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Why can grain spoil if it is stored with too much moisture?

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A large red and yellow combine harvester cuts through a golden wheat field, leaving neat rows of harvested grain behind. — Cereal farming Combine harvesting wheat.

Grain spoilage due to high moisture content is primarily driven by the biological process of respiration and the subsequent growth of microorganisms.

When grain has a high moisture content (typically over 16 percent), the rate of respiration increases. In large bulks, the heat generated by this respiration cannot be easily dissipated, causing the internal temperature of the grain to rise. This temperature increase further accelerates the respiration rate, creating a cycle that leads to damaged, hot cargo.1

Additionally, high moisture levels create a favorable environment for the development of molds and fungi. These microorganisms require a specific amount of available water to grow and reproduce; for instance, molds generally cannot thrive if the water activity is below 0.8. Moisture is also essential for the germination of fungal spores and the initiation of infection, which can lead to significant storage rots. Once these conditions are met, the grain becomes vulnerable to further degradation, including loss of weight and milling value, and may attract various pests that thrive in the heat produced by the spoilage process.123