Preservation of stored grain by aeration

12 December 201920 min reading

“Spontaneous heating of moist grain can be prevented by cooling the grain mass using aeration. The release of or desorption of fumigants at the end of a fumigation can be achieved with relatively low air flow rates. Refrigerated air aeration can be used when ambient air may not be sufficient to protect grain against mites and insects, control self-heating of grain, or preserve the germination capacity and quality of stored grain in warm climates. A recent development is the use of wireless sensors for monitoring temperature and relative humidity to enable the technical personnel assess the effective use of aeration.”

Shlomo Navarro, Green Storage Ltd. Hagit Navarro, Green Storage Ltd.

INTRODUCTION An acceptable practice to reduce the commodity temperature is to use mechanical aeration by means of fans. It can be easily applied to stored grain or granular commodities. Aeration may be defined as “the forced movement of ambient air of suitable quality or of suitably conditioned air through a grain bulk for improvement of grain storability” (Calderon, 1972). Aeration is also called “active,” “mechanical,” “low volume,” or “forced” ventilation, since fan power is used to deliver the air. Aeration should be distinguished from “passive” or “natural” ventilation, which takes place in grain bins with an open manhole, or in granaries with open doors or windows, and is due to natural or convective air currents. Passive aeration takes place also in corn cribs, used traditionally in tropical and subtropical climates. Wind forces ambient air to flow through corn (maize) cribs, causing slow drying of damp unshelled corn and other grains.

Aeration is a widely used method for the preservation of stored grain. It is used to modify the grain bulk microclimate and to render it unfavorable for the development of damaging organisms in the grain, and at the same time to create favorable conditions for the sustained preservation of grain quality. Within the concept of the storage ecosystem, the role of aeration is to modify one or more of the abiotic factors (temperature, humidity, atmospheric composition) and thereby to “condition” the stored grain to improve existing conditions in the grain bulk by moving air of suitable quality through the grain mass.

Forced aeration is an effectively applied method in commercial-scale bulk storage of grain and takes advantage of two important physical properties of the grain bulk: 1. Porosity of the grain bulk: For most cereal grain, the intergranular void volume is 35–55% of the grain bulk volume. The porous nature of bulk grain permits forced air to pass through and come into contact with almost all grain kernels in the bulk. 2. Thermal insulation property of the grain bulk: Due to low thermal conductivity, the grain mass is self-insulating. This enables maintenance of a “modified microclimate” long after the grain bulk has been aerated.

Although the role of temperature has long been recognized, manipulation of this regulation by aeration techniques was first brought into focus in the early 1950’s in stored grain management programs in the United States. Pioneering engineering work of U.S researchers such as Foster (1953), Robinson et al. (1951), Shedd (1953), and Holman (1966) and research on technological aspects of aeration by Hukill (1953), and more recently by Cuperus et al. (1986), Arthur and Casada (2005, 2010), and Reed (2006), form the basis of modern grain aeration systems. Aeration technology is used to modify the grain bulk microclimate to reduce or eliminate the development of harmful or damaging organisms in the grain by reducing and maintaining grain temperatures at safe levels below humidity levels which support microflora activity. Aeration helps sustain favorable storage conditions for the safe preservation of grain quality. Since then, several authors have reported their findings on aeration carried out in temperate climates. The knowledge accumulated over the last four decades has formed the basis for the present-day aeration technology reviewed by Navarro et al,, (2012). At present, forced aeration of grain is one of the most effective sustainable, non-chemical method in use for the control of stored grain conditions, biological activity, and grain quality losses.

OBJECTIVES OF AERATION The objective of aeration is to maintain the quality of bulk grain in storage. Although aeration can improve storage conditions, aeration does not improve intrinsic quality attributes of grain.

COOLING THE GRAIN BULK FOR PEST SUPPRESSION Cooling grain is the primary objective of grain aeration (Reed and Arthur, 2000, Reed and Harner, 1998) when discussing pest suppression. Stored grain insects are of tropical or subtropical origin and require fairly high temperatures, typically 24° to 32°C for development. Grain infesting insects are sensitive to low temperatures. Stored product insect development is generally stopped below 16°C; there is little insect survival above 43°C. In the southwestern and south-central U.S., temperatures of wheat, rice, and sorghum at harvest can range from 32° to 43°C, depending on the specific crop and location. During fall harvest in the northern U.S., grain temperatures around 10° to 18°C are typical.

