What water activity measures and why it matters
Water activity (aw) is the fraction of water molecules free to participate in microbial growth, ranging from 0 (bone dry) to 1 (pure water). It differs from moisture content: a high-salt dried fish might be 30 percent water by weight but have very low aw because salt binds the water molecules, making them unavailable to bacteria.
Each microbe requires a minimum aw to grow. Salmonella and E. coli need aw above 0.95, most molds require 0.8+, and xerophilic fungi can survive as low as 0.6. This is why salt, sugar, or drying extend shelf life by reducing aw, not just by reducing total moisture.
Practical shelf-life boundaries
Foods at aw above 0.86 support pathogenic bacterial growth and require refrigeration or preservation. Between 0.86 and 0.6, mold and yeast dominate, but bacteria slow. Below 0.6, most microbes stop entirely, which is why jerky, dried fruit, and properly dehydrated goods store at room temperature for months.
Water activity also predicts texture changes. Low-aw foods (below 0.5) absorb moisture from humid air and go stale or soggy. This is why crackers, cookies, and powders are packaged in moisture barriers and why opened foods should be resealed quickly in humid climates.