Managing Grain in Storage

The goal of postharvest grain drying, handling, and storage operations is to preserve the harvest quality of the grain and to add value by removing impurities and identifying and segregating lots with special characteristics when appropriate. For agricultural products, quality loss may occur due to poor drying techniques, improper handling, or lack of proper storage environments resulting in deterioration from cracking, splitting, mold growth, insect damage, sprouting, loss of germination, or dry matter loss from respiration. Large grains such as corn — especially when dried at high temperatures — are particularly susceptible to physical damage during handling. Physical damage also makes grain more susceptible to invasion by storage fungi and insects.

All postharvest operations attempt to maintain the initial quality of the harvested grain. During storage, grain must be protected from deterioration or attack by molds, insects, rodents, and birds. Drying and handling operations must prevent physical or chemical deterioration. The physical protection provided by modern grain storage structures should eliminate serious bird and rodent damage. Molds and insects cannot be physically excluded from grain with current storage designs. They can be controlled through grain temperature and moisture management.

Retain Grain Quality While in Storage

Some grain will be stored for many months or even more than a year due to low grain prices. Maintaining grain quality during this extended storage will require extra care and management.

Grain that will be stored for an extended time needs to be good quality grain.

The outer layer of a grain kernel is the pericarp, or seed coat, and provides protection to the kernel. If the pericarp is damaged, the kernel is more susceptible to mould growth and insect infestations. This reduces the expected storage life of the grain. Broken kernels and foreign material should be removed by cleaning the grain before storing it. Segregation based on size and density occurs as grain flows into storage. Fines accumulate in the middle unless a functioning distributor spreads them throughout the grain. Unloading some grain from the center of the bin will remove some of the fines and help level the grain in the bin. Also, immature kernels have a much shorter expected storage life. Grain test weight may be an indicator of maturity and storability.

Assure that the storage facility is clean, and insects are not living in aeration ducts, under perforated floors, or in handling equipment or debris around the facility. Fumigate the empty bin to kill insects under the floor or in aeration ducts if an infestation occurred during the previous year. Also, consider applying an approved residual bin spray and a grain protectant to repel potential insect infestations if storing grain during warmer portions of the year.

Manage Moisture Levels

Mould growth requires moist conditions, usually above about 70% relative humidity, and warm temperatures. To reduce the potential for mold growth, the grain moisture content should be below the equilibrium moisture content (EMC), at 60% to 65% relative humidity.

The EMC of maize at 21 degrees Celsius and 65% relative humidity is about 13.5%, and at 10˚C, the EMC is about 15%. If you can keep stored maize below 10˚C, you can store it at 15% moisture. But if the temperature will be warmer, then the recommended storage moisture content is about 13.5%.

Grain going into long-term storage should be dried and cooled rapidly after harvest. The allowable storage time (AST) is an estimate of the life of the grain until it has deteriorated enough to affect grain quality.

Manage Temperature In Storage

It is important to monitor grain temperature and to keep stored grain cool and dry by regular aeration or by turning it. High moisture and warm temperatures in grain allow for the rapid growth of insects, fungi and the possible production of mycotoxins.

Grain stored at lower temperature has less risk of losing quality:

• Lower temperatures allow moist grain to be stored safely for longer periods.
• Even temperature of the grain mass in the silo prevents moisture migration.
• Hotspots are prevented from developing.
• Mould growth is slowed, and insect development is reduced dramatically.
• Cool stored seed retains its viability and vigour for a longer period.

Grain is a very good insulator. When it is undisturbed, it holds temperature well. If warm grain is placed into storage and left undisturbed, convection currents develop and cause hot spots and moisture condensation. The greater the temperature differential (the difference between the temperature of the grain and the outside temperature), the stronger the convection currents. The stronger the convection, the greater the effect of heating and condensation on the grain. This is particularly evident when stored grain is not leveled and the grain bulk forms a peak.

The cycle of convection currents in bin-stored grain when ambient air outside the bin is cold and the grain is warm.

In the bin:

• The surface of the grain bulk forms a peak.
• Grain at the surface and just below the surface has high moisture content.
• Warm grain is located in the centre of the grain bulk.

Arrows in the image represent the convection currents:

• Cold air flows down from the surface of the grain, along the interior of the bin wall. The flow of cold air surrounds the warm grain.
• At the bottom of the grain bulk the cold air is drawn to the centre of the grain by an upward flow of warmer air. The upward flow is a convection current created at the centre of the grain bulk.
• As the cold air is drawn to the centre of the grain, it warms up and flows up to the surface of the grain bulk where there is moist grain.
• The warm air is cooled as it reaches the surface, condenses, and the cycle is repeated.

Check the temperature of the bin every two weeks. Measure temperature by using temperature sensing cables that are permanently installed or by probing the grain with an electronic sensor device:

• Check the grain at least every two weeks until it has been cooled for storage and every two to four weeks during storage.
• Verify that the moisture content is at the recommended storage level.
• Check the grain temperature.
• Inspect for insects.
• Look for indications of storage problems such as condensation on the roof.

Aeration of Grain in Storage

Aeration is the stored grain management technique of forcing cool air at low airflow rates through the grain mass in order to equalize the grain temperature and in the process effect cooling.

Before aeration is commenced, the properties of air in respect to temperature and relative humidity must be conducive so that no wetting of grain occurs.

In this respect there are automatic aeration devices if desired:

• The top silo vents, hatches and manhole covers should be opened to discharge the air out of the bins.
• During aeration exhaust air and grain surface are monitored to determine when cooling is complete.
• Aeration must be carefully controlled to prevent wetting of the grain and introduction of insect pests.
• Plant operator maintains record of the activity on the prescribed format.

