Safe storage and handling

Aim

The aim of this module is to be aware of the storage requirements to maintain soybean seed at optimum quality.

Learning outcomes

After completion of this module participants will be able to:
  • Understand how storage conditions can affect the quality of stored soybean seed
  • Identify the optimum storage facilities and equipment for soybean seed
  • Determine the best management practices for soybean seed storage

About the author

Philip Burrill is a development agronomist with Department of Agriculture & Fisheries (DAF Qld.) based at the Hermitage research facility, Warwick on Queensland’s Darling Downs. Philip works with AgriScience Queensland’s Postharvest Grains protection research team, whose research specialises in stored product pest control and maintaining grain quality. His primary roles include running grain grower and industry workshops, providing research results and practical information on grain storage. He also conducts grain storage research field trials at the Hermitage research facility and grower’s on-farm storage facilities. Philip is northern regions leader for the National Grain Storage Extension project supported by GRDC. ‘Safe storage and handling of soybeans’ module updated August 2022 

1. Summary

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2. Storing oilseeds

Choosing to store oilseeds on-farm requires attention to detail as there are limited pest control options available, compared to storing cereal grains.

To retain soybean market value care must be directed at maintaining seed quality, visual appearance and freedom from contaminants such as mould, insects and unregistered chemicals.

The handling of whole soybean seed involves conveying and transporting from the farm to end-user. It is possible for soybean seed to be handled as many as five times from when they leave the farm until processing.

The structure of a soybean seed makes it susceptible to splitting and breakage during mechanical handling. The percentage of broken soybean seeds can be as high as 4.5% with a free fall drop of 30 m. Bucket elevators and belt conveyors result in less damage when compared to the standard grain auger. When augers are used, run them to capacity to reduce seed damage.

Over-dry soybean seed or seeds that has been subjected to several rain events just prior to harvest are also more likely to split. The storability of soybean seed is affected by the degree of damage to the seed coat. Damaged seed favours storage insect pest and moulds. Inspect soybean seed for mechanical and other forms of damage prior to storage. If the amount of broken or split seed is high, it may be prudent to separate the broken or split seeds by grading. Gradings can then be sold or used first rather than stored with the better-quality seed.

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3. Seed quality and moisture content in storage

Three major factors that affect the storability of soybean seed are moisture content, temperature, and duration of storage. The general condition of the product and amount of foreign materials also affects their storability.

Moisture content

Moisture is perhaps the most important single factor affecting storage of soybean seed. Soybean moisture content at harvest time usually ranges between 12% and 15% (wet basis). Seed above 13% moisture content should be dried to reduce the risk of deterioration. At high moisture contents, there is a rapid increased in seed respiration leading to spontaneous heating, mould development, germination damage and oil quality deterioration. For long-term storage (> 6 months) moisture content below 12% is recommended.

Like all grains, soybean seed is hygroscopic and will either lose (desorb) or gain (adsorb) moisture from the surrounding air. With soybean seed in storage, aeration systems require careful management as fans left running for too long under certain temperature and relative humidity conditions can result in damage by adding significant moisture to stored seed. 

Table 1. Soybean equilibrium moisture content (EMC) at various temperature and humidity conditions. Example: If soybean seeds were constantly aerated with ambient air at 20°C and 70% relative humidity the seed would gradually move toward a moisture content of 12.8%. 

Temperature

Temperature is another important factor influencing soybean seed storage. Growth of fungi and undesirable oil quality changes can increase with higher temperatures in both whole soybean seed and meal products. Insect pests in storage also develop and reproduce best between 25°C and 35°C. Aim for storage temperatures close to 20°C or below. At 15°C and below all major storage pests become inactive and breeding ceases. 

Soybean seed with a higher moisture content (e.g. 14–14.3%) can be stored for over two years without mould damage if maintained at very cold temperatures of 5°C to 8°C. In contrast bulk soybean seed stored at 30°C can lead to mould development in the storage in a few weeks and severely damage the seed in a few months. Dry seeds (e.g. 10.5%) are unlikely to have mould problems develop. 

Use aeration cooling to store soybean seed at less than 20°C and if possible, less than 15°C during winter months. This limits or stops insect breeding, mould development and oil quality deterioration. Regular monthly testing of seed temperatures in storage using a grain temperature probe is highly recommended. 

Aeration cooling soybean planting seed in storage also helps maintain seed germination quality. Select a good quality, automatic aeration controller. This ensures aeration fans are running during optimum ambient conditions. Automatic controllers provide an improved level of reliability to cool storage conditions. 

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Duration of storage

Deterioration of soybean seed and soybean meal in storage is a combined function of its moisture content, temperature, and duration of storage. Therefore, to reduce the risk of seed quality damage, a combination of low product moisture, low temperature, and a short storage period are desirable.

