The water content is 73%-90%, vegetables contain 65%-96% water, fish contain 70%-80% water, and meat contains 50% water. Some foods contain less water, such as milk powder containing 3%-4% water, and sugar containing 1.5%-3% water. The water in food exists in two forms: free water and colloidal water. The freezing of water in commercial refrigeration can reduce the activity of food and keep food fresh. Food juice and cell sap contain free water, colloidal water is the water that forms the water film around the colloidal particles, and the freezing point of colloidal water is lower than that of free water. After the food is frozen, during the thawing process, free water is easily re-absorbed by the food tissue, but the colloidal water cannot be completely absorbed by the tissue. The moisture in food creates conditions for the reproduction of microorganisms. Therefore, in order to reduce the moisture of food and prevent the reproduction of microorganisms, the moisture in the food must be removed or frozen. At present, water activity (Aw) is used to estimate the water content in the medium that can participate in the chemical reaction. The mass percentage of food water content cannot directly reflect the safety conditions of food storage, and water activity can directly reflect the food storage conditions. Water activity refers to the ratio of the vapor pressure of water in a liquid state to that of pure water, that is, only free water in food can dissolve soluble components (such as sugar, salt, organic acids, etc.). The vapor pressure of water in a solution state decreases as the soluble components increase. Therefore, the vapor pressure of water in a liquid state in food is lower than that of pure water. The water activity of food is less than 1. Different microorganisms need different water activity ranges for reproduction. The lowest water activity limit for most bacteria is 0.86, for yeast is 0.78, and for mold is 0.65. The water activity of many fresh foods is above 0.9, which is within the water activity range of bacterial reproduction, so fresh food is a perishable food. After frozen food, the water activity decreases after the water freezes. This is also a reason for inhibiting the reproduction of microorganisms. Therefore, freezing is the most commonly used storage method for food.

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