At temperatures below 21°C, population growth of most storage insects is significantly suppressed. Grain temperatures of 16° to 21°C are considered “safe” for insect management, because feeding and breeding are slow. Complete life cycles at these temperatures take three months or more, so insect population growth remains insignificant. Insect damage caused under these low temperature conditions is minimal (Flinn et al., 1997; Stejskal et al., 2019).

The crucial control parameter for mite pests is not only temperature, but establishing an equilibrium relative humidity (ERH) below about 65%. About 12.5% moisture content (MC) for wheat at 25°C suppresses mite development (Cunnington, 1984, Navarro et al., 2002). Temperatures required to suppress mite development in damp grain 14%) to 16% MC wet-basis) are obtainable in temperate climates. Maintaining low uniform grain temperatures is too expensive at the bulk periphery when mean ambient temperatures are favorable for mite development. Although cooling moist grain is unlikely to prevent moderate mite infestation, aeration is expected to minimize “hot spots” and heavy mite populations associated with them.

SUPPRESSION OF MICROFLORA GROWTH Low temperatures are required to prevent mold damage in moist grain. Temperatures below 5°C are needed for the suppression of most mold development. For suppression of Penicillium molds, temperatures must be below 0°C. Most fungi do not grow at relative humidity below 70%, which is equivalent to roughly 13% MC for cereal grains at typical storage temperatures. The moisture content threshold is lower for oilseeds. In practice, mold growth is dependent mainly upon interstitial air humidity. Although cooling grain may not seem like an efficient method for controlling mold, at lower grain temperatures, mold damage is reduced.

Fig. 1. Simulation algorithm developed by Centaur, predicts airflow patterns, temperature and moisture content profiles leading to the development of excess moisture in the upper layers of the grain.

MAINTENANCE OF SEED AND GRAIN QUALITY Low kernel temperatures are desirable for better maintenance of seed and grain quality. Studies have shown that the lower the temperature (within certain limits), the longer the seeds maintain full viability. A rule of thumb (Harrington, 1973) states that a seed’s life span in storage is doubled for each 5°C decrease in temperature (within the range of 0° to 50°C) and for each 1 percent decrease in seed MC (within the range of 5% to 14%.)

Seeds are commonly stored with equilibrium relative humidity from 30% to 40% with good results. For extended storage times of seeds, Vertucci and Roos (1990) recommend the best storage MC is between 19% and 27% equilibrium relative humidity.

EQUALIZATION OF TEMPERATURE THROUGH-OUT THE GRAIN BULK Because of self-insulating properties, grain placed in storage during summer harvest retains initial harvest temperatures for a long time before cool weather arrives in the fall (except for grain near bin walls, exposed conical base, or at the surface). It is recommended that harvest heat be removed by night time suction aeration as soon as ambient temperatures are 8° to 11°C below internal grain mass temperatures to prevent condensation and minimize insect activity at or near the grain surface. The initial cooling should be followed by additional aeration when generally lower ambient temperatures will allow cooling the entire grain mass below 21°C.

Fig. 2.  Temperature (oC) and relative humidity (%) readings recorded using a Centaur remote sensor inserted in a silo capacity of 800 tons containing paddy rice under aeration in Arkansas, U.S.A.

PREVENTION OF MOISTURE MIGRATION IN THE GRAIN BULK As the ambient temperature drops during the cool season, the surface (and peripheral) layers of the grain become considerably cooler than the internal grain mass. Temperature gradients are established in the grain bulks that can lead to convection currents that circulate air through the intergranular spaces. In large bulks, the cold dense air settles along the outer walls. The warmer air (which contains more moisture than cool air) moves toward the colder upper surface of the grain bulk.

When the warm air reaches the cool layers of the grain bulk, moisture condenses and creates wet layers or spots in the grain. Studies (Montross et al., 2002, Montross and Maier 2001) suggest a moisture equilibration theory for the mechanisms involved in this moisture movement in a non-aerated grain mass. Using the finite-element model they developed, additional large-scale trials to demonstrate the effect of significant temperature gradients on moisture condensation due to convection currents that carry moisture into the cool layers of the grain bulk. On the other hand, the traditional natural convection hypothesis suggests that the natural convection currents in the grain bulk alone are sufficient to cause large amounts of moisture to “migrate” to cooler layers or the cooler surface grain, where the air cools to “dew point” and deposits excess moisture, slowly increasing the grain moisture content in the upper parts of the grain bulk. An example of the above phenomenon is presented in Fig. 1. Particularly, the prediction algorithm developed by Centaur (Centaur Analytics, 2018) can predict such airflow patterns and the corresponding temperature and moisture content profiles. Equalizing grain bulk temperature by aeration, such moisture migration is prevented.