Foreign material present

Foreign material in soybean seed is all materials that pass through a 4.75 mm or 3.2 mm round-hole sieve and all materials other than soybean grain remaining on top of sieves. See the three standards for Edible milling, manufacturing or crushing grade soybean for more detail. 

Fine foreign materials tend to segregate during silo filling and occupy the central region of the stored seed mass in the silo. Meanwhile, large and lighter materials will accumulate close to silo walls. The process of material segregation inside a silo is illustrated below.

Schematic diagram of how light and heavy foreign materials segregate when seed is loaded into a storage silo.

During aeration of the silo, air will flow around regions with a higher concentration of fine foreign materials and favour moving through areas of clean seed and large foreign materials. This results in non-uniform flow of air during aeration.

Areas of reduced air flow in the stored seed mass are potential sites for hot spots that provide an ideal environment for insects and moulds. Hence, cleaning soybean seed to remove some of the foreign materials prior to storage will minimise this risk. Pre-storage cleaning could be a valuable component of quality maintenance for stored grain.

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Product condition and history

Sound, whole soybean kernels will store better than kernels damaged by cracking, splitting, insects and fungi. While storage pest insects will attack whole soybean seeds and cause significant damage, it is the split kernels that provide the most attractive food source to a larger range of storage pests and fungi. 

A sound seed coat provides some degree of protection against attack. Grain handling systems that minimise soybean cracking and splitting are an advantage. Belt conveyors are recommended as they handle the seeds gently and can move the seeds over greater distances without damage. The amount of broken and split seeds increases with every handling operation during harvest, storage, processing and export. 

4. Deterioration indicators

Heating

Heating is a common indicator of a problem in stored grains and oilseeds. High oilseed temperatures normally indicate high seed moisture, microbial growth, or insect activity. If left unchecked, this may lead to heat-damage to oil quality and visual quality ‘bin burnt’ downgrades. 

Heating due to high moisture contents in cereal grains peaks at about 58°C. In soybean grain, however, as heating progresses above 50°C, the oxidation of the oil becomes a self-sustaining process. Temperatures in the hot spots may rise above 150°C. At these extreme temperatures, charring of the seeds occurs, and spontaneous combustion is possible if sufficient oxygen is present at the hot spot. Hot spots must be cooled or dissipated promptly. Moving the seed is one strategy. Do not run aeration fans on soybean seeds if a fire has already started, as this makes the situation worse. 

A temperature monitoring system such as OPI moisture cables® inside a silo containing bulk soybean seed may be a valuable investment for monitoring both seed temperatures and moisture conditions (EMC). These stored seed monitoring systems, with sensors inside the silo, also provide a regular check on the performance of aeration cooling equipment, ensuring it is operating effectively to achieve desirable cool storage conditions. 

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Change in colour and general appearance

In general, sound soybean seeds are plump with bright, uniform colour and are free from unusual spots and shrivelled appearance. Discoloured soybean seed is usually associated with inferior quality and lower market value. The change in colour is usually associated with higher moisture, temperature and presence of moulds. This deterioration process can be detected by periodic drawing of samples from the stored grain. If detected, check that the aeration cooling system is operating effectively and consider transferring the seeds to another silo. The conveying process cools the grain and breaks up hot spots. Keep in mind the risk of cracking or splitting soybean seed through the additional handling operation. 

Mustiness and off-odour condition

Musty odour usually indicates an advanced stage of insect or mould infestation and should be dealt with immediately. The soybean seed should be aerated to remove the bad odour and to cool the seed. Use the grain at the earliest opportunity. Fumigate if insects are present. A sharp odour may indicate rancidity due to chemical changes in the oil component of the seeds.

Presence of storage insects

Regular monthly inspections and sampling of all grain storages is essential to check both seed quality and for the presence of storage pests. Sieve seed samples taken from the bottom and top of silos. Identify and record any pests detected. Using storage insect probe traps (pitfall traps) located near the top of the stored seed mass is also a helpful tool for early pest detections. While checking for pests, measure the seed temperature with a probe. 

 Seed sampling equipment shown below.  

Grain spear and insect sieve.
Insect sieve.
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Lumping and caking

Lumping and caking indicates an advanced stage of fungi invasion in soybean grain. In metal silos, caking usually occurs on the walls or at the top of the stored seed mass due to ‘sweating’ or ‘moisture condensing’ on inner surfaces of cold silo walls and the roof at night. The condensing moisture drips from these internal surfaces over many weeks and is absorbed by seeds, resulting in sprouting and mould growth. 

