How much do you know about frozen fruits and vegetables

Using high-quality vegetables for blanching, once the vegetables turn dark green, they can be picked up and cooled in cold water. After cooling, the vegetables are placed in a centrifuge to dry, and then frozen for about 45 minutes in a cold air environment between minus 33 ° C and minus 40 ° C. The formation time of the vegetables varies, and then cold water is added to form thin ice on the surface, which can prevent oxidation.
The purpose of blanching is to kill fungi and achieve the purpose of preservation. Water above 40 degrees Celsius can also inactivate biological enzymes in vegetables, thereby slowing down biological physiological reactions. The shorter the freezing time, the better the quality of frozen fruits and vegetables.
The freezing process and its principle: When the water in fruits and vegetables decreases from its initial temperature to a freezing point below 0 ℃, it changes from liquid to solid, that is, it freezes. As the temperature decreases, ice crystals increase, and fruits and vegetables become hard and frozen. The freezing point of fruits and vegetables varies depending on their type (Table 1). Most vegetables begin to freeze at a temperature of around -1 ℃. When the temperature continues to decrease to about -5 ℃, 70-80% of their water freezes, causing the fruits and vegetables to harden. At temperatures ranging from -1 to -5 ℃, a large amount of ice crystals form, which is called the stage of larger ice crystal formation. If the temperature further decreases to -30~-35 ℃, the freezing process is completed. At different temperatures, the moisture in fruits and vegetables has corresponding freezing rates (Table 2). Therefore, to completely freeze the water in fruits and vegetables, it is necessary to lower their temperature to the eutectic point, which is the temperature at which the freezing rate of water reaches 99.99%.
The frozen processing of fruits and vegetables involves the same steps as drying vegetables and storing them in cans, such as selecting raw materials, cleaning, peeling, cutting, and blanching. After blanching, fruits and vegetables should be cooled and rinsed with clean water in a timely manner. Then, they should be rapidly frozen at a low temperature of -30~-35 ℃ using either the blast freezing method or the fluidized bed freezing method. The packaging adopts polyethylene plastic bags or plastic boxes, which are divided into two types: pre freezing packaging and post freezing packaging. Generally, dry packaging is used, but pre freezing packaging can also be wet packaged with 2% salt water, such as frozen processing of mushrooms.