Water Activity, Explained Properly
Water activity is the number that actually determines whether dried food keeps. It is not the same as moisture content, and the difference is the whole reason the concept exists.
Moisture content versus water activity
Moisture content asks how much water is present. Water activity (aw) asks how much of it is available — free to support microbial growth rather than bound up by salt, sugar or the food's own structure.
Two foods with identical moisture content can have very different water activities, and it is the second number that microbes respond to. Pure water is 1.0; the scale runs down from there.
The thresholds worth knowing
- Below about 0.91 — most bacteria stop growing.
- Below about 0.85 — the general target for shelf-stable jerky, and the level below which many regulatory frameworks stop treating a product as potentially hazardous.
- Below about 0.70 — most moulds and yeasts stop growing.
- 0.55–0.65 — where most properly dried fruit lands.
What the estimator actually models
The water activity estimator applies Raoult's law to a salt and sugar solution: aw is approximately the mole fraction of water among all dissolved particles. Salt counts double because sodium chloride dissociates into two ions; sucrose counts once because it does not.
| Solution | aw |
|---|---|
| 100 g water, nothing added | 1.0000 |
| 100 g water + 30 g sugar | 0.9845 |
| 100 g water + 5 g salt + 20 g sugar | 0.9603 |
| 100 g water + 10 g salt | 0.9419 |
| 50 g water + 15 g salt | 0.8439 |
Notice how much more work salt does than sugar. Ten grams of salt drops aw further than thirty grams of sugar, because salt splits into two particles and has a much lower molecular weight — so gram for gram it contributes far more particles to the solution.
The misreading to avoid
A realistic marinade lands around 0.96, well above the 0.85 threshold, and it is easy to conclude either that the marinade is unsafe or that more salt is the route to shelf stability. Neither follows.
Jerky gets below 0.85 because drying removes water, not because the marinade is salty. The estimator describes a solution; the finished product is a different thing. Salt contributes to the final figure and drying does the overwhelming majority of the work — which is also why a batch that looks dry but is under-dried in the middle is the one that spoils.
Where the model is simplified
Raoult's law is an ideal dilute-solution approximation. Real marinades contain proteins, acids and other solutes it does not model, and non-ideal behaviour grows at higher concentrations — exactly where curing brines operate.
It also cannot measure a solid. Water activity in finished jerky is a laboratory measurement made with a dedicated meter, and no calculation from a recipe substitutes for it. Commercial producers measure; home producers dry thoroughly and store carefully.
How to use it anyway
As a teaching tool it earns its place. Adjusting the salt and sugar and watching the number move builds an intuition for why cures are formulated the way they are, and for the relative contribution of the two.
What it should not be used for is deciding that a batch is safe. That decision rests on the process — the heat step, thorough drying, and proper storage — not on an arithmetic estimate of a marinade. Guidance on home-dried meat comes from your national food-safety authority, and the food-safety guide covers the parts that matter most.
Why salt beats sugar, in numbers
The table shows 10 g of salt outperforming 30 g of sugar, and the mechanism is worth spelling out because it explains why cures are formulated the way they are.
Water activity depends on the number of dissolved particles, not their mass. Sodium chloride has a molecular weight around 58 and splits into two ions, so a gram contributes roughly 0.034 moles of particles. Sucrose has a molecular weight around 342 and stays whole, so a gram contributes about 0.003 — more than ten times fewer.
Gram for gram, salt is therefore an order of magnitude more effective at lowering water activity. Sugar earns its place in a cure for flavour, texture and browning rather than for preservation.
Reading aw alongside the other numbers
The estimator is most useful next to the yield estimator, because between them they describe what drying actually does. Five pounds of beef becoming 1.25–2 lb is roughly 60–75% of the mass leaving as water, and that removal is what takes the finished product from a marinade's 0.96 down toward the 0.85 threshold.
Seeing those two figures together makes the point better than either alone: the marinade contributes, and the drying does the work.