Common causes for this problem are poor management of the aeration cooling system or, leaving the silo ‘sealed-up’ with the roof not vented. Attempting longer-term storage of grain with high levels of moisture or foreign materials can also lead to problems. A leaking silo roof is not usually the cause.  

Chemical changes in storage

Stored soybean seeds may undergo chemical changes even under ideal storage conditions. One common indicator of chemical change is the level of free fatty acid (FFA) present. An increase of FFA above 1% may translate into lower oil quality. Decline in soybean seed germination and vigour can be rapid. Careful attention to storage temperature and moisture conditions will minimise these problems.

5. Types of storage

Ideal storage for soybean seed is a well-designed cone based, ‘sealable, gas-tight’ silo fitted with an ‘aeration cooling’ system. 

Soybean seeds in silos should be aeration cooled during storage. Only ‘seal-up’ a silo for a short period during the actual phosphine fumigation exposure time (e.g. 10 days). Refer to label requirements. 

Consider ease of silo hygiene and cleaning to deal with any grain residues in an empty storage. It is important to minimise cross-contamination with other grain types and reduce carry-over of grain storage pests between seasons. 

Design the storage facility to minimise damage to soybeans seed when moving in and out of silos. 

For all types of storages, including grain sheds, extra caution should be taken to prevent rainwater ingress and potential grain contamination from rodents, animals and birds.

Standard aeration cooling fan on silo.
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6. Hygiene – Structural treatment

There are no spray-on, chemical insecticide structural treatments suitable for storages to hold soybean seeds. Soybean is classified as an oilseed, with limited chemical treatment registrations. Due to the oil content, seed is prone to absorbing chemical treatments. After processing, chemical contaminants can be detected in the final soybean oil product. 

Warning – if oilseed buyers detect unregistered chemical residues, this can have serious long-term negative consequences for oilseed sales into both domestic and export markets.

A thorough physical clean of empty storages is recommended. This could include washing out with a pressurised water if the storage is suitable. Carefully dispose of any old grain residues or gradings so storage pests do not breed in piles of old grain and simply fly back to the storage facility. 

Insects survive in any sheltered place with grain residues, including silos, auger hoppers, field bins and inside headers. These attractive locations for pests should be cleaned out when equipment is not in use.

Poor grain hygiene enables storage insect pests to survive between seasons and draws rodents and other pests to the storage area.

Following the physical cleaning of storages, a diatomaceous earth (DE) treatment can be applied internally to deal with the few remaining storage pests hiding in small crevices. DE structural treatments are suitable for pest control in grain storages used for oilseeds, pulses or cereals. Diatomaceous earth (amorphous silica) is a naturally-occurring mined product with insecticidal properties. Products such as Dryacide® can be applied as a dust or slurry spray onto internal surfaces of storages and equipment as a ‘non-chemical’ treatment to reduce insect pest carry over. 

There are some pyrethrin + piperonal butoxide based insecticide products registered for moth control in oilseed storage areas and sheds. They can be applied as structural surface spray or fogging/misting treatments. They are not oilseed treatments and should only be used as the labels directs. Prior to using any of these insecticides in storages for export soybean grain, it is important to check that potential buyers and importing countries accept their use and have appropriate maximum residue levels (MRLs) in place. 

As a guide, the following web sites provide information in regards registered chemicals and MLRs for oilseeds, including soybean. 

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7. Aeration

Aeration cooling should be considered an essential management tool for storing soybean seeds.

Aeration:

Air flow through grain bulk.

Aeration cooling

Fans that provide low air flow rates of around 2–4 litres per second per tonne (L/s/t) can cool seed and provide uniform moisture conditions in storages. If managed correctly, aeration allows safe storage of soybean seed at moisture levels a little above receival standards for several weeks.

Well-managed cooling aeration typically sees seed temperatures fall safely to around 20°C and below within three to five days.

Regular checking of stored soybean grain is essential. Make visual inspections, check seed moisture, sieve for insects, and use a temperature probe or grain monitoring cables inside the silos to monitor seed temperatures.

Standard aeration cooling fan.
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Automatic aeration controllers

Some aeration cooling fans are turned on and off manually, while others use a timer clock. Both of these systems can often lead to serious mistakes. Investing $8000 to $12,000 in an automatic controller that selects the best ambient air conditions to have fans operating is usually worthwhile. One unit controls aeration fans on all the silos at the facility. The aeration controller continually checks ambient air temperatures and relative humidity and may select suitable air from only 2 or 3 days in a week or fortnight. On average it aims to select approximately 100 hours per month of the optimum ambient air to run the aeration fans. 

Automatic aeration controller.

Standard aeration fans operation

There are three stages of aeration fan cooling

Some automatic aeration controllers (as shown above) have a ‘Auto’ mode that will automatically cycle the fans through the three stages outlined above. Settings for aeration drying can be included. 

