Monitoring grain temperature and aerating grain

It is important to monitor grain temperature and to keep stored grain cool and dry by regularly aerating it through mechanical aeration or by turning it. This section explains how monitoring and aeration work together to both prevent and control insect infestations.

Moisture and warm temperatures are the main factors that cause grain to deteriorate. They allow the growth of moulds and the development of an environment that is very suitable for insect development.

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 may develop and cause hot spots and condensation. The greater the temperature differential (the difference between the temperature of the grain and the ambient temperature) the stronger the convection current. 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 pile forms a peak.

This diagram shows 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 pile forms a peak.
• Grain at the surface and just below the surface has a high moisture content.
• Warm grain is located in the centre of the grain pile.

Arrows represent the convection current.

• 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 pile 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 pile.
• As the cold air is drawn to the centre of the grain, it warms up and flows up to the surface of the grain pile where there is moist grain.
• The warm air is cooled as it reaches the surface, condenses, and the cycle is repeated.

Moisture content of grain at time of storage

Consider the moisture content of the grain at the time of storage. If the moisture content is 13 percent or higher and the grain remains warm for a long time, it can be damaged by fungal activity. Fungi may subsequently produce mycotoxins. However, when grain with a high moisture content is cooled to below +10°C, it can be stored for months without moulds forming and causing it to deteriorate.

How to monitor temperature

Check the temperature of the bin every two 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.

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. Under very warm conditions, 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.

Other aeration 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 two to four weeks until the grain temperature reaches +15ºC. This procedure involves removing about one-third of the grain from the bin and putting it back in the bin.