Water Activity

Water activity (Aw) measures how much of the water in food is available to support microbial growth, chemical reactions, and enzymatic activity. It’s measured on a scale of 0 (bone dry) to 1.0 (pure water). The critical distinction is that water activity differs from water content: food can have substantial water by weight and still have low water activity if that water is bound to solutes (sugars, salts, proteins) and not available for microbial use.

This is why salt-cured meats and jams are shelf-stable despite containing significant moisture. The salt or sugar binds water molecules, lowering water activity below the thresholds at which pathogens and most spoilage organisms can grow.

Why Water Activity Matters for Home Preservation

Most food safety thresholds in preservation science are defined in terms of water activity rather than water content:

Aw	Microbial activity
1.0-0.95	Most bacteria grow freely
0.94-0.90	Staphylococcus aureus growth slows; many pathogens still active
0.91-0.86	Salmonella and E. coli growth minimal; molds and yeasts still active
0.85	Generally accepted lower limit for bacterial pathogen growth
0.80-0.75	Most yeasts and molds cannot grow
0.70-0.60	Range for properly dehydrated vegetables and fruits
0.60	Generally accepted lower limit for mold growth
Below 0.60	Essentially stable; microbial growth not possible

Practical Applications

Dehydrating: The goal is to reduce water activity to below 0.60. This is achieved by reducing moisture content to approximately 10-20% of original weight for most vegetables, 15-25% for fruits. The “leathery but not moist” standard for dried tomatoes or “brittle and crisp” for dried herbs corresponds to these water activity levels, but exact Aw requires laboratory measurement.

Jams and preserves: High sugar content (typically 60-65% by weight in finished jam) binds water and reduces water activity to around 0.75-0.80, inhibiting most spoilage organisms. This is why high-sugar jams processed in a water bath canner are shelf-stable.

Curing and salt preservation: Salt draws water out of food through osmosis and binds remaining moisture. Properly salt-cured products have water activity below 0.85. The combination of low Aw and acidity in fermented and cured products explains their shelf stability.

Why “dry-looking” doesn’t mean safe: A partially dried piece of jerky that appears dry on the outside may have moisture pockets inside with water activity above 0.85. This is why the USDA recommends finishing jerky at 275°F for 10 minutes after drying - to ensure the interior reaches a temperature that kills pathogens, since surface appearance alone is insufficient.

Home Measurement

Calibrated water activity meters are laboratory and professional tools starting around $500. Home food preservers cannot easily measure Aw directly. Instead, the guidelines from tested recipes (National Center for Home Food Preservation, USDA complete guide) establish processing times and conditions empirically proven to achieve safe Aw levels.

For dehydrating, following tested time and temperature guidelines for specific foods is the practical substitute for Aw measurement. For canning, following tested recipes and processing times in tested sources (NCHFP, Ball Blue Book) ensures safety through combined mechanisms of acidity, heat treatment, and vacuum seal.

Temperature and Water Activity

Water activity is temperature-dependent - higher temperatures allow microbial growth at lower Aw than would be safe at lower temperatures. Properly dehydrated food stored in a hot environment (above 85°F) may be less stable than the same food stored at 60°F. This is one reason cool, dark storage is recommended for all preserved foods, not just those where temperature directly affects product quality.