Moisture refers to the presence of water surrounding the food and the available water within the food's system. Microorganisms require water to grow, and its abundance helps them spread and multiply even faster. Low water availability can inhibit microbial activity by lowering intracellular water potential and thus reducing hydration and activity of enzymes. 

One preventive control that involves moisture is reducing the moisture content of foods through drying. This food preservation method uses heat to remove moisture, making the food more stable and protected from microbial growth. In some cases, dry foods can reabsorb moisture from the air, which will make them prone to contamination again. This is especially true for high-moisture conditions. To prevent this, food service businesses maintain a very low humidity in storage areas by using dehumidifiers.

Food manufacturers commonly provide the directive “store in a cool, dry place” on labels. This guidance aims to avert the absorption of moisture by foods from their surroundings, a factor that inclines their susceptibility to contamination. The susceptibility of a food item to decay is closely connected to its moisture content and the level of water activity. This principle underscores the practice of food drying. As a rule, foods possessing minimal moisture content exhibit enhanced shelf stability and a reduced vulnerability to microbial spoilage.

Water activity (aw) gauges the available water and is quantified on a scale from 0 to 1.0. Bacteria, yeast, and moulds experience rapid multiplication in environments with high water activity levels surpassing 0.86. Foods preserved using salt or sugar, such as beef jerky or jams and jellies, exhibit diminished aw due to the dehydrating impact of these components on microorganisms, curbing their growth.

Water activity in various foods food as below:

Corn flakes 0.2

Dry pasta 0.5

Flour 0.67

Crisp bacon 0.75

Jam 0.8

Cheese 0.85

Bread 0.95

Raw meat 0.98

Water 1.0