Freezing process of IQF fruit and vegetables 2017/2/12

1 Cooling

Pure water in the process of IQF freezing and cooling, often cold phenomenon, that is, the temperature dropped to freezing point (0℃) below, and then rose to the freezing point began to freeze. In the process of abc, the water to release the sensible way to cool; when the cold to c point, due to the formation of ice crystals, the release of latent heat of the phase change the temperature of the sample quickly rose to 0℃, that process cd, Water in the equilibrium conditions, continue to precipitate ice crystals, continue to release a lot of curing latent heat. At this stage, the sample temperature is kept at a constant freezing temperature of 0°C; when all the water is frozen, the cured sample is cooled at a faster rate (ef).

2 Subcooling

In the process of freezing and cooling food, there will be subcooling phenomenon, but the emergence of such subcooling phenomenon, with the freezing conditions and the nature of the product are quite different, and the fruit and vegetable water is a solution state, its Freezing point is lower than water, the general fruit and vegetable food freezing point temperature is usually between -3.8 ~ 0℃, so the freezing curve and pure water freezing curve is quite different.

3 Crystallization

Water in the food from the liquid into a solid ice crystal structure, that is, the water temperature in the food after falling to the cold spots, and then rose to the freezing point, and then began to change from the liquid to the solid state, the process of crystallization. Crystallization consists of two processes: the formation of nuclei and the growth of crystals.

① the formation of nuclei. After reaching the subcooling temperature, a small part of the water molecules to a certain degree of combination into particles of particles, that is, crystal nuclei, which is the basis for crystal growth.

② crystal growth. Refers to the water molecules in order to bind to the nucleus above, so that the process of increasing crystal. The freezing curve of the food shows the relationship between temperature and time during the freezing of food. AS stage for the cooling stage, the food after the cold phenomenon, where the temperature drop release sensible heat. BC stage for the crystallization stage, at this time most of the water in the water into the ice, the entire process of freezing most of the heat (latent heat) in this stage release, slow down, the curve is flat. CD stage for the ice to the final temperature, ice continues to cool, the remaining water to continue to freeze.

If water and ice are present at 0°C and keep the temperature constant, they will be in equilibrium and coexist. If you continue to exclude heat, it will promote the conversion of water into ice without the formation of nuclei, that is, in the original ice crystals continue to grow and expand. If there is only water at the beginning and no nuclei exist, the crystal nuclei need to be formed before the crystal grows. The temperature must drop below the freezing point to form nuclei, followed by icing and volume growth. Crystal nuclei are the basis for the formation and growth of ice crystals, and icing must first have the presence of nuclei. Nuclei can be spontaneous formation, can also be added, and other substances can also play the role of nuclei, but it should have the same form with the crystal nucleus in order to make water molecules in orderly arranged in its surface combination.

The IQF freezing rate of food is related to temperature, the type of food, the lower the temperature, the higher the freezing rate of food, and the different kinds of foods have different freezing rates even at the same temperature. Usually the temperature of the food should be reduced to -55 ~ -65℃ or so, all the water will be solidified, from the cost of freezing considerations, the process generally do not use such a low temperature, about -30℃, most of the water can be crystalline, Moisture is mainly free of water, freezing the food at this temperature, has reached the frozen storage requirements. In the freezing process, most of the food in the -1 ~ -5 ℃ temperature range, most of the free water has formed ice crystals, generally the temperature range of food that the largest ice crystal production area.