 19.628 kJ/mol. Moreover, the effect of drying temperature on browning degree and surface microstructure was also analyzed. The conclusions of this article provide theoretical support for the analysis of water migration laws and the optimization of the pitaya heat pump drying process.