Food Water Activity
Compute a food's water activity (aw) from the equilibrium relative humidity, aw = ERH/100. Water activity — the 'free water' available for reactions and microorganisms — is the most important conservation factor: below aw 0.6 no microorganism grows; bacteria stop at ~0.90, molds at ~0.70. Unlike total moisture, it explains why honey (moist but with low aw) does not spoil. Enter the equilibrium relative humidity (%).
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Food water activity
Why does honey, sweet and moist, keep forever, while a cut of meat with less water in it rots within days? The answer lies not with total moisture, but with water activity (aw) — the share of the water that stays free and available for chemical reactions and for microorganisms, instead of bound to sugars, salts and proteins. It is measured through the equilibrium relative humidity the food creates in a closed headspace: aw = ERH/100. The scale runs from 0 (bone dry) to 1 (pure water), and the thresholds decide everything: below 0.60, no microorganism grows at all (cereals, honey, powdered milk); pathogenic bacteria stop at around 0.90; molds hold on down to about 0.70. That is why salting, sugaring, drying and dehydrating preserve food — every one of them lowers the aw, locking the water away. Water activity also governs the rate of spoilage reactions (fat oxidation, Maillard browning, vitamin loss), each with its own optimum range. Alongside temperature and pH, it stands as one of the pillars of food preservation. Enter the equilibrium relative humidity.
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.