Definition
A 0–1 scale indicating how accessible the "free" water in a food is for microorganism and chemical reactions. aw = water vapor pressure on the product / vapor pressure of pure water at the same temperature. In chocolate and confectionery storage, aw < 0.6 is targeted; above this, mold, fungi, and bacterial growth begin. Differs from total moisture content — bound water is unavailable to microorganisms.
Detailed Explanation
Water activity (aw) is a fundamental parameter for microbiological safety and shelf life in food science. It distinguishes the difference between the total moisture in food and the active (microorganism-accessible) moisture.
Microorganism growth thresholds: • aw < 0.60 — no microorganism can grow (dry biscuits, chocolate) • aw 0.60–0.85 — molds and yeasts can grow (confectionery, dried fruit) • aw 0.85–0.95 — bacteria grow (cheese, bread) • aw > 0.95 — pathogenic bacteria grow (fresh food)
Example aw values: • Chocolate: 0.45–0.50 • Chocolate dragee (coated): 0.40–0.55 • Cocoa powder: 0.35–0.45 • Nuts: 0.55–0.70 • Dried fruit: 0.65–0.80 • Hard cheese: 0.85–0.92 • Fresh fruit: 0.97–0.99
aw is directly influenced by ambient relative humidity — Equilibrium Relative Humidity (ERH) = aw × 100%. That is, a product stored at 50% RH equilibrates toward aw 0.50. The 40–50% RH target in chocolate dragee production exists for aw 0.40–0.50 equilibration.
Additionally, aw also influences chemical reactions: lipid oxidation (fat rancidity) is minimum at aw 0.30–0.40, accelerating at lower or higher values. Therefore, aw control is critical for both microbiological safety and product quality.
