Grain quality cannot be improved during storage. Grain improperly harvested and dried will remain of low quality no matter how well it is stored.
The main objective of proper grain storage is to maintain the quality that existed immediately after harvesting and drying.
In cereal grain, a loss in quality and quantity during storage is caused by fungi, insects, rodents and mites. Respiration may, in certain cases, contribute to a loss of dry matter during grain storage, but those losses are minor compared to those caused by living organisms.
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Fungi, or moulds, are the major cause of spoilage in grain. Losses caused by fungi in cereal grain are related to a decrease in germination, discolouration of the seed, heating and mustiness, biochemical changes, the possible production of toxins and loss in dry matter. All these changes may occur without the mould becoming visible to the naked eye.
The two groups of mould that affect grain quality are field mould and storage mould.
Field moulds invade kernels while the grain is still in the field. They cause the discolouration often observed in plants exposed to moist weather before harvest. In addition to affecting grain appearance, field mould may reduce seed germination.
Field mould damage is completed by the time the grain is harvested and there is relatively little a producer can do about it. Once the grain is dried, these moulds die or become inactive.
Storage mould is prevalent in storage facilities when the grain moisture content is too low for field moulds: usually less than about 20 percent.
The moisture and temperature requirements of these moulds determine the safe storage period.
By controlling moisture content and temperature, mould growth is restricted and grain can be dried without significant spoilage. Grain temperature and moisture content determine the allowable storage time, which is the length of time grain can be kept before it spoils.
For long-term storage, grain should be dried as soon as possible after it comes from the field. More dried grain goes out of condition because of uncontrolled grain temperatures than for any other reason.
Improper temperature control inside the bin causes moisture to migrate from one part of the grain mass to another, where the moisture can accumulate and cause spoilage.
Although moisture migration problems can occur whenever grain temperatures vary considerably, the most critical time occurs when warm grain is stored in cold winter temperatures. This is especially true for large bins.
In the fall, when the air temperature cools down, the grain along the bin wall cools more quickly than the rest of the grain.
The difference in temperature starts air moving down the wall and toward the centre of the bin.
As the air moves through the grain it becomes warmer and begins to pick up moisture from the grain. When the warm, moist air hits the cool upper surface of the grain, condensation occurs.
In the spring, the problem is reversed. Heat from the sun on the outside of the bin causes moisture currents to move up and into the bin. Condensation then occurs on the bottom of the bin.
Modern grain management uses aeration to control grain temperature and reduce moisture variations. Aeration forces air through the grain. It is not a grain drying system and should not be used as one.
Grain drying or rewetting is usually insignificant during grain aeration because the cooling or warming front moves through the grain about 50 times faster than a drying or wetting front.
Grain can be tempered (cooled or warmed) by either negative or positive aeration systems. With either system, a cooling or warming zone moves through the grain. The movement of the tempering zone completely through the grain is one cooling or warming cycle.
Once a cycle has been started, operate the fan continuously until the zone moves completely through the grain. The time required to complete each cycle depends almost entirely on the aeration airflow rate.
In a positive pressure system the tempering zone starts at the bottom of the bin and moves up. When air is moved upward, aeration progress can be easily determined by checking the grain temperature at the top centre. Also, with an upward airflow, the fan can be started immediately and air leaving the duct will keep the perforations clean.
Airflow rates for aeration are normally one to two litres of air per second per cubic metre of grain. The time for one cooling or warming cycle to completely pass through grain can be estimated by the formula below.
Also, producers should strive to ensure that the following conditions are met during aeration:
- For best results, the bin floor should be at least 15 percent perforated.
- Grain put into the bin should be within one percent of dry.
- Producers should screen the grain to reduce foreign material and fines because they increase resistance to airflow and reduce total air flow.
- Attempt to level the top of the grain to promote uniform airflow through the grain mass.
- Operate the fan long enough to complete a cycle, either cooling or warming.
In the fall, grain should be cooled as quickly as possible. Start aerating as soon as the grain is in the bin, check the grain temperature and turn off the fan when the grain temperature is less than 5 C above the outside temperature.
Check the grain periodically for condensation or heating. If the grain will be kept over winter, turn the fan on again when the outside temperature drops and aerate until the grain temperature is less than 5 C above the outside temperature. Continue these cooling stages until the entire volume of grain is close to 0 C.
In winter, after turning the fan off, cover the opening with metal, canvas or plywood to keep out rodents. When the outside temperature is close to that of the grain, run the fan for a day or two.
In spring, if the grain is being held over the summer, a series of warming stages is recommended beginning in April. Continue until the grain temperature is about 10 C.
By June the grain temperature should be 10 C. Check the grain periodically and run the fan during cool, fair weather when the outside temperature is lower than the grain temperature.
If there are signs of heating or hot spots, no matter what the season or the weather, run the fan continuously until no heating can be detected.
Most dry grain will form a peak at an angle of 16 to 20 degrees when a bin is centre filled without a distributor.
Although it is tempting to store those extra bushels, keep in mind they interfere with uniform aeration and add to the moisture migration problem.
Peaking also makes it difficult and dangerous to enter the bin for observation. Because of dust and high temperatures during the summer, never enter the small space between roof and grain. Shifting grain may block the exit.
If the grain has peaked when filling the bins at harvest, remove the grain in the peak immediately for long-term storage. Lowering the centre cone of the bin improves airflow through the centre. Probing and sampling are also made easier and safer. Some fines will also be removed.
Producers should consider installing temperature-sensing units in large grain storage bins.
Temperature sensors accurately trace the progress of aeration cooling or warming cycles and help identify hot spots within the grain mass.
