Chilling: a quality control alternative
October 01, 1993
by Teresa Acklin
This article is adapted from a presentation by Jan Hellemar, vice-president for PM-Luft AB, Kvanum, Sweden, to the Grain Elevator and Processing Society's 1993 annual conference. What is a grain cooler and how does it work?
A grain cooler is like a big air conditioning unit with an air fan large enough in capacity to blow air through the tallest silo.
A grain cooler is used in the same way as an aeration system, but with the major difference that grain can be cooled to a safe storage temperature at any time, regardless of ambient conditions.
A grain cooler can help avoid respiration losses, can reduce the need to use chemicals to protect grain and can prevent the build-up of molds and fungi.
Using artificially cooled air for chilling grain is not a new concept. The first experiments started more than 30 years ago, both in the U.S. and Europe.
Several problems were involved in the first tests, and the major one appeared to have been the failure to control relative humidity. But in the late 1970s, the technology reached a point where the equipment was reliable, and the concept became more popular.
Today, grain cooling with artificially cooled air is widely used in Europe. In the mid 1980s, the use of grain coolers started to spread through Asia, Africa and South America.
In Europe, grain coolers are used to reduce drying costs, to increase drying capacity and to allow storage at trading moisture level without storage problems and losses. In Asia and Africa, the main concern is temperature reduction.
Grain coolers may be used only in bulk storage facilities, either upright silos or flat storage buildings. In bulk facilities, the goal is to create a climate that will maintain grain quality and will avoid or at least minimize storage losses.
Because grain is a living material, it creates its own problems. It breathes or respires throughout the storage period, and breathing increases heat. Fungus and insects compound that increase.
Diagram 1 shows the importance of temperature in the determination of storage life. Temperature is, in fact, more critical than moisture content.
It is well-accepted that allowances must be made for shrinkage when storing a living product such as cereal, and shrinkage sometimes can get out of control. One reason, frequently overlooked, is respiratory or breathing losses.
Diagram 2 shows the breathing losses in maize of different moisture contents at various temperature levels. At 17% moisture content, the monthly weight loss at 32°C is nearly 1.5%; but when the same maize is stored at a temperature of 12°C, the loss is only about 0.25%.
This means if 10,000 tonnes of maize with 17% moisture content is stored at 32°C, breathing losses would total about 150 tonnes per month. Compare this result with the 25 tonnes that would be lost at the lower temperature.
INSECT CONTROL AND HEALTH CONCERNS.
Sooner or later, most of the world's cereal grain production ends up as food, either directly, as in bread and breakfast cereals, or indirectly through livestock feed.
Today, we try to prepare our food in more healthy ways. Organically grown produce is commanding higher prices and gaining significant market share. Why does this not apply to cereal?
Ten years ago, insecticide sales in the U.S. alone totaled more than U.S.$6 billion, and today's figure probably is much higher. Another interesting figure is that every year, insects cause more than $1 billion worth of damage to stored grain.
The consuming public in general is unaware of how we treat our cereal grains. But they are becoming more interested and informed, which may force change unless we move to clean up our act.
Grain-eating insects will not complete their life cycles fast enough to create any real problems if the grain is cooled to about 17°C or below. To prevent a build-up of insects, it is necessary to reduce temperatures to safe levels immediately after receipt of the grain. We don't have time to wait for favorable ambient conditions because in some areas they never arrive.
Grain cooled to a safe storage temperature will hold that temperature through long storage periods, even though ambient conditions are highly unfavorable.
Another advantage of a grain cooler is that it can prevent the growth of many types of fungi and molds. Once these develop, germination capacity is lost, and insect problems probably have developed.
The heat of respiration from fungi and other storage micro-organisms increases grain temperature and produces a consistent increase in the organisms' growth. The use of a grain cooler can prevent this cycle, as cooling can be done just after harvest, before infestation, regardless of ambient conditions.
GRAIN DRYING AND COOLING.
Wet grain must be dried. To dry grain from a high moisture level down to 20% is easy and does not cost a lot. But grain drying starts to become energy and time consuming when moisture levels are reduced below 20%.
