Moisture and Water Activity

bacteria, and molds all require minimal levels of moisture for growth. In general, most wine-borne yeast and bacteria need more moisture than molds. During solidification of cultivation media, agar binds water and lowers free moisture (a, ) or water activity. As such, it is important to adhere closely to the supplier s recommendations regarding the amounts of agar to include in formulations because a could be reduced too far to support growth. 

Moisture may be further lost during the process of pouring plates. For instance, immediately pouring agar upon removal from the autoclave promotes condensation on the inside cover of Petri plates. This loss of water yields very hard agar due to a lower a. To avoid this problem, it is recommended that media be transferred to a 45°C/H3°F to 50°C/122°F 

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According to the glass transition theory, lipids are dispersed in the free volume of the food matrix composed of carbohydrates and protein polymers. In the rubbery state, the lipids react readily with oxygen and become oxidized. In the glassy state, however, the lipids are stable to oxidation because they are encapsulated and there is no free volume. The glass transition temperature, which determines when the food matrix changes from one state to the other, increases with a decrease in moisture and water activity. In many foods the... 

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