Keeping grain cool

by Emily Wilson
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Keeping grain cold is a stored commodity management tool that has been researched and tested since the 1960s. Fewer storage problems are reported in colder seasons and colder climates.

Research has shown that controlling the temperature and relative humidity favorably impacts grain storage conditions. Grain chilling is now a common technique to preserve quality and protect against insects, and is widely used in Europe and Asia.

Dirk Maier, a professor in the Department of Agricultural and Biological Engineering at Purdue University, West Lafayette, Indiana, U.S., has been working with temperature control of stored grains for about 13 years. "Grain temperature is the major stored-grain management tool that regulates insects and molds," he said.

Temperature in grain storage facilities is commonly controlled by aerating grain with ambient air, said Jan Hellemar, vice-president of sales for PM-Luft, a manufacturer of grain chillers based in Kvanum, Sweden. However, the ambient conditions must be right, he said.

"For safe storage conditions, you need grain temperature below 15°C (60°F) and that is hard during the summer months," Hellemar said. "Extensive use of ambient air will result in over-dried grain, meaning loss of weight and money, because the relative humidity of the air cannot be controlled. As opposed to aeration, a grain chiller controls temperature and relative humidity regardless of ambient conditions."

Several factors affect grain in storage, including handling, moisture, bacteria, mold and fungi , insects and natural respiration of the grain. Chilling grain can control each of these damaging factors.

"Ripe grain continues to live after the harvest — it breathes," said Heinrich Brunner, a recently retired salesman for Sulzer Escher Wyss, Lindau, Germany, which also manufactures grain chilling machines.

As heat increases, he explained, so does the respiration of grain, resulting in the loss of grain weight.

Frieder Barth, Sulzer's international sales director, further explains the effects of respiration. "The insulation properties of grain prevent the migration of heat from the inside of the grain bulk to the outside," he said. "The temperature keeps rising due to respiratory activity of the grain, creating a paradise for insects and mold fungi."

After a storage bin is cooled to approximately 10° to 18° C (55°F to 65°F), insect activity ceases, as does growth of mold and fungi, and fumigation becomes unnecessary, Brunner said. "The insects fall into a state of hibernation and are no longer active," he said.

"Originally, grain cooling was developed for buffer storage of moist, freshly harvested grain waiting to go through the dryer. Nowadays, more dry than wet grain is being cooled, just for protection against insects."

Cooling the grain mass also avoids having to turn the grain to keep it cool, Hellemar added. This reduces grain dust and prevents broken kernels.

"If a chiller is being used, the grain stays in the same bin until it is time to ship it out," he said.

Drying costs and times are also reduced as the grain begins to dry while cooling down. "For each 10°C temperature reduction, expect a .5% to .75% reduction in moisture content," Hellemar said. "Cooling will increase drying capacity by 50% minimum, and cuts drying costs by more than 50%."

In addition, cooling equalizes the moisture content from kernel to kernel. "The longer the cooling period, the greater the equalization," he said. "This reduces the opportunity for molds and fungi to form and degrade grain quality."

COOL RESULTS. Grain cooling technology also has been known to increase head yield and husking capacity for rice and reduce cracking problems in corn and rice while reducing the need for fumigants, thus increasing worker safety.

Lundberg Family Farms in Richvale, California, U.S., has used a grain chiller on its 1.78 million bushels of rice storage for more than five years, and plans to buy an additional cooler within the next year. Jim Stewart, manager of Lundberg's drying and storage division, said the chilling unit has greatly reduced fumigation costs and improved rice quality.

Lundberg Farms was spending U.S.$75,000 to U.S.$100,000 for carbon dioxide to fumigate its organic rice each year, Stewart said. Now, the company spends less than U.S.$15,000 a year for fumigation.

Although the chiller works extremely well to keep the rice fresh and insect-free, warm patches can form near the steel silo walls and these areas need to be fumigated, he noted.

Stewart said the grain chiller also has improved rice quality. "We found there was equalized moisture content and better head rice (also known as the whole rice kernel)," he said. "You're paid based on head quality."

The ease of milling increased, as did yields. "The millers don't have to make as many adjustments," Stewart added. "They can set the machines and go."

HOW IT WORKS. Purdue's Maier describes how the grain chilling process works. "In a grain chilling system, ambient air is ducted over a bank of refrigeration coils in order to decrease its temperature," he said. "In this process, the relative humidity of the air is increased. The chilled air is reheated slightly to match the equilibrium relative humidity of the stored grain."

The best time to begin the cooling process on a grain bin is right after filling or, if possible, simultaneously to the time of filling so that grain will not begin respiration in the bin.

The cooler blows air through the duct system into the bottom of the silo. The cold air moves up through the silo gradually. When the cooled air meets the warm grain, a temperature decline begins. At the same time, the moisture content of the cooled air increases and a certain amount of drying takes place. Once the top layer has reached the required temperature, the whole silo is chilled and you can move to the next silo.

Both Sulzer's and PM-Luft's grain chilling units are portable and are easily moved from silo to silo.

