Effects of osmotic dehydration pretreatment on the drying and characteristics of pineapple slices after microwave hot air rolling bed drying

This study aimed to investigate the effects of osmotic dehydration (OD) pretreatment on the drying and quality of pineapple slices dehydrated using microwave hot air rolling bed drying method. The result suggested the OD pretreatment significantly reduced the moisture content of the food material before drying. It also slightly increased the moisture ratio decreasing rate in the early stage of drying based on the formation of microchannels during the pretreatment. The pretreatment also increased overall temperature and facilitated the transition of free water to bounded water during drying. Based on the pretreatment, the undesired browning of pineapple slices during drying was significantly inhibited. However, the OD pretreatment with 60 % (w/v) sucrose solution caused significant dehydration of the sample before drying, finally resulting in more dramatic browning. Compared with the sample without OD pretreatment, the dried pretreated pineapple slices exhibited an increased amount of collapsed cell structures and pores, due to the cell deformation and water diffusion during the pretreatment. These microstructure changes finally contributed to their increased hardness, springiness and chewiness. Although the pretreatment caused a slightly higher ascorbic acid loss in the final dried food materials, the product with an improved sugar-to-acid ratio was prepared, indicating its improved acceptability. Considering the overall characteristics profile of the dried pineapple slices, the OD pretreatment using 50 % sucrose solution for 24 h was the optimum condition to develop promising dried pineapple slices products.