Higher temperatures are used to hasten this process, which increases fuel consumption and the risk of quality loss. High dryer temperatures mean high grain temperatures, which extend the cooling time and contribute to storage problems later.
A grain cooler can be used in conjunction with a grain dryer, or sometimes can replace a dryer.
High-moisture grain, if chilled to a low temperature, can be stored safely while waiting for the dryer. Depending on the required final moisture content, cooling alone is sometimes adequate.
The dryer can be stopped at higher moisture and temperature levels, followed by immediate cooling. All or part of the cooling normally done in the dryer can be omitted, provided grain is cooled adequately in the storage bin as soon as possible.
Use of a grain cooler as described above will increase dryer capacity and will extend the drying season to allow for larger volumes to be handled safely. Energy consumption also will be reduced.
The cooling process produces a certain, very predictable drying effect. The higher the grain's temperature and moisture content, the greater the evaporative drying.
For example, if your grain is to have a final moisture content of 15%, stop the dryer at 16% to 16.5%. Cooling to 13° from 38° will produce an additional moisture reduction of 1% to 1.5%.
AERATION AND COOLING.
A common way of cooling grain is to use fans moving ambient air. This is a good system, providing Mother Nature is cooperative and you have the fan capacity to do the job when ambient conditions are right.
To obtain a good result from aeration, you must have a good duct system through which chilled air can be blown; a good duct system is one that will deliver adequate volumes of chilled air.
When blowing cold air through a mass of grain, a cold zone is established. This moves up through the grain mass until it reaches the top, cooling the entire stock.
With aeration, a new zone is created every time the temperature changes, extending the time needed for the fans to adequately cool the grain. The more you run your fans, the more you dry your grain. This can explain many shrinkage problems.
Aeration also does not account for relative humidity. All grains have their own equilibrium between moisture content and the relative humidity of the cooling air flow. If the relative humidity is too low, excessive drying results. If the relative humidity is too high, remoistening occurs.
A grain cooler is equipped with a computer system, in which all common grains and their equilibriums are recorded. The operator selects the type and moisture content of the grain to be cooled, and the computer calculates the correct temperature.
After the operator sets the proper temperature and starts the equipment, the cooler will operate at the set temperature and relative humidity, regardless of changes in ambient conditions.
TURNING GRAIN AND COOLING.
Another common method to control heat is to turn the grain. If no other alternative is available, turning creates some temporary reductions in temperature.
The costs for electricity, workpower and equipment are easy to calculate, but there are other costs to be considered.
One silo must be kept open to have turning ability. Each time grain is turned, dust is created, a problem in itself as well as adding to shrinkage. Some breakage also will occur, adding to overall shrinkage.
If a grain cooler is used, the grain is cooled in the silo, where it remains until shipment.
A grain cooler of today must be able to cool ambient air, reduce the relative humidity of the cooled air, blow air through the grain and keep the temperature and relative humidity on the same level throughout the cooling process regardless of ambient conditions. In addition, the air blown from the cooler must be clean.
Grain is a very good insulator, and once a low temperature is reached, this temperature will be maintained for long periods of time.
On average, grain with 15% moisture content in a concrete silo and cooled to 9° to 12° can be stored for six to seven months under summer conditions before rechilling is required. In a steel silo, the same grain can be stored for four to five months before rechilling.
One common concern when talking about grain cooling is condensation.
Most of the cooling using a grain cooler is done at or soon after harvest. At this time of year, the ambient temperatures are not very low, and grain temperatures are high. Under such conditions, the condensation that does occur will be on the outside of the silo.
If cooling takes place when ambient temperatures are low and grain temperatures are high, the risk of condensation at the roof is high. This requires the use of a roof exhauster, which will exhaust air and will draw air in and over the roof to eliminate condensation.
Another concern might be condensation forming on the kernel when loading out cool grain on a hot day. But experience proves this does not occur.
Grain coolers can eliminate or reduce the costs of turning, fumigating and drying grain. In addition, shrinkage and dust are reduced, safety is increased and bins do not need to be kept empty for turning.
Because of these savings, grain coolers most of the time have paid for themselves within two to three years, depending on the type of grain, the location, energy costs, etc. Grain coolers have an expected life of at least 15 years.