Volume shrinkage mechanism for combined vacuum freeze drying-hot air drying of diced apples

The combination of vacuum freeze drying (FD) and hot air drying (HAD) has been observed to markedly reduce the volumetric shrinkage of hot air dehydrated products. While the FD can be attributed to concurrently optimizing energy consumption. However, the appearance quality of combined drying materials is still unstable at present. This study aims to clarify the shrinkage mechanism of apple products during the FD-HAD. Four points of moisture transition (moisture contents of dry basis were 1.00, 0.76, 0.53, and 0.33 g/g, respectively) were selected for the combined drying of apples. Shrinkage, texture, microstructure and pore distribution were determined in the dehydration products. The water migration and distribution of the samples were analyzed at the HAD stage using low-field nuclear magnetic resonance (LF-NMR). The results showed that the shrinkage of FD-HAD samples was significantly (P<0.05) better than that of HAD ones. The moisture transition point shared a significant (P<0.05) effect on the shrinkage of the samples (6%-45%). There was no outstanding volumetric shrinkage when the moisture content at the transition point was below 0.53 g/g. The samples were more shrinkable, harder and less crisp, as the moisture content increased at the transition point. There was a great variation in the center collapse. The decrease was found in the porosity and average pore size, with the increase of moisture content at the transition point. FD-HAD treatment saved energy more substantially (23.58%-28.95%), compared with the FD. The unit energy consumption decreased gradually with the increase of water content at the conversion point. The shrinkage of the sample in the FD-HAD occurred in the HAD when the moisture content at the conversion point was greater than 0.53 g/g. The sample was divided into two phases of rising and falling in the HAD, where the volumetric shrinkage of the sample mainly occurred in the rising phase. A three-phase transition of water was found in the sample from the ice crystals to the liquid and water vapor, which was then removed from the sample. There was a rapid decrease in the free water, and little change was measured in the content of bound and immobile water. There was a larger humidity difference between the center and the surface of the sample. The water was migrated from the center to the surface. The ice crystals inside the sample were basically removed by sublimation, and then to reduce the melting of ice crystals, when the moisture content at the transition point was less than 0.53 g/g. The small amount and range of free water and its migration resulted in a small shrinkage of the sample. In summary, the controllable transition point of co-drying controls shrinkage can be expected to reduce the amount and extent of ice crystal melting, as well as the free water migration in the HAD phase of the sample. There was less damage to the microscopic pores caused by moisture migration. This finding can provide the basic data and theoretical reference for the precise regulation of the appearance quality of FD-HAD samples and energy saving.