frozen vegetable processing
Frozen vegetable processing
The processing method of vegetables preserved by low-temperature quick-freezing. After the pretreated vegetables are quick-frozen at a temperature of -30 to -35°C or lower, they are packaged in plastic bags, boxes and other containers, or packaged and then quickly frozen, and then stored at a temperature of -18°C for frozen storage. , In order to inhibit the growth and reproduction of microorganisms and the activity of enzymes, so that vegetables can be preserved for a long time.
The freezing process and its principle When the moisture in vegetables is lowered from the initial temperature to the freezing point below 0 °C, it changes from a liquid state to a solid state, that is, freezing. As the temperature decreased, the ice crystals increased, and the vegetables became hard and frozen. The freezing point of vegetables varies by species (Table 1). Most vegetables start to freeze at a temperature of about -1°C. When the temperature continues to drop to about -5°C, 70 to 80% of their moisture freezes, and the vegetables become hard. That is, at a temperature of -1 to -5 °C, a large number of ice crystals are formed, which is called the maximum formation stage of ice crystals. If the temperature is further reduced to -30 ~ -35 ℃, the freezing process is completed. At different temperatures, the moisture in vegetables has corresponding freezing rates (Table 2). Therefore, in order to completely freeze the moisture in vegetables, the temperature must be lowered to the eutectic point, that is, the temperature at which the freezing rate of moisture reaches 100%.
Vegetable quick-freezing process In the quick-freezing process of vegetables, the steps of raw material selection, cleaning, peeling, cutting, and blanching are basically the same as those of dried vegetables and canned vegetables. After the vegetables are blanched, they should be cooled and rinsed with water in time, and then rapidly frozen at a low temperature of -30 to -35 °C by blast freezing method or fluidized bed freezing method. The packaging is made of polyethylene plastic bags or plastic boxes. There are two types of packaging before quick-freezing and after quick-freezing. Generally, dry packaging is used, but 2% salt water can also be added for wet packaging before quick-freezing, such as the quick-freezing of mushrooms.
