Drying Wheat to Prevent Spoilage and Sprouting
May 01, 1997
by Teresa Acklin
Following a few quick tips can assure satisfactory results.
Harvesting wet wheat can boost test weight, prevent field sprouting and increase yields on double-cropped fields. But grain handlers who accept wet wheat need to pay close attention to some specific handling and drying skills to prevent spoilage and loss of quality.
Unheated air-drying is limited to grain moisture of 16% to 18% in a bin equipped with a drying fan and fully perforated floor. If stirrers are available, grain with a maximum moisture of 20% may be dried without adding heat.
If drying with unheated air, the fan should be turned on as soon as the bin floor is covered, and it should be run continuously day and night, rain or shine, until the grain moisture in the upper layers is less than 15%. Tables 1 and 2 on page 55 summarize the equilibrium moisture content (ECM) of soft and hard wheat at various relative humidity and temperature combinations; the ECM is the grain moisture content at which no gain or loss of moisture will occur at specified ambient air temperature and humidity levels.
Note that the higher the temperature and the lower the relative humidity, the drier the wheat will become. Soft wheat will reach lower moistures than hard wheat given the same air conditions.
Once the moisture content drops below 15%, the ambient air loses its drying potential, and the fan should operate only when conditions are favorable. To reach the desirable moisture content of 13% to 14%, fan operation between 8 a.m. and 10 p.m. usually succeeds.
Heat is required for wheat moistures above 20%. Drying should be accomplished fairly rapidly to reduce the risk of mold development and subsequent spoilage. To be safe, a 60- to 90-centimeter deep batch in a bin should be dried with about 1 to 5 cubic meters per minute per tonne to 14% in 24 hours. No more than 2°C to 11°C heat should be added.
High-temperature batch or continuous flow dryers also will work well for wheat. Because of the high airflow rates in these dryers (50 to 125 cubic meters per minute per tonne), supplemental heat may not be necessary.
If heat is used, the air temperature should be throttled by firing the burner for only short time intervals, or by firing it continuously at a reduced fire rate (for example by changing the gas burner orifice).
In a bin drying system, the difference between moving air through maize versus wheat is significant. For the same airflow per tonne, wheat can be filled only 60% as deep as maize. Thus, 3 meters of wheat is equivalent to nearly 5 meters of maize in terms of resistance to airflow.
As a rule of thumb, a bin filled with 3 meters of wheat will require one horsepower of fan capacity per 28 tonnes of grain to deliver 1 cubic meter air for every tonne of wheat. Thus, a 9-meter diameter bin filled to 3 meters with 152 tonnes of wheat would require a 5.5 HP fan.
However, this relationship does not hold above 3 meters of depth. Doubling the depth of wheat to 6 meters cuts the airflow by 70% to 0.3 cubic meter per minute per tonne.
This article is based on a grain quality fact sheet prepared by Dirk E. Maier, agricultural engineering department, Purdue University, West Lafayette, Indiana, U.S.Table 1 Equilibrium moisture content of soft wheat at different drying air temperatures and relative humidities
Table 2 Equilibrium moisture content of hard wheat at different drying air temperatures and relative humidities
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