Installing a grain chilling system in a storage facility is relatively simple, manufacturers say. Most facilities already have the necessary components in place.

"A good duct system or aeration system in the bottom of the silo or flat storage is needed," said Hellemar. "Most silos already have this as well as openings on the top of the silo where warm air can escape."

An exhaust fan is sometimes necessary, added Barth. "An exhaust fan only has to be installed when moist grain during cooler ambient conditions has to be cooled, such as during the corn harvest in autumn," he said. "An exhaust fan will be advantageous, but only necessary during morning operation or during big temperature differences between day and night."

The cooling time depends on the size of the cooling unit and the amount of stored grain, but elevated silos and warehouses of any size and type can be cooled.

Barth recommends cooling the grain temperature to approximately 10° to 18°C, depending on the location and type of grain. After that, the cooling unit can be turned off and moved to another bin for cooling, he said.

It is important to close the air inlet pipe to the bin, Barth explained, otherwise warm ambient air gets onto the already cold grain and causes damage.

Because grain is a good insulator, once it is chilled, it will stay cool for several months even during summer periods, Hellemar said. However, it is necessary to periodically rechill the grain.

"Re-chilling is a quick process as it will only reduce the temperature a few degrees compared to the larger initial temperature reduction," Hellemar said. Recooling in necessary after a temperature increase of 3° to 5°C (5° to 9°F).

The time before recooling depends on the moisture content (m.c.), silo size, and ambient conditions. In tropical areas, Barth suggests the following intervals: every six to eight months for 12% to 15% m.c.; three to five months for 15% to 17% m.c.; one to two months for 17% to 19% m.c.; and every 15 to 25 days for 19% to 21% m.c.

COLD CASH SAVINGS. Proponents of grain chillers say the technology can save a facility a substantial amount of money.

Approximately 10% of all grain stored in the United States is lost to insects, according to Hellemar. "That's something like half a billion U.S. dollars in lost value," he added. "How much is lost to other temperature-related storage problems, no one dares to talk about. However, it is a fact that every elevator has to deal with a certain amount of shrinkage. Shrinkage is everything from grain dust, breathing losses, over-drying or too low moisture content, moulds, fungi, and rodents."

In developing countries, it is not uncommon to have up to 40% grain losses after harvest, Hellemar said. "If the temperature could always be kept on a safe level, related losses would be reduced by 85% to 90%," he said.

After the initial capital costs of a grain chiller, manufacturers say the machines.

"Experiences gained over the past 25 years show that if cooling is applied in the proper manner, the total investment costs for cooling, including the required air distribution system, are paid back in one to two years time," Barth said.

Sulzer started researching grain chilling in 1965 and put its first units into operation in 1967. Barth estimates that 9,000 units have been installed worldwide and that 90 million tonnes of grain are chilled each season.

PM-Luft has been manufacturing coolers for about 25 years. Hellemar said their customers see a quick return on investment costs.

A major reason for the quick pay off, he said, is that a cooling unit requires less energy than the traditional aeration process, which is run almost entirely throughout the grain's storage. Instead of paying to dry, continuously aerate, turn and fumigate grain, chilling grain requires only periodically running the cooling unit.

Brunner said the amount of energy needed for cooling depends on various factors, such as the moisture content and temperature of the product. "Moist grain is easier to cool than very dry grain," he said. "Other factors are the ambient air temperature and the relative humidity."

He estimates that it costs approximately 3 to 6 kWh per tonne of grain in areas with temperate climates, and approximately 8 to 12 kWh per tonne in extreme tropical conditions.

PM-Luft estimates that it costs about (U.S.) one-half of a cent per bushel to chill grain.

Although cooling benefits all grain storage, the pay back period is usually shorter for corn and rice, Hellemar said.

"Corn and rice are very sensitive," he said. "If a dry corn or rice kernel is exposed to air that is forcing the kernel to re-absorb humidity, fissures will appear resulting in cracked/broken kernels. A broken kernel often has only half of the value compared to a whole kernel."

In addition, corn has a high fat and oil content, causing it to heat up quickly.

The Asian market in particular uses grain chillers for rice because of the high humidity in that climate, Hellemar said. "The consumers there care a great deal about taste," he said. "Due to the fatty acids in rice, there are changes in the taste over long storage unless you cool the rice."

The organic foods industry also is interested in grain chilling because chemicals cannot be used for insect control. "That makes temperature control extremely important," Hellemar said. "Europe, for instance, is planning for 15% to 20% of the total grain production to be organic within 15 years from now."



Estimated costs: traditional handling vs. grain chilling

Traditional handling:

Drying, from 17% to 15%


Aeration, 5.5 cents/bushel


Moisture, 14% after aeration, 1.5% shrinkage


Turning, three times at 1 cent/bu each


Fumigation, one time, 5 cents/bu


Total cost


Grain cooler:

Cooling, .5 cents/bu at 5.5 cents/kWh


Shrinkage, drying shrinkage 1.5%






Total cost


Total savings using grain cooler


Source: PM-Luft. All figures in U.S. dollars.