Ralph E. Kolb
CEO
FrigorTec GmbH

Improper storage causes over 20% of the global grain harvest to spoil each year—mainly due to insect infestation, mildew, and poor temperature control. This article outlines the science behind grain’s biological behavior after harvest and explore practical, proven solutions—from aeration and exclusion strategies to the increasingly important use of grain cooling. With the right technologies and consistent monitoring, we can reduce spoilage and safeguard both food quality and supply security worldwide.

Grain storage is essential for preserving the harvest after it is reaped until it is ready for consumption or processing. It helps bridge the gap between harvest seasons and allows farmers to store surplus grain during abundant years, ensuring a steady supply during lean periods. However, according to the Food and Agriculture Organization of the United Nations (FAO), the annual spoilage of the grain harvested worldwide is over 20 %. The majority of this loss results from insect activity and mildew growth. Through optimized storage management, many millions of tons of grain, oilseeds, rice, corn, and other cereals can be saved from spoilage.

The state-of-the-art storage practice, especially for large quantities of grain, is bulk storage in warehouses, silos or grain bins. This method is efficient, cost-effective, and suitable for both farmers, commercial grain handling and grain processing industries. The quality of grain is significantly affected not only by its nature, but also by the conditions under which it is stored. It is often underestimated that grain is a biologically active commodity. Grain goes on living after the harvest; it breathes. 

CHALLENGES IN GRAIN STORAGE

In grain’s cellular respiration, oxygen is absorbed, and carbohydrates are then converted into carbon dioxide, water and heat. The consequence is loss of dry substance. The rate of the activity is dependent on the grain’s moisture content and temperature as shown in urePicture 1 (Jouin, 1964). Respiration becomes exponentially intensive as the temperature and moisture content of grain increase. Loss of grain quantity by respiration may be as high as 0.5 to 3 % according to the temperature and moisture content of the grain during the storage.

Picture 1: Heat generation during grain storage modified according to Jouin (1964)

Management of grain’s moisture content and its temperature are the only ways to avoid losses and danger. If it is neglected additional risks by infestation of bacteria, mites, insects and fungi are developed. Other consequences are grain damage by water condensation inside a storage which leads to caking of grain as shown in Picture 2.

Picture 2: Caking of grain inside a storage

During storage the highest risk is the infestation of insects. More than 80% of the postharvest losses are caused by weevils as stated by FAO. Each grain has a specific dominate weevil with nature to attack that grain preferably. The growth rate of insects is exponential at the optimal condition of their environment. The main driver for their nature is the right temperature as shown in Picture 3. Favourable temperature levels according to their nature will lead to optimal development with the most damage to the grain. Any lower or high temperature affects their lifecycle by reducing the fertility as well as the time of their lifecycle. If the temperatures are low enough, the insects go into diapause – hibernation – and cannot harm the stored crop.

Picture 3: Weevil developments according to the temperature of their environment (FrigorTec)

The damage of the grain is mainly mass loss by consuming the grain and the nesting inside the grain kernel. In addition, there is the risk of contamination by excrements and bacteria. It is absolutely necessary to monitor insects and act with serious grain management about it.

The third main danger for grain storage is the development of fungi which depends on the temperature, the relative humidity and the grain’s moisture content as shown in Picture 4. Grain of more than 14% moisture content create fungal development at the right temperature because the level of relative humidity is high enough to allow their growth.

Picture 4: Fungi development according to the temperature, relative humidity and grain’s moisture content (Lacey, J.; Hill, S.T.; Edwards, M.A., 1980)

Thereby the danger of fungal contamination is not only the deterioration of the grain but the increase of mycotoxin which affects the health of humans and animals. Proper grain protection is important and must be considered mainly by managing the proper moisture content of the crop in combination with the right temperature level if the acceptable moisture content can’t be assured or maintained. It is important to consider that respiration causes rise of water by reduction of grain’s dry matter leading to higher moisture content by the nature of grain.

GRAIN MANAGEMENT AND PROTECTION

Some management techniques are covered up here to understand what they contribute to grain protection. It must be assumed that the grain which is stored has been harvested at the proper maturity and moisture content. 

Since pests are one of the main problems of grain storage and number one concern of grain protection, the setup of the grain storage is essential. Therefore, the exclusion strategy is used in pest control which refers to the practice of preventing pests from entering grain storage. Exclusion techniques can involve the use of physical barriers, such as screens, seals, and caulk to close gaps and cracks that pests could use as entryways. The idea behind exclusion is to deny pests access to the interior of storage and helping to prevent infestations from occurring in the first place. It is an essential component of integrated pest management, a sustainable and environmental-friendly approach to pest control that aims to minimize the reliance on chemical treatments and focus on prevention and non-toxic solutions. 

