Effect of Different Pretreatments on the Vacuum-freezing Characteristics of Apple Slices
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Abstract:
The vacuum-freezing characteristics of apple slices were investigated after freeze-thaw, blanching, and ultrasound pretreatments versus no pretreatment (four groups). The characteristics studied included: rate of water loss, temperature, moisture state in tissues, and cell structure. The water loss rate among the four groups after 40 min of vacuum-freezing was 46.48%, 27.49%, 24.80%, and 22.92%, respectively. The results showed that water loss rate of the freeze-thaw group was significantly higher than that of the other three groups (p<0.05). The difference in water loss rate between the untreated and blanching groups was significant (p<0.05), whereas no significant difference was noted between untreated and ultrasound groups (p>0.05). The process of vacuum freezing apple slices is divided into three phases: depressurization flashing, ice forming, and deep freezing. The final freezing temperature of the freeze-thaw, blanching, and ultrasound and no pretreatment groups was -30.40 ℃, -27.42 ℃, -26.84 ℃, and -26.50 ℃, respectively. The transverse relaxation time peak T23 value of free water in low-field nuclear magnetic resonance (LF-NMR) of apple slices pretreated with freeze-thawing and blanching shifted significantly to the left, whereas the binding force on free water in tissues was enhanced. Freeze-thaw and blanching pretreatments caused greater damage to the structure and morphology of cells and tissues of apple slices compared with ultrasound and no pretreatment. The differences in the vacuum-freezing characteristics of the four groups were mainly due to the different extent of damage to the structures and tissues in apple slices caused by each pretreatment.
