Water Activity and Shelf Life of Foods

Salting or curing has the same effect. A saturated solution of common salt has a water activity of close to 0.75. Thus, by adding sufficient salt to foods, the water activity can be lowered to a level where most pathogenic bacteria are inactivated, but the moisture content remains high. 

Intermediate moisture content foods (IMFs) such as pet food and continental sausages rely on fats and water-binding humectants such as glycerol to lower water activity. Fat, which is essentially hydrophobic, does not bind water, but acts as a filler for IMFs to increase the volume of the product. 

The effect of several humectants is for each to sequester an amount of water independently of the other humectants that may be present in the food. Each thus lowers the water activity of the system according to the equation of Ross (1975)  

For example, the water activity of a food with a moisture content of 11% (wet basis) and a salt content of 3% (wet basis) can be calculated as follows 100 g of the food comprises 77 g of water, 20 g of bone dry matter, and 3 g of salt. The contribution to the water activity due to the salt can be calculated (according to Raoult s law of dilute solutions), using the molecular weights of water (18) and salt (58.5), as  

The water activity for the salt-free solid matter of the food is found from its sorption isotherm at that moisture content, e.g., aw0 = 0.90. Thus the water activity of the salted food is  

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