Cleaning & Storage of Grain

If the moisture content of the grain is too high, it is mandatory to apply a drying under consideration of the targeted storage time. Regardless of whether drying is necessary or not, cleaning the grain is an important step that must be carried out before each storage.

Dust, chaff, broken or unripe kernels, impurities and dirt occupy space of the costly storage and add risk to the storage of grain by their moisture or contamination property. There are different cleaning technologies available which act according to the physical property of the difference between good grain and unwanted material. The cleaning capacity must be considered to match the volume of the incoming grain to avoid delays. A proper setup of the machinery and monitoring of its operation is required. Uniform grain with limited moisture variation and without unwanted materials optimize the storage condition, maximize the storage volume and lead to a good base for grain protection.

It is essential to monitor the grain during storage to take appropriate measures to protect the grain in the event of changes in storage conditions, such as rising temperature or humidity. The most common monitoring is to measure the temperature inside a grain bulk. It observes the condition of the grain as its respiration or activity by insects and moulds are indicated by an increase of the bulk temperature. A three-dimensional grid of measuring points enables the early identification of hot spots, ensuring that appropriate countermeasures can be taken. If cables with probes are used it is mandatory to fix the cables at the ground to avoid its movement by the loaded grain and misplace the cable.

Aeration

A minimum of grain protection is applied by aeration. It involves the controlled movement of ambient air through the stored grain mass. Aeration systems typically consist of ducts or perforated pipes installed within the grain mass. These ducts or pipes are connected to a fan or blower located outside the storage structure. When the aeration system is activated, the fan draws outside air into the ducts or pipes and then pushes it through aeration screened canals or full perforated floors into the grain mass. At the exit of the air flow are vents or exhaust fans which are opened or activated during the operation. The particular design and dimensions of the aeration system are supplied by the designer of the bulk storage.

Aeration helps to control the temperature of the grain mass. During storage, temperature fluctuations can occur due to external climate changes, but mainly by the natural respiration process of the grain or the activity of weevils and moulds. By forcing air through the grain, aeration dissipates excess heat and maintains a more consistent temperature throughout the storage period. Usually, aeration is very effective if cold and dry air is available. It may be critical to use an aeration when the outside air is hot and humid as it may boost the respiration of the grain or lead to moisture increase. It is advised that the temperature of the applied air is at least 7 Kelvin lower than the grain’s temperature to avoid issues. Therefore, the application of the aeration is weather dependent. Aeration may lead to a reduction of grain moisture content up to 3%. Thereby it should be understood that aside of the water also the dry matter loss by respiration takes place as mentioned above.

Fumigation

One of the most applied grain protection techniques is fumigation. Fumigation of grain is a pest control method used to eliminate or control insect infestations and other pests in stored grains. It involves the application of fumigants, which are gaseous chemicals with pesticidal properties, to the grain mass inside a storage facility. Fumigation is typically used as a last resort when other pest management strategies have proven ineffective or when infestations are severe. It should not be used as a routine treatment to solve infestation risk. The target of fumigation must be to stop the infestation of insects totally. Failure will create resistant insect population. The success of fumigation is determined by the selection of the right chemical according to the infectious species of insects, by applying constant and effective concentration during a sufficient time at the right ambient condition. Proper management by professional and qualified staff is mandatory. The procedure must be applied strictly according to the specification as fumigants are dangerous by their toxicity. 

The application procedure starts with the evaluation of the threshold level for the specific insect by proper monitoring throughout the storage. If it is reached the right fumigant must be chosen according to the way of application and he ambient conditions. The storage must be airtightly sealed to ensure effectiveness. The test of the airtightness is essential and usually performed by a pressure test. The area must be locked and marked. The gas is applied until effective concentration is achieved and the concentration must be monitored during the treatment. Fumigant is added if the concentration reduces to maintain effectiveness. The duration of the fumigation is according to the specification to reach 100% kill of adults, larvae and eggs. If ambient condition changes, the application must be adjusted. After the effective time did pass, the storage is to ventilate sufficiently, and the quarantine period of the grain is to consider before it can be used or sold.

