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Grain Storage Practices to Maintain Grain Quality

10 July 20148 min reading
Kenneth HELLEVANG, Ph.D., P.E. Agricultural Engineer - North Dakota State University Grain storage includes preparing the structure and the grain for storage, monitoring the stored grain and operating the aeration system to control grain temperature. The first step is cleaning the structure, especially if there was an insect infestation in the storage previously. Insects are frequently in aeration ducts, under perforated floors and in grain or debris in the structure. It may be necessary to fumigate the structure to assure that insect infestations do not remain. The structure should be treated with a residual bin treatment to discourage insects from entering the structure. Do not forget to clean the exterior perimeter of the structure. Preparing the grain for storage includes bringing the grain to the proper moisture content, temperature and grain condition. The storage life of the grain due to microbial growth is dependent on both the grain temperature and moisture content. If the grain can be kept cool, it can be stored at higher moisture contents. For example, cereal grains at 18 percent moisture have an estimated Allowable Storage Time (AST) of 200 days at 40 degrees Fahrenheit but only 30 days at 70 degrees. The grain will show a measurable quality loss at the end of the AST so the grain should be used before the end of the AST. The estimated allowable storage time (AST) of grain is available in tables or charts. Allowable storage time charts for cereal grains, soybeans and malting barley are at https://www.ag.ndsu.edu/graindrying/publications-grain-drying-and-storage. These tables estimate the length of time the grain can be stored, but should only be used as a guideline and the grain must be monitored. AST is cumulative. If half the storage life is used at a warm grain temperature and then cooled, the AST at the cooler temperature is only one-half of the value in the table. The optimum temperature for both microbial growth (spoilage) and insect infestations is about 80 degrees Fahrenheit, so if possible limit the time that grain is at that temperature. If the grain will be stored at a warm temperature, it needs to be dry enough to limit mold growth. Generally the air relative humidity level between the grain kernels must be under about 70 percent to limit mold growth. The grain equilibrium moisture content (EMC) associated with a warm air temperature and relative humidity of 60 to 65 percent provides the recommended maximum grain moisture content required for storage at warm temperatures. Equilibrium moisture content tables and equations are available from a variety of sources. EMC charts are available at https://www.ag.ndsu.edu/graindrying/publications-grain-drying-and-storage. The EMC of hard red spring wheat at 70 degrees Fahrenheit and 60 percent relative humidity is about 13.3 percent and the recommended storage moisture content is 13.0 to 13.5 percent. The EMC for corn is about 12.8 percent and the normal summer storage moisture content is 13 to 14 percent. The recommended moisture content to maintain germination of seed or grain such as malting barley may require a moisture content that is lower than needed based on the EMC. The EMC of barley at 70 degrees Fahrenheit and 60 percent relative humidity is 11.8 percent. The recommended warm storage is 12 percent moisture content. The estimated allowable storage time of 12 percent moisture malting barley based on maintaining germination is about 60 weeks at 70 degrees Fahrenheit and 25 weeks at 80 degrees Fahrenheit. The allowable storage time of 13 percent moisture barley at 80 degrees Fahrenheit is only about 16 weeks. Grain must be at or drier than the maximum recommended moisture content to maintain the germination at warm temperatures. Because grain moisture content is so important to safely store grain, it is important to assure that the moisture measurement is accurate. It is important to assure that the sample of grain being measured provides an accurate representation of the larger quantity of grain. Grain moisture content is not even across a field or within a truck. Grain segregates based on size and density as it flows into any container, so the method of sampling must collect a representative sample. It is also critical to know the operating range of the moisture meter and to follow the recommended procedure. Some meters are not accurate on cold or hot grain, some do not adjust for grain temperature, and a moisture gradient within the kernel may cause an error. If the sample is not at the meter standard temperature or if the grain has a moisture gradient due to drying or rewetting, the sample should be placed in a sealed container for several hours to permit the grain to warm to the standard temperature and for any moisture gradient to equalize within the kernel before the moisture content is measured. The goal is to store grain at cool temperatures to extend the allowable storage time. Grain is an excellent insulator, so the majority of the grain will stay warm or cool unless air is moved through the grain. Stored grain should be cooled during the fall as outdoor air temperature cools. Run aeration fans when the average (daily maximum + minimum / 2) outdoor air temperature is 10 to 15 degrees Fahrenheit cooler than the grain temperature. Cool the grain to about freezing or near the average minimum winter temperature for winter storage. In the spring, warm the grain to about 40 degrees Fahrenheit. The goal during spring and summer is to keep the grain cool to enhance the AST and limit insect activity. Cover the fans and ducts when the fans are not operating. A natural chimney effect will draw in outside air that will warm the grain with warm outside air unless the fan is covered. Also wind blowing into uncovered fans and ducts will ventilate stored grain. If the air is warm and humid, the air will warm and add moisture to the grain. At temperatures below about 50 degrees Fahrenheit insects are dormant and at temperatures near or below freezing insects can be killed if grain is stored for an extended period. Insect death occurs more rapidly at colder temperatures, so insects can be controlled by regulating grain temperature if the climate permits. Solar heat gain on the sunny side of the bin and on the bin roof can warm the stored grain. The space above the grain should be ventilated similar to the attic of a building with air entering near the eave and exiting near the peak. Alternately, an exhaust fan mounted on the roof with adequate air intake vents can cool the space under the roof. Grain warming is limited to within a couple feet of the bin wall but may extend several feet into the grain at the top due to air currents. This makes keeping the top of the bin as cool as possible important. Stored grain should have a level top surface to minimize the amount of grain warmed at the top of the bin. Peaked grain has more exposed surface area than a level surface. The number of hours required to cool a bin of grain can be estimated by dividing the number 15 by the airflow rate in cubic feet per minute per bushel. The number of hours needs to be adjusted by the ratio of the test weights of the grain being cooled and that of wheat or soybeans. The cooling time to cool 48 pounds per bushel barley using an airflow rate of 0.2 cubic feet per minute per bushel would be about 60 hours (15/0.2 X 48/60). Outdoor temperatures will vary during the aeration cycle, but outdoor temperature is the primary factor when determining when to operate the fan. Typically the grain loses a little moisture during cooling, so generally the air relative humidity during aeration is not a concern as long as the fan is only operated long enough to cool the grain. Do not operate the fan when it is foggy, raining or snowing if possible. It is critical to monitor the stored grain. Measure the grain temperature and moisture content and check for insect infestations. Temperature sensors in the grain are recommended to assist with monitoring the grain temperature. However because grain is an excellent insulator, only the grain temperature next to the sensor is being measured. Place the temperature sensor cables in grain storage where problems are expected such as the sunny side of the bin, in the middle of the bin and near the top of the grain. The monitors do not replace checking the grain to look for indicators of problems. Check the stored grain at least every two weeks until the grain has been cooled for winter storage, every 2 to 3 weeks during winter when temperatures are below freezing, and at least every two weeks during spring and summer. There are safety hazards when working around or with grain. Assure that all personnel are aware of the hazards such as grain entrapment and respiratory hazards of exposure to mold or fungi in grain and grain dust and proper safety practices.
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