Grains are hygroscopic in nature. Grains absorb or desorb moisture based on temperature and relative humidity of air surrounding them and continue to absorb or desorb moisture until it attains equilibrium with the surrounding air. This final moisture content of grain is called Equilibrium Moisture Content (EMC). This EMC value is different for different grains based on RH and Temperature of surrounding air.
Understanding EMC concept of grains is very important to understand how grain will behave in various temperature and relative humidity conditions. In general relative humidity of 65% or less is considered safe for storage of grains.
Storage period of grains depends upon the moisture content of grains. As a rule of thumb 9% to 13% moisture content of grains may give a storage life of 8-11 months, however there still may be some infestation problems, which have to be tackled with effective fumigation. When the grains are at 14% to 18% moisture level, the storage period is reduced to only 4-8 weeks due to problems of mold growth resulting in discoloration of grains. Below 9% moisture content Grains can be stored safe for more than one year but it may result in loss of germination.
Temperature and Relative Humidity Monitoring are the two very important aspects of Grain Management inside Silo. A rate of change of temperature more than 5.5 Deg C per week indicates that there is spoilage inside grain silo. Respiration rate of grains increases exponentially when grains start to spoil and mold / insects develop inside the silo. Increase in respiration produces additional moisture, which further deteriorates the situation. Hence continuous monitoring of Temperature and Relative Humidity inside Grain Silo is very important to keep the grains safe.
EMC of paddy under different storage conditions is replicated here for reference.
Yellow colored areas are good humidity and temperature conditions for Paddy Storage.