Grain storage moisture targets for cereals and oilseed rape

Food safety risk increases at high moisture levels. Learn about moisture content (MC) targets and how equilibrium relative humidity (ERH) affects spoilage risk in grain stores.

• Food safety risk increases above 14.5% MC in cereals
• If grain is stored below 13% MC, moisture uptake is considerably limited, effectively preventing mites breeding
• Dry grain with a high moisture content (at or above 18% MC) immediately
• Commercial grain is commonly traded at moisture contents of 15% and above

The safe time to achieve the recommended moisture content will depend on the grain temperature. The impact of any particular temperature and moisture content combination can be assessed using the AHDB safe storage time calculator.

From a safety perspective, it is always better to store grain at a lower temperature. This is because the grain exchanges water with the surrounding air and, in enclosed spaces, this continues until a balance (the equilibrium relative humidity – ERH) is reached. As the temperature decreases, so does the ERH. At 5˚C, wheat at 14.5% MC has an ERH of 56%. The same grain stored at 25˚C at the same MC has an ERH of 66%. Mould growth and mite reproduction stop below 65% ERH.

The relationship between grain (wheat) moisture content, temperature and ERH is shown in the table, below.

Note: Mould growth and mite reproduction stop below 65% ERH (green squares).

Cereals and oilseed rape have different relationships between moisture content and equilibrium relative humidity. Oilseed rape has lower moisture content targets.

• For prolonged, stable storage, dry oilseed rape to 7.5–8% moisture content, as soon as possible
• Do not dry oilseed rape below 6% – the lowest acceptable moisture content

• Monitor at several locations (same each time)
• During winter, record at least monthly
• Allow a safety margin

If moisture content increases in a localised area of 2% or more in a week, identify the cause – e.g. condensation, leaks, hotspots or insects – and take necessary action.