不同预处理方式对苹果片真空冻结特性的影响

Effect of Different Pretreatments on the Vacuum-freezing Characteristics of Apple Slices

为了解预处理对苹果真空冻结过程特性的影响,采用冻融、热烫、超声波与未预处理4种预处理措施,进行苹果片真空冻结试验,分析各组在失水率、温度、组织内水分状态以及细胞结构等方面的差异。结果表明:4组苹果片40 min真空冻结失水率分别为46.48、27.49、24.80和22.92%,冻融组失水率明显高于其他3组(p<0.05),热烫组与未处理组也有显著性差异(p<0.05),超声组与未处理组无显著差异(p>0.05);苹果片真空冻结过程分为降压闪发段、冰晶生成段和深层冻结段3个阶段,4组冻结终温依次为-30.40℃、-27.42℃、-26.84℃与-26.50℃;冻融和热烫处理使苹果片LF-NMR自由水横向弛豫时间T23峰位向左明显偏移,组织内自由水束缚力增强;冻融和热烫预处理对真空冻结前、后的苹果片细胞组织结构形态的破坏影响程度均高于其他2组;4组物料真空冻结特性所表现出来的差异性主要是由于物料组织结构产生不同程度破坏影响所致。

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.