Drying Characteristics and Process Optimization by Heat Pump Drying of Green Sichuan Pepper

In order to investigate the drying characteristics of green Sichuan pepper by heat pump and optimize the drying process parameters to improve the quality of dried green Sichuan pepper, the single-factor experiments and orthogonal tests on the drying process of green Sichuan pepper were conducted in this study. Temperature, air speed and laying thickness were defined as test factors, and effective moisture diffusion coefficient, photosynthetic pigment unit mass content and color difference were determined as evaluation indexes. Six commonly used drying mathematical models were used to fit the orthogonal test data nonlinearly, respectively. The results showed that the Page model had the best fit to the test data and it was the best model to describe the drying of green Sichuan pepper by heat pump. Under the constant temperature drying condition, the effect of temperature on the effective moisture diffusion coefficient and color difference was highly significant (P<0.01). The higher the temperature, the greater the change of effective moisture diffusion coefficient and color difference. The effect of air speed on color difference was significant (P<0.05). The higher the air speed, the greater the change of color difference. The effect of laying thickness on color difference was highly significant (P<0.01). The higher the laying thickness, the greater the color difference. However, under different temperatures the unit mass content of photosynthetic pigment slightly decreased, and then increased, and then rapidly decreased until it reached stability. The higher the temperature, the faster the change of the unit mass content of photosynthetic pigment, which was not beneficial to the quality improvement of dried green Sichuan pepper. By comprehensively the effects of temperature, air speed and laying thickness on the color difference, unit mass content of photosynthetic pigment and effective moisture diffusion coefficient, the optimal parameters of drying process were determined as temperature of 40 ℃, air speed of 0.3 m/s and laying thickness of 11.9 mm. Under these conditions, the best color quality of dried green Sichuan pepper was achieved, with a color difference of 20.01 and a photosynthetic pigment unit mass content of 2.9601×10⁻⁴ mg/g. The results of the study could provide reference for the application of heat pump drying process of green Sichuan pepper.