Both the moisture content of the grain, and the relative humidity of the surrounding air, affect microbial growth and spoilage. Relative humidity of 100 percent indicates that the air contains all the water it can normally hold at that temperature, whereas a relative humidity of zero percent indicates that there is no water in the air—in other words, the air is completely dry.
Grain will attempt to establish equilibrium moisture content with the surrounding air. Because grain is hygroscopic, it will exchange moisture with the surrounding air until the vapor pressure of the moisture in the grain and that of the air reach a state of equilibrium. If grain comes to equilibrium, with air maintained at a relatively constant environmental condition, the grain moisture content is referred to as the equilibrium moisture content (EMC) corresponding to the existing air conditions. Grain stored outdoors in constant contact with atmospheric conditions will reach its equilibrium moisture content with the surroundings very quickly.
On the other hand, if the grain is surrounded by a relatively limited amount of air (such as occurs in the interstitial space of a grain mass in a storage silo), the air will reach moisture equilibrium with the grain without any significant change in the grain moisture content. The relative humidity of the air, in this situation, is referred to as the equilibrium relative humidity (ERH) corresponding to the existing grain moisture content at the prevailing temperature. All equilibrium moisture properties are a function of temperature; that is, the properties change with changes in temperature.