Grain Cooling

Grain cooling is a post-harvest practice in agriculture that involves reducing the temperature of freshly harvested grains to a safe and optimal level for storage. It is an essential step to preserve the quality of grain and prevent spoilage during storage. Grain cooling is particularly crucial in regions with warm climates or when grain is harvested at high temperatures, as heat can accelerate the deterioration of grain by respiration and promote the growth of pests and moulds. Further water condensation inside bulk storage is prevented at any time if grain cooling takes place. The specific cooling requirements for different grains may vary depending on factors such as grain type, initial temperature, ambient temperature, humidity, and storage duration. It’s essential to monitor and manage the cooling process carefully to achieve the desired storage conditions and preserve grain quality effectively. If the ambient is not in favourite to use outside air, the grain cooling must be performed by a grain cooler. 

Picture 5: Application of grain cooler at silo

The technology operates with a mechanical cooling process constantly providing cold and dehydrated air completely weather independently. The grain cooler is connected to the grain storage and the conditioned air is blown into the grain bulk. The air flow passes the grain and removes the heat of the grain. The heated air exits the storage, as shown in Picture 5. The grain cooling continues until the entire bulk is cooled to the desired temperature which is in general around 13°C. Then the grain cooler is turned off and the air inlet and vent openings at the roof are closed. The cooled grain is kept in the storage until unloading. If the temperature increases during storage, a new cooling process can be started. The temperature remains stable over a long period because grain has low thermal conductivity and respiration activity of the grain is suppressed by the low temperature. 

The grain cooling takes place already when the screen of the aeration system is covered by grain. It continues until all grain has reached the safe level of low temperature. If grain is added the operation will start again, which allows the grain protection even for silos which are regular loaded and unloaded. The grain cooler unit can be moved from storage to storage, so the required machinery is reduced to one or just a few units. As the process is reliably controlled, the air conditioning remains constant. That allows to make use of the physics of air and the hygroscopic property of grain to achieve a moisture control of the grain or do intentionally a limited grain drying of up to 0.5 to 2% dependent on the initial moisture content of the grain. A retrofit of the grain cooler is always possible for storage system using an aeration system. Then only the blower of the aeration system must be removed to give space for the connection of the grain cooler. 

SUMMARY

After harvesting, grain continues to respire, consuming oxygen and converting carbohydrates into carbon dioxide, water, and heat. This process can lead to accelerated spoilage. Pests, especially beetles, are responsible for over 80% of these losses. They thrive in warm conditions and reproduce exponentially at optimal temperatures (15 - 30 °C). Therefore, professional grain storage is essential to reduce spoilage, maintain grain quality, and achieve optimal market prices.

Effective quality assurance begins even before storage. The first line of defence against pest infestation comprises physical barriers, such as shields and seals, as well as sealing compounds to close gaps and cracks, and bird nets. Additionally, the facility must be thoroughly cleaned before each storage period to remove dust, chips, contaminants, and broken kernels. The grain itself should also be cleaned and pre-dried if the moisture content is too high.

After storage, various methods can be used to protect the grain from spoilage due to insect infestation. Aeration with unconditioned outside air provides a minimum level of protection. However, it is important to note that aeration can promote the grain respiration or increase moisture levels in hot and humid outside air.

Fumigation is typically a last resort, used when other pest control measures have failed or when the infestation is severe. It should not be used routinely to control pest infestation. Due to the high level of protection and preparation required, fumigation is generally labour-intensive and costly.

In addition to traditional methods, the use of grain cooling is becoming increasingly popular worldwide. Based on scientific research, this process optimises storage conditions in terms of temperature and humidity, while requiring minimal labour and energy. It effectively prevents insect infestation and the formation of mould and mycotoxins. It also reduces the dry matter loss and has a drying effect.

Professional grain storage guarantees the availability of high-quality grain and prevents significant spoilage. Grain is extremely important to us, both as a food source and as animal feed.

About FrigorTec 

FrigorTec provides solutions for grain cooling, crane air conditioning, hay drying, insect heat treatment, and specialised cooling systems. GRANIFRIGOR™ grain coolers offer reliable cooling for grains, rice, maize, seeds, and oilseeds, protecting them from insect infestation, fungi, and mycotoxins – all without chemicals. These units have been sold in over 80 countries since 1963 and are known for their reliable, energy-efficient operation. 

All products are developed and manufactured at the company’s headquarters in Germany. With branches in the USA, Singapore, India, France, Austria, and worldwide partners, FrigorTec provides a strong sales and service network. As a result, FrigorTec has earned an excellent reputation and is trusted by clients worldwide for its innovative cooling solutions.

Email: info@frigortec.com
More information: www.frigortec.com