Approaching winter weather allows for cooling of grain in most of the United States, Central Europe and China, and pretty much all of Canada and Northern Europe to well below 5°C (41°F). In the tropics and subtropics, grain temperatures typically remain between 25-35°C (77-95°F), which are ideal growing conditions for stored grain insect pests. Grain needs to be at sufficiently low moisture content to be safe from mold spoilage, which requires drying after harvest but remains a challenge during storage due to high relative humidity of ambient air.
So this is as good a time as any to review a stored grain preservation technology that allows for cooling of grain year-round independent of high ambient temperature and relative humidity conditions.
Ambient versus chilled aeration
Aeration of grain with ambient air after harvest is an important tool to maintain quality and minimize post-harvest losses due to biological deterioration caused by insect pests and mold spoilage. However, to be effective, ambient temperatures of less than 20°C (68°F) and relative humidity of less than 70% are required. Such conditions are usually not available during the summer harvest season of wheat and other small grains in temperate climates and during most of the year in subtropical and tropical climates when rice, maize, wheat and other staples are harvested. This increases the dependence of stored grain managers on chemicals, specifically fumigants and insecticides, to suppress and prevent insect pests.
An alternative, non-chemical technology is the aeration of stored grain with air that is chilled by large capacity air conditioning systems. So called grain chillers are typically mobile and may be moved between storage structures to cool grain below about 15°C (60°F) soon after harvest independent of local weather conditions.
Grain chillers function similarly as air conditioners but are high capacity and portable (see Figure 1). They may be connected to storage structures such as silos and warehouses via existing aeration fans and operate on three-phase power. Once cooling of a silo or warehouse is complete, a grain chiller can be moved to the next structure, or depending on size and chiller capacity, one grain chiller may be connected to more than one silo or warehouse section at a time.
Grains such as wheat and rice are excellent insulators, so once cooled a grain mass would typically not need to be rechilled. For example, wheat harvested in the Great Plains of the United States could be chilled in July to 15-17°C (59-63°F) before ambient aeration in the late fall can be used to lower grain temperatures further for storage through the winter and beyond. Paddy rice in Malaysia could be chilled to 20-22°C (68-72°F) soon after harvest and remain cool for four to six months before mill capacity catches up.
Chilled aeration is not a new technology but commercial equipment has not been widely available in North America until recently. The technology was first introduced in the United States in the early 1960s and then again in the late 1980s with the first commercial unit sold for the cooling of milled rice in California in 1993. Most units sold in the United States are used for the conditioning of bulk seed and the cooling of pesticide-free rice, wheat, food maize as well as organic grains, oilseeds and pulses, which has been documented to be economically feasible.
In the past 20 years only one company (Tornum, a Swedish company selling the PM Luft Grain Cooler) has distributed grain chillers in the United States. Recently, a German company (FrigorTec) has introduced its Grainfrigor grain chillers to the United States with rice producers and processors as the initial target market. In the fall of 2014, researchers at Kansas State University received a Brazilian-made grain chiller from the company Coolseed, which has been utilized to investigate the feasibility of chilling wheat during the summers of 2015 and 2016 with partial funding from the Kansas Crop Improvement Association.
At Iowa State University, we are utilizing a 3D ecosystem model that was developed over a number of years by my research group with the help of several Ph.D. students. The system allows us to investigate the technical feasibility and economic viability of ambient versus chilled aeration for the preservation of stored grain quality for weather conditions in any geographic location.
Benefits of chilled grain aeration
Lower grain temperature and moisture content in stored grains, whether cooled with ambient or chilled aeration, have several benefits. They reduce dry matter loss that is caused by seed respiration of individual grain kernels, which converts starch (carbohydrates) plus oxygen into carbon dioxide plus water and heat. The higher the grain moisture content, the greater the dry matter loss and the more heat and water are generated in the stored grain mass. Fortunately, below 13% in starchy grains (wheat, rice, maize) and below 8% in oilseeds (soybeans, sunflower, canola) seed respiration is minimal. Therefore, drying of grain to safe storage moisture contents and subsequent cooling is critical for preserving quality and maximizing allowable storage time.
Grain chilling also is used to complement the drying process either by holding harvest-wet grain in a hopper-bottom silo as a buffer ahead of the dryer, or to remove additional moisture from hot grain through evaporative cooling after transfer out of the dryer, which can increase dryer capacity by 20% to 30% and improve grain quality by minimizing stress cracks in maize and fissures in rice.
Recent grain chilling research
Recently completed research at Kansas State University utilizing a Coolseed grain chiller documented that temperatures were lowered in a 1,350-tonne wheat silo from approximately 28°C (82°F) in mid-August to a minimum of 17°C (63°F) in 175 hours. Chilled wheat remained below 20°C (68°F) through early fall and had substantially less insect activity than wheat stored in a second 1,350-tonne wheat silo that remained above 25°C (77°F) while aerated with ambient air through the end of September.
Recent applications in California with maize, Arkansas with paddy rice, soybeans and maize, Kansas with wheat and maize, Pennsylvania with organic popcorn, New York state with buckwheat, and wheat in Mexico utilizing Grainfrigor grain chillers reflect increasing commercial interest in the technology in North America.
The benefits of grain chilling versus ambient aeration for paddy rice stored under tropical climate conditions of Costa Rica also have been investigated using computer simulation. After six months of storage in Costa Rica, ambient aeration was not able to reduce average stored paddy rice temperatures by more than 4°C (7.2°F), i.e., from 35°C (95°F) to 31°C (88°F). In comparison, grain chilling was predicted to lower the average paddy temperature to below 15°C (60°F) in less than 120 hours which would result in effective insect suppression.
The grain chilling technology requires a substantial investment into a physical asset that has to pay off through real and perceived benefits. Previous research has determined that the operational costs of grain chilling is typically lower than the cost of ambient aeration plus at least one fumigation treatment. However, given the initial cost of purchasing a grain chiller, the net present cost (NPC) over 10 years is higher than the cost of ambient aeration plus one fumigation treatment. If multiple fumigations per storage season are required (as is typical in tropical and subtropical climates) and the real threat of insect resistance to phosphine is considered, incorporating the grain chilling technology into an overall quality management and food safety strategy generates long-term benefit. It also allows for cooling of grain year-round independent of high ambient temperature and relative humidity conditions.