It is important to monitor grain temperature and to keep stored grain cool and dry by regular aeration or by turning it.
High moisture and warm temperatures in grain allow for the rapid growth of insects, fungi and the possible production of mycotoxins.
How temperature and moisture work
Grain is a very good insulator. When it is undisturbed, it holds temperature well. If warm grain is placed into storage and left undisturbed, convection currents develop and cause hot spots and moisture condensation. The greater the temperature differential (the difference between the temperature of the grain and the outside temperature), the stronger the convection currents. The stronger the convection, the greater the effect of heating and condensation on the grain. This is particularly evident when stored grain is not leveled and the grain bulk forms a peak.
The cycle of convection currents in bin-stored grain when ambient air outside the bin is cold and the grain is warm.
In the bin:
- The surface of the grain bulk forms a peak.
- Grain at the surface and just below the surface has high moisture content.
- Warm grain is located in the centre of the grain bulk.
Arrows represent the convection currents.
- Cold air flows down from the surface of the grain, along the interior of the bin wall. The flow of cold air surrounds the warm grain.
- At the bottom of the grain bulk the cold air is drawn to the centre of the grain by an upward flow of warmer air. The upward flow is a convection current created at the centre of the grain bulk.
- As the cold air is drawn to the centre of the grain, it warms up and flows up to the surface of the grain bulk where there is moist grain.
- The warm air is cooled as it reaches the surface, condenses, and the cycle is repeated.
Determining the moisture content of grain
Grain with a moisture above grade requirements can create an environment conducive to insect and fungal growth and development if it is not managed. Grain left unmanaged may increase in temperature and subsequent convection currents can cause surface condensation.
To determine the moisture content of grain at storage, take samples from bins every 3 to 4 weeks after storage if grain is not managed (no aeration or turning). Samples should be taken from several areas of the bulk and be kept in sealed plastic containers prior to testing.
Check the temperature of the bin every 2 weeks. Measure temperature by using temperature sensing cables that are permanently installed or by probing the grain with an electronic sensor device.
If devices for measuring temperature are not available:
- Assess the general temperature by inserting a metal rod into the grain at the top of the pile near the centre. The rod should reach at least one metre into the grain.
- Leave the rod for approximately 30 minutes.
- Remove the rod and, with the palm of the hand, test it for warmth at various points from the tip. Any section of the rod that feels warm to the touch is an indication of heating and grain spoilage.
How aeration systems work
Aeration systems preserve stored grain and keep it dry by reducing the temperature of grain and reducing moisture migration. Appropriate aeration can prevent convection currents and condensation from occurring.
It is important to consider the physical characteristics of grain when considering aeration and drying. Factors such as grain class and storage configuration impact the static pressure and thus the aeration fan requirements. In general, as static pressure increases (e.g. increased height of the storage or change in the class of commodity in storage), the amount of time required to properly aerate also changes. Please consider consulting with your aeration system provider to determine aeration or drying times for the bin type or commodity that you have in store.
The movement of cooling and drying fronts through crops ventilated with air during autumn.
How to use aeration systems
Stored grains should be aerated as soon as possible after harvest, particularly if aeration can reduce the bulk temperature below 18°C. When the ambient temperature falls below that of the grain bulk, initially during the early evening, night, and early morning, you can use aeration to reduce the temperature of the grain.
If you aerate grain when the ambient temperature is above 20°C and the temperature of the grain is above 30°C, the odors produced will be more attractive to insect pests. Sanitation involving cleaning and treating grain is very important in preventing problems.
As well as preventing insect infestations, aeration is also very effective in controlling them. Once the grain temperature is reduced to below 18°C (which prevents insect feeding and reproduction) a further temperature reduction can be used to cause mortality. Refer to the disinfestation time periods required at low temperatures table in Physical control of grain insect pests.
Other cooling methods
If an aeration system is not available, turning grain outside the bin is an alternative to aerating it in the bin when the ambient air temperature falls below 15°C. Turn the grain every 2 to 4 weeks until the grain temperature reaches 15°C. This procedure involves removing about one-third of the grain from the bin and putting it back in the bin.
Grain chillers can be used to reduce the temperature of the grain when outside temperatures are above 20°C. Grain chillers reduce the temperature and moisture of air used to aerate the grain. Once grain is chilled it remains cool for extended periods, due to its natural insulating properties.