Corn Drying
The three general types of grain dryers used on the farm are:
- in-bin
- batch
- continuous flow
No single drying system is superior. Grain dryer selection is dependent on desired features, including drying capacity, grain quality, fuel/drying efficiency (BTUs per volume of water removed), convenience, manpower required to run the dryer, ability to dry a variety of crops, maintenance required and capital cost.
All dryers move “dry” air past the grain to evaporate moisture within the kernel and carry the water vapour away. Heat is added to this drying air to reduce its relative humidity, thereby increasing its ability to pick up moisture. Wet grain can be dried at higher temperatures, without damaging the corn, because the corn is cooled as the moisture evaporates from the kernels. As the grain dries, it will approach the temperature of the drying air. The longer grain kernels are in contact with this heated air, the drier and hotter the kernels will get.
Corn dries as the moisture from the inside of the kernels is evaporated from the kernel surface. Most of the moisture inside the kernel exits through the tip end of the kernels. The first few points of moisture can be easily removed using relatively little energy. Further moisture must be removed from deep within the corn kernels. As the outside layers of the kernel dry, the moisture must migrate out from the moist centre. This moisture does not move to the surface as quickly as it is being evaporated from the surface of the kernel by the drying air. This results in higher energy requirements to remove the last few percentage points of moisture.
Drying Temperatures
A range of drying temperatures can be used to dry corn but should not exceed the maximum suggested air temperatures in Table 1, Maximum suggested air temperatures for drying corn of various end uses. The maximum recommended drying temperature depends on several factors, including the final end use of the grain, initial moisture content of the grain, type of grain and type of dryer.
Table 1. Maximum Suggested Air Temperatures for Drying Corn of Various End Uses
| End Use | Maximum Drying Temperature (°C) |
| Seed corn | 45 |
| Starch milling | 70 |
| Industrial uses, non-ruminant feed | 90 |
| Cattle feed | 120 |
Viability is destroyed when the actual grain temperature exceeds approximately 50°C. Reduction in nutritional value occurs when grain temperature reaches 90°C–100°C.
Kernel Quality
Stress cracking can be reduced by taking corn hot out of the dryer, allowing it to steep and then aerating the corn with a minimum of 6.5 L/sec/m3 (0.5 CFM/bu) airflow. Both stress cracking and physical kernel damage are influenced by the speed of moisture removal and maximum kernel temperature, coupled with the rate of cooling after drying.
In addition to maintaining grain quality, using this system of dry-aeration or cool-aeration can increase the throughput of the drying system. Many farmers in Ontario practice “cool-aeration,” where corn is removed hot from the drier, transferred to a storage bin and cooled slowly. In this way, hot corn is continuously being added to the top of the final storage bin and slowly cooled.
Natural-Air Drying
Natural-air drying of corn is possible in most parts of southern Ontario. This method of drying corn is well suited for livestock operations to produce high-quality corn that is free of stress cracks. Good management of a natural-air drying system is critical to success.
Minimum Requirements for Natural-Air Drying:
- full aeration floor in the bin
- level grain surface across the whole bin
- minimum airflow of 26 L/sec/m3 (2 CFM/bu), preferably more
- corn 25% moisture content or less
- clean corn with no cob pieces or fines
- accurate moisture reading of the corn in the bin
- accurate outside air temperature and relative humidity measurement
- an understanding of corn equilibrium moisture content
- coring the bin (auger out some grain) after filling. The best way is to remove a couple of loads from the bin. This establishes the flow funnel and removes the highest concentration of fines from the centre of the bin. Clean these loads before placing them back into the bin. Even if the loads are put right back in the bin without cleaning, the resistance to airflow will be less than if the bin had not been cored.
- an on/off switch for the fan
When to Run the Fan
Fan operation in a natural-air corn-drying bin is slightly different than for other air-dried crops.
- Once there is sufficient corn in the bin to hold the perforated floor down, the fan can be turned on.
- Run the fan continuously for the first 3 weeks after the bin has been filled or until the first drying front has come through the top of the bin.
- The first drying front emergence will be evident when there is a noticeable drop in the moisture content of the corn at the top of the bin.
- Before this drying front passes through, the corn at the top of the bin will remain at harvest moisture levels and may even increase slightly compared with the corn drying further down.
- If the fan is shut off for an extended period of time at the start of the drying process, there is a risk that the drying front may stall and will not move upwards once the fan is turned on again. This will result in spoilage occurring above the drying front.
- Once the first drying front passes through the top of the bin, begin to manage the fan operation, using the equilibrium moisture chart for corn, see Table 2. Equilibrium moisture content.
- Run the fan any time the outside conditions will still allow the wettest corn in the bin to dry. At times, this procedure may add some moisture to the corn at the bottom of the bin. This temporary rewetting of the bottom corn will actually dehumidify the air so it can do more drying up higher in the bin.
[text box] Rain or shine, the fan should not be turned off until the first drying front has passed through the whole bin.
The corn may not reach the desired moisture content before freezing weather arrives. Trying to accomplish natural-air drying in below-freezing temperatures is very slow and inefficient.
The last few points of moisture may have to be taken out in early spring. Some livestock producers never finish drying the corn any further after winter, as it processes and stores well as feed at the higher moisture levels.
Table 2. Equilibrium Moisture Content for Corn Exposed to Air
| Temperature °C | Relative Humidity (% Wet Basis) | ||||
| 50% | 60% | 70% | 80% | 90% | |
| 0 | 13.7 | 15.1 | 16.6 | 18.4 | 21.3 |
| 5 | 13.1 | 14.4 | 15.9 | 17.8 | 20.7 |
| 10 | 12.5 | 13.8 | 15.4 | 17.3 | 20.2 |
| 15 | 11.9 | 13.3 | 14.9 | 16.8 | 19.8 |
| 20 | 11.5 | 12.8 | 14.4 | 16.4 | 19.4 |
| 25 | 11.0 | 12.4 | 14.0 | 16.0 | 19.0 |
Humidistats are available that will activate the fan at preset humidity levels. The operator can adjust and set the relative humidity level at which the fan is activated. Bins with stirrators will have fairly uniform moisture levels throughout the whole bin as a result of the mixing that has occurred.
Corn at moisture levels greater than 25% can also be dried in a natural-air bin. This is accomplished by only partially filling the natural-air bin, resulting in an airflow of 52–78 L/sec/m3 (4–6 CFM/bu). Producers who need corn for feed in late September can harvest headlands and put this in the bin. The warm temperatures in late September, combined with higher CFM/bu airflow enable this corn to be dried in a couple of weeks.
Equilibrium Moisture Content
Researchers have developed equilibrium moisture content tables that predict the final moisture content of corn when exposed to air at a certain temperature and relative humidity, see Table 2, Equilibrium moisture content for corn exposed to air. For example, to determine the equilibrium moisture content of corn exposed to outside air at 10°C and 70% relative humidity, find the point at which the 10°C line and the 70% relative humidity line intersect. This point (15.4%) will be the equilibrium moisture content.