Moisture Content and Equilibrium

Moisture content can be expressed in two ways. On a wet basis, the water is given as a fraction of the total weight of the moist grain; this is the basis normally used in grain trading and in the storage moisture targets quoted for cereals. On a dry basis, the water is given relative to the weight of dry matter alone; this is often used in drying and engineering calculations. The two give different numbers for the same grain, so it is important to know which basis is meant.

Because water moves between grain and air, grain left in a given environment tends toward an equilibrium moisture content determined by the air’s temperature and relative humidity. Grain drier than this equilibrium will slowly take up moisture from the air, while grain wetter than it will slowly give up moisture. This is why storing grain in cool, dry air keeps it safe, and why grain can re-wet if exposed to humid conditions.

Equilibrium moisture content underlies both drying and storage. Drying works by surrounding grain with air whose equilibrium is lower than the grain’s current moisture; safe storage works by holding grain in equilibrium with cool, dry air so that moulds cannot grow. The relationship differs among commodities, so oilseeds, for example, are stored at lower moisture than cereals.

Water activity describes how available the water in grain is to microorganisms and to chemical reactions, rather than how much water is present in total. It is closely related to the relative humidity that grain would create in a sealed space around it. Water activity is often a better guide than total moisture to whether moulds can grow, because microbes respond to available water; keeping water activity low is what makes dry grain microbiologically stable.

Moisture is measured in practice with electronic moisture meters, which are quick but must be calibrated and are verified against reference oven-drying methods that determine moisture by weight loss on heating. Reliable measurement matters because storage safety and trading value both hinge on the figure.