Water Activity and Product Stability

Water activity is the measure of how much of the water in a product is actually available, rather than how much is present. Written aW, it runs from 1.0, pure water, down towards 0. Fresh meat sits at about 0.99. Salt and drying pull it down, and everything that can grow in meat has a level below which it cannot.

The numbers are worth knowing because they are the whole argument for drying. Most spoilage bacteria stop around 0.95. Clostridium botulinum stops at roughly 0.94 to 0.97 depending on type. Staphylococcus aureus is the stubborn one, growing down to about 0.86, which is why it appears in every discussion of fermented sausage. Most moulds and yeasts continue below that, which is why a shelf-stable salame is still a mouldy-surfaced product rather than a sterile one. Below about 0.85 a product is generally considered shelf-stable, and the traditional dried salumi land between 0.85 and 0.91.

Available is the important word. A product can hold a great deal of water and still be stable, because salt binds water to itself and sugar does the same, so the water is present but not accessible to a microbe. This is why salt lowers aW immediately, before any drying at all, and why the salting step is doing safety work from the first day rather than only seasoning.

Measuring it directly needs a water-activity meter, which most home curers do not have, so weight loss is the practical stand-in. It is a good one because the relationship holds for a given formulation: a piece that has lost 38 per cent of its starting weight has reliably crossed into the range that matters. That is the reason the house floor is expressed as a weight-loss figure rather than an aW figure, and the reason a piece must be weighed at hanging and logged.

The last point is the one people miss: aW is not uniform inside a product. A case-hardened salame can measure 0.88 at the rind and 0.95 at the core, and it is the core that decides whether it is safe. This is why the drying has to be even, and why a firm outside is not evidence of a finished inside.