Drying characteristics and quality of button mushrooms during microwave freeze drying

Abstract: Button mushroom is the most widely cultivated and consumed mushroom throughout the world and it contributes about 40% of the total world production of mushroom. Button mushrooms are extremely perishable and their shelf life is only about 24 h in ambient conditions. Various physiological and morphological changes occur after harvest, which make these mushrooms unacceptable for consumption. Hence, they should be consumed or processed promptly after harvest and for this reason the mushrooms are traded mostly in dried form in the world market. The convective drying method is the most commonly used to dry mushrooms. Nevertheless, due to long drying time and high drying temperature at industrial scale, the problems of darkening in color, shrinkage, loss in flavor and decrease in rehydration ability easily occur. In recent years, microwave freeze drying (MFD) has been investigated as a potential method for obtaining high quality dried food products with low energy consumption. Thus, to achieve faster drying rate with high product quality, MFD was applied to dry button mushrooms. The drying curves, effective moisture diffusivity, rehydration ratio, shrinkage ratio, vitamin C retention ratio, energy consumption and the sensory evaluation based on the fuzzy reasoning were investigated, which were considered to reflect the drying and quality characteristics of button mushrooms under different microwave loading levels (0.25, 1.0 and 0.75 W/g) and system pressure (50, 100 and 150 Pa). The drying models were obtained by the nonlinear fitting of drying curves. The weighted comprehensive evaluation of button mushroom MFD processes were also carried out based on energy consumption, drying time, rehydration ratio, whiteness index and vitamin C retention ratio. Results showed that, when changing the microwave loading level, the reduction ratio of the minimum value to the maximum value of drying time, effective moisture diffusion coefficient, whiteness index, rehydration ratio and shrinkage ratio were higher than those obtained by changing the system pressure. It meant the microwave loading level had a more significant influence on drying rate and physical quality index than the system pressure. However, the system pressure had more obvious influence on nutritional content index, energy consumption and sensory evaluation compared to the microwave loading level. Through the nonlinear fitting function of Origin pro 8.5, we found the R-square of Henderson and Pabis model fitting curve was the highest, which implies the Henderson and Pabis model could well describe the moisture variation in drying process; the effective moisture diffusion coefficient of button mushrooms during the MFD was in the range of 3.423×10-10-5.654×10-10 m2/s, and it was on the 10-10 m2/s order of magnitude and more influenced by microwave loading level. In addition, the products obtained under high microwave loading level and system pressure would be not accepted by consumers. Taking energy consumption, drying time, rehydration ratio, whiteness index and vitamin C retention ratio into account, 0.25 W/g microwave loading level with 100 Pa system pressure was proposed as the favorable condition for button mushroom MFD. The results provide a theoretical basis for the selection of the suitable processing technology for the MFD of button mushroom.