Moisture Sorption Isotherms

An isotherm links the moisture held by a product to the surrounding equilibrium vapour condition at one temperature. It helps explain why equal moisture percentages can have different water activities.

Salt, sugar, protein, fat and structure alter water binding and the shape of the curve. A cured sausage and a whole muscle do not share one universal isotherm.

The moisture-gain path may differ from the drying path, a behaviour known as hysteresis. Processing history must therefore be stated when the curve is applied.

Changing temperature shifts the relationship and can affect apparent equilibrium. Data collected at one temperature should not be transferred blindly to another.

Isotherms support drying design, packaging selection, storage prediction and method development. They do not replace direct endpoint measurement for a specific batch.

Use product-appropriate data, state temperature and branch, and verify against measured moisture and water activity before making safety or shelf-life decisions.