The Meaning Of Water Activity To Food

The water in food exists in two forms: free water and bound water. Microorganisms produce and multiply on food, and the water that can be used is free water, not the total water content of food (%). Because part of the water is combined with protein, carbohydrates and some soluble substances, such as amino acids, sugar, salt, etc., this combination is useless to microorganisms, so the water content lacks scientific guidance for food production and preservation. Therefore, it is proposed to use water activity (Aw value) to represent the water that can be used by microorganisms in food.

Water activity (Aw value) is the ratio of the fugacity of water in the solution to the fugacity of pure water, and can also be approximately expressed as the ratio of the partial pressure of water vapor in the solution to the vapor pressure of pure water. It refers to the state of the presence of water in the food, reflecting the degree of binding or dissociation of the water with the food. The smaller the value, the higher the degree of binding.

Water activity refers to the amount of free water in the product, which is the basic data for the growth of enzymes and microorganisms. Water in products, such as food, is restricted to different ingredients, such as protein, salt, and sugar. The chemically bound water does not affect microorganisms. The more bound water, the less water vapor can evaporate. Therefore, the more water content in the product does not mean that the partial pressure of water vapor on its surface must be higher. The equilibrium relative humidity must be higher, and the microorganisms must be more active. Water activity has a great influence on product stability, such as resistance to microorganisms, fragrance retention, powder agglomeration, chemical stability, physical properties, and food preservation. Generally speaking, the lower the water activity of a food, the longer its shelf life. But there are exceptions. If the water activity in the fat is too low, it will accelerate the rancidity of the fat.

It is easy to see from the definition of water activity that water activity is extremely important in predicting the safety of food and predicting the growth of microorganisms, biochemical reaction rate and stability of physical properties. By measuring and controlling the water activity of food, the following can be done:

　　 (1) Predict which microorganisms are the potential sources of corruption and pollution;

　　 (2) Ensure the chemical stability of food;

　　(3) Minimize non-enzymatic oxidation reaction and non-enzymatic oxidation of lipids;

　　 (4) Elongase activity and vitamins in food;

　　 (5) Optimize the physical properties of food, such as texture and shelf life.