Control of water activity is a common method of preventing the outgrowth of microorganisms. Food generally have water activities that range between 0.1 for dried foods to 0.96 for fluid foods. Different organisms require different levels of free water in order to growth, with bacteria requiring more free water than yeasts and molds. There are some exceptions. Intermediate moisture foods depend upon the addition of large quantities of sugars or polyols to reduce the A w to below 0.85 and on packaging to prevent the growth of yeasts and molds.
Sugar is the most common carbon source.
In starch based foods, the action of amylases will frequently increase the available sugar source. Organisms that spoil refrigerated products are generally psychrotrophic, and frequently belong to the genus Psuedomonas. The amount of time required for an organism to reach a specific number is dependent upon the initial population, the lag time and the generation time. Under ideal growth conditions, microorganisms can double their number in about 30 minutes. In processed foods, organisms surviving have generally been stressed and commonly may exhibit a lag time of 2-4 days.