Final answer:
Water activity (aw) measures available moisture for microorganism growth relative to pure water and is influenced by both the moisture content and other factors like added solutes. The reduction of aw, for instance by adding salt or sugar, decreases microbial growth, which is key to food preservation.
Step-by-step explanation:
The relationship between water activity (aw) and moisture content is a crucial concept in understanding how microorganisms grow and how food preservation works. Water activity is defined as the ratio of the vapor pressure of a substance to the vapor pressure of pure water. When the aw is high, bacteria grow well with an aw of 0.97-0.99 required, while fungi such as Aspergillus spp. can grow in a lower range of aw around 0.8-0.75.
To control and prevent microbial growth, food can be dehydrated or solutes like salt or sugar can be added, effectively lowering the aw even if the actual moisture content remains the same. This is because solutes bind to water molecules, reducing the water's availability for microorganisms and decreasing the water activity without necessarily changing the total amount of water present.
Relative humidity affects evaporation rates and water vapor content in the air. 100% relative humidity means that the air is fully saturated with water vapor, and further water cannot enter the vapor phase unless temperature or pressure conditions change. Organizations aim to modify this through the use of moisturizers containing either occlusives or humectants to influence aw and moisture content.