Effects of prefreezing on the drying characteristics, structural formation and mechanical properties of microwave-vacuum dried apple

The effects of prefreezing on the drying rate, internal structure and mechanical properties of apple fruit processed by microwave-vacuum drying (MVD) were evaluated. The drying rate of the prefrozen sample was approximately 1.2-1.3 times higher than that of the nontreated sample. In the frozen-thawed tissue, damage to the cell wall structure and cell membrane due to ice crystal formation during freezing was confirmed; thus, improvement in the drying rate was suggested to be the result of accelerated water transfer in the tissue. Structural observation using X-ray computed tomography showed that the prefrozen MVD sample had a porous internal structure with larger voids than air dried or nonpretreated samples. Alterations in the mechanical properties, such as higher maximum stress and number of peaks during the puncture test, indicating a softer and crisper texture, were observed in the prefrozen MVD sample.