Aeration drying

Well-designed, purpose-built high air flow aeration drying systems with air flow rates of 15–25 L/s/t can reliably dry soybean seeds. During aeration drying, fans should force large volumes of appropriate air through the seed bulk for many hours each day (approx. 14–22 hours). This ensures drying fronts are pushed quickly through the full depth of seed, so seed at the top of the silo is not left sitting at very high moisture contents. 

Soybean is well suited to aeration drying as ambient air can reduce moisture content without the potential risks of using excessive heat associated with hot air driers. See Table 1. Soybean EMC. Monitor seed moisture content regularly and take care that seed in the silo base is not over dried.

Do not try to use aeration cooling fans with very low airflow rates of only 2–4 L/s/t to dry high moisture soybean seed. Reliable aeration drying requires high airflow rates of 15–25 L/s/t.

Automatic aeration controllers are available (as shown) that can run fans at optimum ambient conditions for either seed cooling or drying functions. Ensure the controller has a good quality humidity sensor.

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Heated air drying

For hot air drying of soybean seed, fixed batch, recirculating batch, or continuous flow dryers are all suitable for reducing moisture content. Always consider seed blending first if low moisture seed is available.

Give close attention to temperature control and duration to ensure seed is not over dried. A wise precaution is to use the minimum amount of additional heat.

The following points are useful for heated air drying:

8. Fire risk

The dust and admixture associated with soybean seed presents a fire risk. Harvesting and drying operations are high risk operations where constant vigilance is required. Good housekeeping in and around equipment and keeping a close eye on problem sites reduces the threat.

In case of a fire, ensure appropriate equipment is at hand and that all operators understand the response plan.

Without careful management, high moisture content soybean seeds and /or high admixture poses a risk of mould formation, seed heating and fire through spontaneous combustion.

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9. Insect pest control

There are several insect pests that will infest stored soybean seeds, usually favouring the exposed surface of the stored seed. Common examples are cowpea weevils (Callosobruchus spp.), rust-red flour beetle (Tribolium castaneum), psocids (Liposcelis spp.), Indian meal moth (Plodia interpunctella) and warehouse moths (Ephestia spp.).

Occasionally soybean bruchids (Bruchidus mackenziei) are seen infesting soybean pods in the field before harvest. They, however, are not considered a pest that will continue to lay eggs and breed in stored soybean seed.

Cowpea weevils (left), rust-red flour beetle (centre) and Psocids (right).
Indian meal moths (left), warehouse moth (centre) and soybean bruchid (right).

Storage pests multiply rapidly when given food, shelter and warm, moist conditions. They can complete their full life cycle in about 3 to 4 weeks under optimum breeding conditions and seed temperatures of 25–35°C.

Only a few treatments are registered for insect control in stored soybean seeds. Phosphine, pyrethrins, diatomaceous earth (DE) and ethyl formate as Vapormate®. Pyrethrins and DE use should be limited to storage area treatments and Vapormate is restricted for use by licensed fumigators only. This leaves phosphine as the key on-farm storage treatment for storage pests detected in oilseeds.

Phosphine fumigation must take place in a gas-tight, well-sealed silo. If the silo passes a standard three-minute pressure test, it shows there are no serious gas leakage points. Given this, phosphine gas can be held at high concentrations in the silo for sufficient time to kill all life stages of the insect pest – eggs, larvae, pupae and adults. 

When purchasing a new silo that may be used for fumigation, ensure the manufacturer builds it to meet the Australian standard for sealable silos: AS 2628. Several silo manufacturers now make aeratable, sealable silos that pass this Australian Standard. 

Phosphine tablets are placed in trays so the spent tablet dust can be removed after fumigation.
Australian standard for sealable silos: AS 2628.

A silo pressure test is important prior to fumigation to check for leaks. The two graphs below show the large differences in phosphine gas concentrations in a gas-tight silo (Silo B) compared with a silo that has minor air leaks (Silo A). 

Gas-tight silo.

Silo with minor leak.

Like most grains, including oilseeds, soybean seed can absorb some phosphine gas during fumigation. It is therefore important to use the full, correct dose rate as shown on the label.

By using phosphine bag-chains, belts or blankets, placement and removal of the fumigation treatment is simplified. If using the standard phosphine tablets, ensure tablets are kept separate from the stored seed by using trays so the spent tablet dust can be removed after fumigation.

If aeration cooling has been in use and the seed temperature is less than 25°C, ensure the fumigation exposure time is 10 days or more. Refer to the phosphine label for further details.

Once the fumigation exposure period is completed, release the top lid, vent the gas for the required period and return to aeration cooling of the stored soybean seed.

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10. Further resources

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