Drying and Cooling Grain: An Update (HGCA Topic Sheet No. 78)

Drying and Cooling Grain: An Update

TS No.78


  • If grain is wet, consider hot-air drying or stirring to reduce time grain is above 18% mc.
  • Cool grain using airflow rate of around 10 m3/hr/t. Either measure airflow rate or obtain data from manufacturer.
  • Record fan hours run. Remember: cooling dose = airflow x hours run.
  • Control fans, ideally using a differential thermostat, especially after warm, dry harvests.
  • If cooling grain with bulk drying fans, run fans for proportionately shorter time as fans blow up to 20 times more air than cooling fans.
  • Consider ways of voiding hot air from any building containing fans. Include adequate louvered vents or link roof fans to cooling/drying fans.

If you are unsure about any of the suggested actions, or want them interpreted for your local conditions, consult a professional agronomist.

Bulk drying

For long-term storage grain should be dried to 14.5% or less.

Ambient-air drying systems should deliver 180 m3/hr/t in order to pass a drying front through grain in ten days. Undried grain, ahead of the drying front, can deteriorate if its moisture content remains too high for too long.

Germination loss has hitherto been the best indicator of biodeterioration. More serious is the possible formation of ochratoxin A (OTA), a carcinogen that may be produced above 18% mc by Penicillium verrucosum.

Bulk dried grain will be around 15-20C due to evaporative cooling.There is a danger that OTA can form at 20-22% mc even if drying is complete in ten days (Figure 1). Action to reduce mc below 18% mc before bulk drying should be considered, especially if harvest backlogs lead to damp grain being stored.

Cooling with bulk dryer

Often the same fans are used for both cooling and drying. However, cooling requires only one twentieth of the airflow compared with drying.

A bulk drying system will take only five hours to achieve the first cooling front of 15-20C after harvest, whereas a low volume aeration system may take up to 100 hours. However, heat from the fans motor limits cooling efficiency. Excessive fan use wastes electricity but grain is unlikely to dampen at airflow rates used for cooling.

A float meter at the grain surface measures airflow for bulk drying, but is too insensitive for cooling rates.

Cooling by aeration

Low volume aeration systems should be used at an aeration rate of about 10 m3/hr/t. This airflow can be calculated from makers performance charts.

Measuring fan hours

It may take up to 300 hours to drop grain temperature below 5C, so it is essential to record fan running time to monitor cooling efficiency. Inefficient cooling, despite many hours of fan use,may be due to incorrect wiring, air leaks, or blowing when air is too warm.

If fans are switched manually, record when fans were switched on and off each day and the total hours. A cheap wired-in hours meter can monitor hours run for one or many fans.

Effects of climate change

In 2004, 15% of grain offered for sale was rejected due to infestation.

The autumn weather of 2004 was considered by many to be too warm to cool grain rapidly enough to prevent infestation.

The past decade has been warmer than the 30-year average (Figure 2), but the day-night average is still below 20C, low enough for the first cooling front to stop the saw-toothed grain beetle from breeding.The real problem is selecting the  most suitable periods for cooling.

Fan control using a thermostat

Grain cools most rapidly if only the coldest air is blown. Grain temperature follows ambient if cooling fans run  continuously. A simple thermostat wired to the fans can be set at 15-20C until the first cooling front is through in around 100 hours; then the setting is dropped to 10C, and finally to 5C.

Use of a differential thermostat is more efficient.This only switches the fan on when ambient is cooler than grain, and would typically be set at a 6C difference. Airflow rates used for cooling will not usually dampen grain, so blowing in damp weather is not a problem.


Previous HGCA publications have outlined the principles of successful grain storage: dry to below 14.5% mc; cool to below 15C within two weeks and to below 5-10C by Christmas.

This Topic Sheet amplifies these rules in the light of feedback from recent HGCA workshops on cooling and drying as  well as difficulties many farmers had to cool grain after the hot, dry 2003 harvest. Key messages are: know your airflow, record fan hours, use a thermostat. Bulk drying fans used for cooling must run for much shorter times than dedicated cooling fans.

Further information:

David Armitage, CSL

Tel. 01904 462026

Topic Sheet Nos. 7, 16, 34, 53

The grain storage guide, revised 2003


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The Home-Grown Cereals Authority (HGCA) has provided funding for this project but has not conducted the research or written this report.While the authors have worked on the best information available to them, neither the HGCA nor the authors shall in any event be liable for any loss, damage or injury howsoever suffered directly or indirectly in relation to the report or the research on which it is based.

Reference herein to trade names and proprietary products without stating that they are protected does not imply they may be regarded as unprotected and thus free for general use. No endorsement of named products is intended, nor is any criticism implied of other alternative, but unnamed products.

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