Improvements of drying rate and structural quality of microwave-vacuum dried carrot by freeze-thaw pretreatment

Freeze-thaw pretreatment was applied to microwave-vacuum drying (MVD) of carrot, and the drying rate, structure of the dried material, rehydration rate, and mechanical properties after rehydration were evaluated. Pretreatment increased the drying rate, owing to the accelerated moisture transfer rate of the tissue because of damage to the cellular structure during freezing. Structural observation using X-ray CT demonstrated the pretreatment was also effective in preventing structural deformation during MVD. The rehydration rate of the prefrozen-thawed sample was markedly increased due to the larger surface area formed with less shrinkage. Initial elastic moduli of the prefrozen-thawed MVD sample and air dried sample after rehydration mechanically determined were significantly lower than those of the non-treated MVD sample and air dried sample, indicating that the mechanical properties do not depend on the drying method, but on the presence or absence of freeze thaw pretreatment.