Process And Principle Of Quick-frozen Fruit

The process of freezing fruit can be divided into 3 stages: the first stage is to reduce the original temperature of the fruit to the temperature where it starts to freeze; the second stage is to freeze the juice of the fruit; the third stage is to reduce the fruit from the freezing temperature to the desired temperature storage temperature. Freezing is carried out gradually from the surface to the inner layer, when the temperature of the thermal center of the product (in a homogenized and isotropic substance, the thermal center falls on the geometric center of the product) is equal to or not higher than the storage temperature 3 ~ 5 ℃, it can be considered that the freezing has been completed. In the juice of fruit, in addition to a large amount of water, various inorganic salts such as potassium, sodium, calcium, magnesium, iron and other organic matters are dissolved, forming a colloidal solution with a freezing point lower than pure water. The highest freezing point of most fruits is -0.8～-2.5℃ (Table 1). When the temperature of the fruit drops to its freezing point, the water is partially separated from the juice, and ice crystals begin to form. As the water gradually separates out of the juice and freezes into ice crystals, the concentration of the juice gradually increases and its freezing point decreases. When the freezing temperature of ice is lowered to the eutectic point of low-soluble ice salt (about below -55°C), the thickest juice is completely frozen. At this time, the colloid is denatured and the reversibility disappears, which is not conducive to the recovery of the product. Therefore, the freezing temperature should not be lower than the reversibility limit of the colloid (usually -18 to -25 °C), and the frozen water content of the fruit is usually controlled at 70 to 80%. Most fruits start to freeze at about -3°C. When the freezing temperature drops to -7°C, about 50 to 60% of the water freezes, that is, in the temperature range of -3 to -7°C, a large number of ice crystals are formed. It is called the largest ice crystal formation zone (Table 2). When the fruit is quick-frozen, it can quickly pass through the largest ice crystal formation zone, and most of the water forms a large number of very fine ice crystals in the pulp cells. The physical-chemical changes are not too strong, and the damage to the pulp tissue is extremely small. When the ice melts, the water can fully penetrate into the pulp tissue, so that the pulp tissue can be recovered well, and a higher quality product can be obtained. It is generally believed that the freezing speed of retail packaging (the speed of the ice front in the product) should be higher than 0.5cm/h, and the freezing speed of individual quick-frozen products should be higher than 5cm/h.