Grain Turning

Grain turning or recirculation is the movement of grain from one bin to another bin - preferably empty. This may be an emergency measure necessitated by rapid increase in temperature of say 20˚C indicated by the temperature monitoring system or programmed operational procedure during which grain is moved from one bin to another, say once every six months.

• The essence of grain turning is to discover the cause of temperature rise if any as well as to monitor and visually monitor and ascertain the quality of the grain.
• During turning the grain is inspected, samples are drawn for quality assessment, the grain is cleaned, cooled, blended and fumigated as it moves to the receiving bin.
• Grain turning must be undertaken carefully to minimise expenses resulting from damage due to grain breakages and other indirect costs.
• The silo operator completes the grain turning forms and updates the bin/stock cards accordingly.

Click here to view a video that explains grain stirring and drying.

Drying of Grain

Those involved with the management, operation, and design of drying systems need to understand the principles of drying and how a particular situation may dictate the desired final moisture content for storage of grain and selection of a drying method.

Drying is usually the most economical choice for successful storage of grain and seed products, especially in the long-term. Economic considerations that influence the decision about the type of drying and storage system to purchase include the following:

• Opportunity for earlier harvest, which reduces the potential for weather- and pest-related field losses while maintaining the quality and quantity of harvested grain and seeds.
• Reducing the net price penalty (dockage) from the sale of high-moisture grain and seeds.
• Increasing options regarding when, where, and for what purpose the grain may be sold or used for feed.
• Risk associated with processing and maintenance of grain between harvest and time of sale are reduced.

In most grain drying systems, ambient air is heated and passed through grain so that a relatively high vapor pressure gradient is produced between the moisture in the grain and the moisture in the drying air. This differential causes moisture to move from the grain to the air that is flowing past the kernel where it then exits from the grain mass to the outside atmosphere. In the most simplistic drying situation, the grain and the air that surrounds it are in equilibrium before the introduction of heated air, and the properties and rate of flow of the heated air entering the grain mass remain constant during the drying period. In such a situation, the heated air transfers its heat to the grain and creates a new equilibrium moisture content based on the new differential vapor pressure. The drying air begins absorbing moisture from the first kernels that the air contacts. This process continues until the drying air, falling in temperature and increasing in relative humidity, can no longer add additional moisture because it is in equilibrium with the remaining grain mass. Simultaneously, the transfer of moisture from the grain to the drying air becomes increasingly more difficult as the grain dries, so much so that stress cracks can occur if the grain is dried too quickly or is overdried. Effectively, the above process produces a drying front and a drying zone. All grain behind the trailing edge of the drying zone would be in a new, stable equilibrium moisture condition with the heated air. The grain ahead of the drying zone would remain essentially in its initial equilibrium moisture condition. The grain in the drying zone would range from its highest moisture content at the start (leading edge) of the drying zone to being driest at the end (trailing edge) of the drying zone. The simplistic drying situation described above seldom happens for very long. In practice, the grain being dried varies during the harvest period (gradually losing moisture in the field during harvest) in temperature and moisture content and may contain differing levels of fines and trash that influence the distribution of airflow. Heat is lost or gained in the grain mass because of changes in ambient air conditions surrounding the mass. Properties of the ambient and heated air also change during the drying process as daily weather changes occur.

Given all of this, there are some generalizations that should be considered when evaluating the drying process:

• If there is sufficient variation in the temperature and relative humidity of the drying air relative to the grain mass, some zones in the grain mass may experience heating, cooling, drying or rewetting relative to other zones.
• Safe storage conditions are reached when all the grain has been dried to a safe equilibrium moisture content (65 to 70% ERH or lower), either by passing the drying front completely through the gain mass or by thoroughly mixing the grain so that the overdried and under-dried grain can equilibrate to a safe storage moisture condition in an acceptable amount of time.
• Drying is most efficient when the drying air has come into full temperature and moisture equilibrium with the grain as it passes through the grain mass.

Pest Control

Application of insecticide and fumigant is done at the time the grain is being loaded into the storage bins either at reception of new crop or during inter-bin transfer. The recommended dosage must be used all the time and the safety regulation always adhered to. Trained personnel should carry out application of fumigants and pesticides.

• Fumigation is the disinfestation using a fumigant i.e. a pesticide that kills in gaseous form.
• Before fumigation is carried out all silo base, sidewall and roof openings must be sealed tightly otherwise the process will fail.
• Application of the fumigant is normally done by automatic pellet dispensers or manually dropped into the bin or conveyor.
• After loading the silo bin with grain, the grain surface may be top-dressed with a dust pesticide for insect control.
• In bulk storage like silos, the problem of rodents is limited to damaging equipment – especially electrical cables. To reduce the incidence of rodents, ensure good hygiene and a clean environment. Sweep clean all surrounding areas such as reception hopper, pits and tunnels and burn sweepings. Keep drains clean and surrounding grasses and bushes cut, chemical and biological control should also be considered.

Click here to view a video that explains the fumigation of grain bags.

Bin Inspection

Once grain has been stored in the bins, regular but frequent inspections at intervals of say once every week or two weeks must be contacted to monitor the grain quality, signs of deterioration or infestation.

• Inspect top of grain mass if accessible, around aeration ducts and vents, at manholes, unload conveyors for signs of live insects or webbing.
• Signs of crawling and flying insects on the top of the grain pits may be an indication of damage already done within the grain pile.
• During the inspection check for signs of rain or water leakage and the integrity of the structure in general.