Effect of Different Combined Freeze-Drying Methods on Components, Nanostructure and Nanomechanics of Cell Wall in Dried Peach Slices

摘要

Four drying methods, including freeze-drying (FD), freeze-drying followed by air drying (FD-AD), freeze-drying followed by microwave drying (FD-MD) and freeze-drying followed by air puffing drying (FD-APD) were in this study performed on peach slices to investigate their effects on cell wall components and its nanostructure (roughness) and nanomechanics (stiffness). The alcohol insoluble residues (AIR) yields of dried peaches in the four drying methods showed significant difference, with highest for FD peaches and lowest for FD-APD. After drying in four methods, total pectin, hemicellulose and cellulose from AIR decreased dramatically compared with those in raw materials. Moreover, contents of three components in FD peaches were significantly higher than those in other drying methods, with lowest for FD-APD peaches. Cell wall materials in FD-MD peaches showed highest contour roughness, followed by FD-APD, FD-AD and FD. Young's modulus was evidently higher for cell wall materials in FD-AD peaches than others, followed by FD, FD-MD and FD-APD. Highest hardness of dried peaches was observed in FD-MD, with lowest for FD, which was consistent with the nanostructure (roughness) of cell wall materials and microstructure of dried peaches. It was inferred that the above differences were probably caused by different conditions in the second drying phase of the four combined freeze-drying methods, such as temperature, pressure, action time, action principle etc. Our results therefore suggested that peach slices dried with single FD method obtained highest AIR yields and component contents, lowest roughness and moderate stiffness in the cell wall materials, as well as lowest hardness and most fluffy porous microstructure of dried samples.