The Crop Storage Guide
The Crop Storage Guide
Cooling Grain is a Priority
Insects, fungi and mycotoxins develop quickly in grain at high temperatures. For this reason, it is important to start cooling using a system such as Pile-Dry or FloorVent Pedestals as soon as grain comes into the store or off the drier and continue for at least 24 hours to remove high heat. This should be done irrespective of weather conditions. Even if it is raining, damp air will not affect the moisture of high temperature grain. It is almost impossible to increase the moisture when cooling grain that is still warm.
Continue cooling with automatic control to make best use of cooler air when it is available during the immediate post-harvest period. It is a common mistake to wait until cold weather comes – doing so increases the risk of condensation caused by heat in the grain meeting cold air above the surface of the grain.
The Importance of Crop Monitoring
Temperature – Safe long-term storage requires regular monitoring and control of crop temperature. Ideally, temperatures should reach 5°C to reduce the viability of disease, insects and moulds. Measuring temperature shows if the cooling system is effective and gives early warning of problems such as hot spots – a rise in temperature can indicate insect activity.
Humidity – Ambient air humidity measurement will indicate the suitability of air for drying grain. Knowing the humidity of the air ensures that drying targets are reached as quickly and energy-efficiently as possible. Damp air will not increase grain moisture, but it will reduce the efficiency of drying systems if damp air enters the grain bulk.
Insects – Low grain temperatures suppress insect breeding and prevent activity. Cooling below 15°C prevents saw-toothed grain beetles developing; below 10°C stops grain weevils; below 5°C prevents storage mite activity. Monitoring stored grain for insect activity will confirm if cooling has been effective, allow corrective action to be taken and costly rejections avoided.
Why ventilate your crop store?
Many agricultural buildings have inadequate external ventilation, or none at all. This makes crop cooling systems inefficient because warm air extracted from the grain is circulated within the building rather than being expelled. The result is poor energy efficiency and slow cooling speeds. Condensation can occur inside the building giving a higher potential for insect infestation.
Leaving the doors open can help the situation, but there is still no through-flow of fresh air. This also increases the security risk at remote sites and is non-compliant with quality assurance schemes since birds and vermin can enter the store.
A building ventilation system, such as StoreVent removes all these risks and ensures maximum energy and cooling efficiency from your crop ventilation system.
Fresh air intake louvres should be installed so that the prevailing wind can enter through them (usually in the gable end above the door). Warm air extraction fans should be at the opposite end or side of the building (not in the same wall as the louvres)
Crop Temperature Monitoring in Bulk Stores
When? Take crop temperature readings once per week until the stored grain has been cooled to 5°C (which should be achievable by December in the UK); and then every two weeks thereafter.
Where? Take one temperature reading for every 100 tons stored 3 to 5 m deep. Use an imaginary 6m x 6m grid over the grain surface and take a reading in the centre of each grid square. This ensures that readings are always from the same place and shows actual changes rather than location differences. In deeper grain, take readings using a 10m x 10m grid. If Pile-Dry or FloorVent Pedestals and Pile-Dry Fans are used to cool grain, measure the temperature at the mid-point between groups of 4 Pedestals since this is the last point to cool.
How deep? Measure crop temperature within the top 1.5-2 m of grain depth. This is where any significant change in temperature will be seen.
Risk Areas of Grain Temperature and Moisture
When storing grain for any period, it is important to be aware of the risk areas of grain temperature and moisture. As the graphic shows, high temperature or high moisture grain is at risk from insect infestation or fungal growth.
There is a safe storage zone (the hatched area on the graphic) which allows moist grain to be stored if it is cooled to about 5°C or very dry grain to be stored at temperatures up to 15°C.
Drying Grain with Ambient Air
If using ambient air for drying, this will be more effective while the grain and air are still relatively warm. The ability of air to dry grain depends on the relative humidity (RH) of the air and its temperature. Warm air can hold more moisture, so is more effective at drying.
The point at which grain can be dried by air of a specific RH is known as ‘the equilibrium relative humidity’. These points can be joined on a graph:
By measuring RH and grain moisture content a decision can be made as to whether the ambient air can dry the grain. In the example above (dotted lines on the graph), air of 77% RH will dry grain down to 17% moisture. To dry grain down to 15%, the air needs to be at 66% RH.
The basic concept to understand is that if ambient air is drier than the stored crop then it can dry it. If it is wetter than the crop, then it will not dry it.
The relationship between RH and grain moisture changes with air temperature and for different crops. The graph data here is based on an air temperature of 20°C. Available data for different temperatures is limited but the principle is that air of a specific RH will dry grain to a lower moisture if it is warmer.
Benefits of Automatic Fan Control
Most automatic fan control systems also monitor the grain temperature automatically. This provides several benefits:
Case Study: Cutting the cost of grain cooling
Grain drying and cooling can be expensive, but temperature differential control can reduce cooling energy costs by up to 40%.
In a project co-ordinated by HGCA (now part of AHDB), farms were surveyed to find out if grain cooling targets were being achieved. The results of using a Martin Lishman Temperature Differential Controller with Pile-Dry Fans were compared with operating the fans manually.
The resulting grain temperatures were similar, but the savings in electricity costs in the automatically controlled store reached 40%. The target temperature was reached more quickly, with the added benefit of reducing the risk of insect infestation.
The comparison of management methods demonstrated the importance of being aware of air temperature in relation to grain temperature. The project proved that suitable cooling air can be available at all times of day, as well as at night. Automatic control makes use of this air, at times when it may not seem obvious that the air temperature is suitable for cooling grain.
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