Structural analysis of vacuum microwave dried shiitake mushrooms based on X-ray CT images and physical properties

This study was designed to investigate the effects of changes in vacuum microwave (VMW) drying conditions on the drying kinetics, microstructures and physical properties of shiitake mushrooms. Under the tested conditions of 3 kPa and 20 kPa, low pressure and high microwave power in particular increased the rate of moisture change of shiitake mushrooms. Morphological observations from X-ray computed tomography (CT) images showed that samples dried at 3 kPa had many more porous structures with smaller pores, while clumps of cells formed and led to the formation of larger pores in samples dried at 20 kPa. Under the 20 kPa condition, the apparent density increased, and the internal porosity decreased, immediately after drying. These results implied that shrinkage of the samples occurred in the early stages of drying. However, drying at lower pressure led to the dried samples with lower densities and higher internal porosities because of the puffing effect.