Equilibrium moisture properties are specific to each type of grain and are important in developing storage recommendations. Grain will dry when the relative humidity of the air that surrounds it is lower than the equilibrium relative humidity, corresponding to the moisture content of the grain. Alternatively, grain will absorb moisture from the atmosphere if the air surrounding it has a relative humidity greater than the relative humidity corresponding to moisture content of the grain.
Climates with high relative humidity make it necessary to take into account the moisture held in the ambient air when aerating grain to avoid increases in moisture in the grain mass. In these situations, it is not recommended to operate the aeration system during extended rainy periods, or periods of extremely high relative humidity.
The relative humidity of ambient air changes on a diurnal (daily) basis. Relative humidity is usually lowest at midday (the hottest part of any day) and highest at night, with the maximum usually occurring just before daybreak. However, since no cooling potential exists during most of the daylight hours, operation of the aeration system is a must at night to take advantage of lower ambient temperatures.
Thus, it becomes a juggling act for the storage operator to maintain low grain temperatures without increasing grain moisture contents beyond acceptable storage levels. These moisture contents are generally accepted to be 14 percent for shelled corn, sorghum and paddy rice, and lower for other crops such as wheat and soybeans. A strategy for operating aeration systems in subtropical climates is based on research from the International Grains Program at Kansas State University. Their recommendations are for the following aeration operations that explain in the next article.