PREVENTION OF HEAD-SPACE AND DOWN SPOUT CONDENSATION Under-roof condensation is a different natural process than moisture migration within the grain bulk. Condensate that drips on the grain involves moisture in humid air, which accumulates in the head-space above the grain bulk and condenses on the under-surface of the bin roof. Bins with sufficient roof vent and open eave gaps (spacing of 1/2 to 1 inch) between sidewall and roof, generally have enough natural ventilation to avoid under-roof condensate. Condensate is especially problematic in bins with eave gaps that are permanently sealed to prevent fumigant gas losses and easy grain access for insects.

PREVENTION OF BIOLOGICAL HEATING OF DRY GRAIN In grain bulks where infestation is localized, insect populations develop in small pockets of grain. The lesser grain borer and the three primary weevil species found in grains in the United States — the rice weevil, the maize weevil, and the granary weevil are characteristic species that develop localized infestations in bulk grains, creating hot spots.

Temperatures of heavily infested grain undergoing widespread heating are typically about 38° to 43°C. When heavy infestations are discovered, the grain should be fumigated immediately to stop insect activity. Then aeration should be used to cool the grain bulk.

PREVENTION OF SPONTANEOUS HEATING OF MOIST GRAIN In warm moist grain (equilibrium relative humidity greater than 70%), respiration can become very intensive due to mold development. High levels of respiration produce a phenomenon called “spontaneous heating.” Heating of the grain bulk is detrimental to grain quality. In spontaneous heating, hot spot temperatures can easily reach 57° to 60°C creating steep temperature gradients between heated and surrounding cool grain. In bulks containing oil rich seeds such as cottonseeds, soybeans, and sunflower seeds at sufficiently high moisture conditions, very high temperatures are generated and “spontaneous combustion” can occur, starting a fire. Do not operate aeration fans if fire is detected (by the smell of smoke or burning grain in the exhaust air stream) in a grain bulk. Aeration promotes fire by supplying more oxygen to the heating spot.

LIMITED GRAIN DRYING BY AERATION A small, but significant drying effect (from 1/4% to 1/2% moisture loss per aeration cooling cycle) is typically experienced, and during long-term aeration (multiple cooling cycles) up to 2% moisture reduction may occur while cooling large grain bulks. Because of the very low flow rates during aeration, the drying front moves slowly, and this small drying effect is usually limited to the grain near the entrance of the aeration air. This grain moisture loss is reflected in a corresponding shrinkage or market weight loss in the grain bulk. This must be considered in grain management as a cost for keeping grain safe for marketing. Aeration moisture shrinkage as well as “invisible” handling loss will affect facility records significantly and should be considered when grain receipt and delivery records from storage facilities or sites do not tally.

REMOVAL OF FUMIGANT RESIDUES AND ODORS The release of or desorption of fumigants at the end of a fumigation can be achieved with relatively low air flow rates. The aeration system can be operated intermittently (in pulses) to flush gas vapors from the grain bulk and storage. Aeration systems can be operated for 10 to 15 minutes every two to three hours to allow interstitial air space to reach equilibrium with the concentration of the fumigant in the grain. Thus, the aeration system can be operated several times to ventilate the storage. Storage odors also can develop in a grain bulk due to hot spots containing insects or moldy grain. Sour odors result from anaerobic activity in the process of fermentation at high MCs (above 18% for cereals). At moderate moisture levels (14% to 18% MC for cereals), musty odors in grain are usually caused by the growth of certain molds. Other odors occasionally found in grain are considered commercially objectionable foreign odors (COFO) because they are odors that are foreign to grain and render it unfit for normal commercial usage. Most odors can be reduced using aeration; however residual odors may linger after repeated aeration cycles. Commercial applications based on pilot laboratory studies have used aeration combined with ozone treatment to reduce off odors in grain.

PRESERVATION BY CHILLING OF GRAIN BY REFRIGERATED AIR Aeration using ambient air may not be sufficient to control fungi on moist grain protect grain against mites and insects, control self-heating of grain, or preserve the germination capacity and quality of stored grain in warm climates, or when warm grain is stored immediately after harvest. In answer to these situations, refrigerated-air units for chilling grain have been developed for commodities that can justify the added expense of refrigerated aeration cooling. In this type of aeration process, ambient air is conditioned by passing it through the evaporator coil and a secondary reheat coil of the refrigeration unit, and then blowing the chilled air into the grain bulk via the existing aeration system. Passage through the secondary reheating coil is designed to adjust the air relative humidity to 60–75% to match the target MC of the dry grain. The amount of reheating and the final air temperature are adjustable by the operator to achieve the desired aeration conditions.

