Effects of different pre-freezing methods and pressure levels on the pore structure and mechanical properties of microwave-vacuum dried apple

The present study describes the effects of different pre-freezing methods with microwave-vacuum drying (MVD) at different pressure conditions (no vacuum or at 1, 3, or 8 kPa) on the three-dimensional pore structure and mechanical properties of dried apples. Slow-freezing pretreatment and lower pressure conditions were shown to be effective in reducing the drying time in MVD. Slow-frozen MVD samples had large pore sizes and high porosity, especially the mean pore equivalent radii of the slow-frozen MVD samples at 1 and 3 kPa were 153.7 and 138.7 μm, which are comparable to that of the freeze-dried sample (137.7 μm). These structural characteristics were thought to confer low hardness and high crispness in the samples. Principal component analysis revealed that the structural properties of the pores in MVD samples were decided not by the pressure condition, but by the pre-freezing method. Overall, the results showed that the combination of slow-freezing pretreatment with MVD under a lower pressure is advantageous for improving the drying efficiency and the texture of the dried samples. The slow-frozen MVD samples were structurally similar to the freeze-dried samples, but there were some differences in the mechanical properties, suggesting that there are factors other than the pore structure that influence the mechanical properties.