Investigation on the rehydration mechanism of freeze-dried and hot-air dried shiitake mushrooms from pores and cell wall fibrous material

Shiitake mushrooms are unique in their porous structure, which could be affected by various chemical/physical changes during freeze-drying process. In this work, rehydration characteristics of freeze-dried products which were pre-frozen at -20 ℃, -40 ℃, -80 ℃, and -196 ℃ (by liquid nitrogen) were explored from aspects of pores and cell wall fibrous material. Although the appearance and rehydration rate of freeze-dried samples was better than hot-air dried samples with drying temperature ranging from 30 ℃ to 90 ℃, the final rehydration ratio was still less than hot-air dried samples dried at low temperature (30 ℃ and 40 ℃) due to the more serious structural damage by freeze-drying. Hydration capacity of the cell wall fiber was increased by freeze-drying, which might be ascribed to the loosen structure of cell wall instead of composition changes. Thus, hot-air drying at low temperature is still recommend and freeze-drying should be further optimized.