Refrigerated aeration has been used for cooling dry grain in subtropical climates when ambient temperatures are too high for successful insect control by aeration with untreated air (Hunter and Taylor, 1980; Navarro et al., 1973). Refrigeration involves considerable investment, but together with the dehumidified air method, it could provide answers to the practicability of aeration for safe commercial storage in tropical climates.

Fig. 3. User interface for monitoring and managing the aeration process of a stored commodity (automatically or manually). Specifically, the user may choose between “manual” or “automatic” mode (left side) while receiving information about the current weather and grain conditions (right side). For the “automatic” mode the user may set the target temperature or the target moisture content.

In many grain-growing areas of the world the MC of grain at harvest is too high for safe storage. This damp grain is readily attacked by molds unless it is protected in some way. The most widely used method of preventing mold growth is to dry the grain to a safe level. Burrell (1982) argued that there is a balance between “safe moisture content” and “safe temperature”: The lower the temperature of bulk grain, the damper it can be safely stored. This leads to the conclusion that drying need not be so stringently applied since the power required to evaporate moisture from a bulk of grain is far greater than that required to cool the same bulk. For example, the energy required to evaporate 6 percentage points of moisture from a grain mass is at least six times greater than that required to cool the same bulk from 25oC to 5oC by using refrigerated air. Therefore, chilled storage may have an economic advantage over drying. This advantage is even retained when storing excessively damp grain, which requires frequent re-chilling, including the high capital investment costs for equipment.

In the major study “Enhancing the Quality of U.S. Grain for International Trade” (U.S. Congress, 1989), the maintenance of low temperature and moisture levels in bulk-stored grain was identified as the primary way to preserve grain quality and prevent damage from molds and insects. Grain chilling is a technology that can be successfully applied under many climatic conditions to preserve grain quality during storage when commodity value and profit margins are sufficient. It permits the short- to long-term storage of grain independent of the ambient conditions. Grain chilling has been applied commercially in over 50 countries during the past 40 years but has only in 1994 gained recognition in the United States (Maier, 1994).

It is estimated that over 80 million tons of grain are cooled annually worldwide with grain-chilling systems. Grain chilling is accepted as a grain conditioning technology in much of Western Europe; currently most new units appear to be marketed in Southeast Asia. In the 1960s grain chillers were primarily used as a means of preserving high-moisture (moist, damp) grain. Later, grain chilling was applied to improving storability of sensitive commodities subject to development of heat foci (hot spots), i.e., for soybeans and maize, and preserving the quality of high-value dry grain, seeds, and edible beans, primarily against mites and insects (Navarro and Noyes, 2002).

RECENT DEVELOPMENTS A significant aspect of effective aeration control is our ability to monitor the temperature in the aerated silos. Similar to fumigation, O2 or CO2, temperature and humidity should be monitored to ensure a successful aeration. As a common practice temperature sensors have been installed to ensure timely cooling of grain in silos. Such monitoring is performed by technical personnel on site at predetermined regular times, particularly during aeration. The conventional method of such monitoring is the visit of the technical personnel on regular basis to monitor the changes in temperature. This method necessitates precious time and travel of the technical personnel, which makes the monitoring expensive.

Wireless sensors for monitoring temperature and relative humidity have been developed to enable the technical personnel record the data (Fig. 2)(Centaur Analytics, 2018). Such development facilitates the analysis of the cooling pattern of the grain bulk without the presence of the technical personnel on site. Since the aeration to cool the grain mass lasts many days and the aerated structures may be located at distant sites then the location of the operator, such development offers an excellent tool that can be incorporated to the package of the aeration equipment. Such wireless monitoring assists the operator in decision making on the necessity of intervention during the treatment. Furthermore, recent advancements in machine learning offer the capability for automatic aeration control through an easy-to-use web interface (Fig. 3) (Centaur Analytics, 2018). These automatic aeration systems combine grain conditions and weather data to produce an aeration plan that minimizes the aeration fan operating hours and maximizes the benefits of the aeration process.

Acknowledgement: Authors thank Mr. Nadav Inbari for his technical assistance in the preparation of this manuscript.

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