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Fall 2019 Wheat Drying (09/12/19)

Sprouted wheat due to excessive moisture leads to low falling numbers.

Sprouted wheat due to excessive moisture leads to low falling numbers. The falling number is a test that measures starch damage in wheat that reduces the quality of baked goods and noodles. Weather conditions play a huge role in preharvest sprouting of wheat and other cereal grains. Periods of cooler, wet weather at the stage when the crop is mature and starting to dry down are ideal for sprouting to occur. Limiting the time mature wheat is exposed to wet field conditions by harvesting and drying reduces the potential for low falling numbers.

Adding supplemental heat when natural-air drying wheat generally is not needed, even during September, for most of North Dakota, but it may be needed if wet conditions continue.  Adding heat reduces the air relative humidity and the final grain moisture content, so it should be used when outdoor air relative humidity is too wet to dry wheat to the desired moisture content. However, adding too much heat causes wheat in the bottom of the bin to dry to a lower than the desired moisture content. Adding heat increases drying speed very little.

Air will be warmed 4 to 5 degrees as it passes through the fan on a bin of wheat when the fan is operating at a static pressure of about 6 inches. Warming air by 5 degrees reduces the relative humidity about 10 percentage points. Warming air that is at 60 degrees with a 70% relative humidity by 5 degrees reduces the relative humidity to about 60%. This air will dry wheat to about 13.5% moisture content with just fan heat. A supplemental heater is not needed if the average (daytime + nighttime / 2) outdoor air relative humidity is less than 70%.

If the average relative humidity exceeds 70%, then a little supplemental heat is needed. Even if the average relative humidity is 75%, the air will need to be heated only 2 or 3 degrees. For example, if 60-degree air has a relative humidity of 75%, warming the air 3 degrees in addition to the 4 degrees from the fan reduces the relative humidity to 59% and permits drying wheat to about 13.5% moisture. The approximate heater size needed for wheat is about 1.0 kW per fan horsepower or 3,413 Btu per fan horsepower.

Only running the fan during the warmer and drier portion of the day lengthens the drying time. The estimated drying time during September is about 35 days using an airflow rate of 0.75 cubic feet per minute (cfm) per bushel with the fan operating 24 hours per day, and 62 days when the fan is operated just during the warmer 12-hour portion of the day. Running the fan 24 hours a day, and adding supplemental heat if necessary, permits drying to the desired moisture content faster and is the preferred option.

Turn off the fans if the weather is foggy or rain is falling. Wheat up to 16% moisture can be without airflow for a few days, but wheat at 18% moisture should not be without airflow for more than a day or two due to the potential for heating and spoilage.

The drying rate is directly proportional to the airflow rate. If drying 16% moisture content wheat using an airflow rate of 1 cfm per bushel takes 21 days, it will take 28 days with an airflow rate of 0.75 cfm per bushel and 42 days at 0.50 cfm per bushel. The airflow rate must be increased to increase the drying speed. Adding heat slightly increases drying speed, but primarily reduces the final grain moisture content. The maximum recommended moisture content for natural-air drying wheat is 18% which requires a minimum airflow rate of 1 cubic feet per minute per bushel (cfm/bu.), 17% requires 0.75 cfm/bu. and 16% requires at least an airflow rate of 0.5 cfm/bu. to complete drying before significant deterioration occurs. However, the drying time is extremely long when using an airflow rate of 0.50 cfm/bu.

The recommended minimum airflow rate is 0.75 cfm/bu. and the maximum recommended initial moisture content is 17%. The maximum recommended wheat depth for drying is about 18 feet. A very rough rule of thumb is that 1 hp of fan is needed for each 1,000 bu. of wheat to obtain an airflow rate of 0.75 cfm/bu. Also, a centrifugal fan is going to be required. The NDSU Grain Drying and Storage website ( has a link to a fan selection program. With the program, you can determine the fan size needed to obtain the desired airflow or estimate the airflow provided by an existing fan.

High-temperature drying is recommended if the wheat moisture content exceeds 17%. However, use caution because high temperatures affect the chemical structure and milling quality of the grain. High temperatures can damage baking quality severely even though the grain kernels appear undamaged. Allowable dryer temperatures will vary with dryer type and design. Refer to the manufacturer’s recommendation for guidance. However, a general recommended maximum drying air temperature for milling wheat in a cross-flow dryer where some of the wheat approaches the drying air temperature is 150 degrees for 16% moisture content and 130 degrees for 20% moisture content wheat. Frequently, a plenum air temperature about 30 degrees warmer is used in dryers where the kernel temperature remains below plenum temperature and the wheat kernel is not damaged.



Kenneth Hellevang

Extension Agricultural Engineer & Professor

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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