Water Activity

Water activity is a ratio, not an amount. It describes the thermodynamic availability of water in a commodity — how readily that water can leave the commodity as vapour, and by extension how readily it can be taken up by a fungal spore, a bacterial cell, or an enzyme that needs free water to act. This is a physically different quantity from moisture content, which simply describes what fraction of the sample’s mass is water, regardless of how tightly that water is held.

Because water activity is a ratio expressed on a scale from zero to one, it carries no unit of mass or percentage of its own; it is reported as a dimensionless fraction. This is deliberate: unlike moisture content, water activity is directly comparable in its physical meaning across very different commodities, even though the moisture percentage that produces a given water activity differs from one commodity to the next.

Water binds to starch, protein, oil, and sugar with different strength. Starch and protein hold water relatively loosely; sugars bind it more tightly; oil holds essentially none. A consequence follows directly: in an oilseed, the water present in the lot is concentrated in the non-oil fraction of the kernel, so the water activity of an oilseed is higher than the same percentage moisture would suggest in a cereal grain. An oilseed and a cereal reported at an identical moisture percentage are therefore not equally stable — the oilseed’s water is, in effect, less diluted and more available.

Water activity changes with temperature even when the moisture content of the commodity does not, because the vapour pressure of water — both in the pure state and within the commodity — is itself a function of temperature. A water activity value measured at one temperature is not automatically valid at another; grain warmed or cooled without any change in moisture can shift into a different water-activity range, which is one reason temperature management in storage matters independently of drying.

Every microorganism capable of spoiling a stored commodity — including the toxigenic storage fungi of greatest concern for mycotoxin contamination — has a limiting water activity below which it cannot grow. This is the physical basis for water-activity-informed storage-stability control: holding a commodity below the limiting value for the organisms of concern is what prevents growth, independent of the moisture percentage that achieves it. Different organisms have different limiting values, and those values are also affected by temperature and by the presence of other organisms.

This entry does not state any limiting water activity value for any organism, because those values are organism-, commodity-, and condition-specific and are established and published by food-safety authorities and commodity-specific guidance rather than asserted generically here.