Cereals
Post-harvest cooling of seeds to temperatures below the insect activity threshold of 12°C is the most widely used preventive method in France for conserving seeds (cereals and oilseeds) without the risk of insect infestation (usually using large-capacity cold rooms). Cooling seeds directly in the storage cell is a lengthy process which must begin immediately after harvesting, as soon as night-time temperatures allow a first temperature plateau at 20-22°C, around one month after placing in the cell. In regions where the drop in temperature during the night in summer is not sufficient to allow ventilation, there are refrigerated air ventilation systems that allow ventilation throughout the day, whatever the outside temperature (Mathie, 2019). Refrigerated air ventilation units are also designed to cool seed storage buildings in summer. In this case, storage buildings need to be well insulated to avoid energy losses. This solution is much less costly than maintaining the building in cold storage, provided that the building is well insulated, especially the roof, with a minimum of openings to the outside. In existing buildings, after reinforcing the thermal insulation, seeds can be stored in sealed enclosures, containers, big-bags or cells at room temperature (up to 24-25°C for cereals, for example, excluding malting barley), bearing in mind that only dry, clean grain free of living insects can be stored at this temperature over the long term. Thus, cooling grain stocks below 20°C is imperative only where there is a risk of insects already being present in the batch to be stored.
In the case of straw cereals at harvest time, there are no dangerous insects for storage - and corn seeds dried at low temperatures are rarely infested when they leave the dryer (desiccation effect on hidden field insects such as the cereal leaf beetle; figure 7). Cereal seeds, placed in an airtight container, can be stored in good conditions without necessarily being cooled below 20°C, and if the circuits and handling equipment at the storage site have been properly cleaned and disinfected before the new crop is received (Leblanc et al, 2013).
Long-term storage under inert gas (carbon dioxide, CO₂ or pure nitrogen, N₂) in an airtight enclosure prevents infestation from either internal or external sources, provided the exposure time is sufficiently long. The lethal exposure time for different insect species in seeds stored under inert gas is highly variable, and depends on grain temperature. Preservation under CO₂, the most common solution, can be carried out simply in big-bags with internal plastic film, airtight after sealing, or in hermetically sealed cells from 50 to 500 tons, equipped with gas introduction nozzles and concentration control and safety systems.
Pulses
Bruchids are already present in seeds at harvest. Infested batches intended for human consumption must therefore be systematically "disinfested" using a residue-free chemical process (gassing with aluminum or magnesium phosphide, or placing in a modified atmosphere with CO₂ or N₂). To avoid treating all batches of legumes at risk of bruchid infestation at harvest, (lentils, broad beans, yellow peas, tarbais beans), sorting between batches at risk of infestation and healthy batches can be facilitated by the use of acoustic detection probes (Tomasini and Fleurat-lessard, 2017) that detect active larvae. In the absence of positive detection, batches are declared healthy, with no need for fumigation or inerting.
For legume seeds, the absence of insects at the time of collection at harvest is imperative, and conservation without the risk of bruchid emergence after storage requires complete disinsectisation of batches that arrive bruched at harvest to avoid any emergence of adult bruchids throughout the conservation period.
Oilseeds
Oilseeds are more sensitive than cereals to high temperatures and also to oxygen content, which favors lipid oxidation and can reduce germination energy, an important factor for rapid seedling emergence, leaving no time for plant diseases to take hold. The hermetic preservation of oilseed under inert gas also helps limit loss of seed vigor at temperatures not exceeding 20°C, provided seed moisture is less than 8% (figure 1).
For oleic sunflowers with a higher oil content than conventional varieties, the sorption balance between air and seed is different (figure 2). Hermetically sealed storage in an inert atmosphere is recommended for these oleic varieties to limit the risk of loss of vigor and even germination capacity in the event of long-term storage.
For oilseeds, storage in hermetically sealed containers offers both mechanical protection of the contents against rodents and insects, and prevents the diffusion of attractive odors into the storage room. The preservation of germination capacity is ensured over a long period of time, provided that the water content of the product is sufficiently low (below the sorption equilibrium threshold of 65% RH).
Soy
The germinative capacity of soybeans is rapidly reduced during storage at high temperatures (28-32°C) in the open air. The lower the residual oxygen content, the smaller the reduction in germination capacity. In an atmosphere with 70 or 80% CO₂, or under partial vacuum, the drop in PG is limited to 15% over one year of storage, whereas it reaches almost 80% in ambient air (at 20.9% O2). At 10% residual oxygen, the loss of germinative capacity can reach 30% of its initial value (Swapna Kumari, 2013).
Generalization of Good Storage Practices (GSP)
For cereals and pulses, it is imperative to dry grains to 14% moisture or less, and to keep them at 20-22°C or below, to preserve their initial quality over a full year of storage.
For oil-rich seeds (sunflower, rapeseed), the maximum water content for safe storage is 8%, and even below 8% for oil-rich varieties (figure 10). Temperature has a greater effect on oilseeds than on cereals or pulses, and oilseeds must always be stored at 20°C or below.
Within these constraints, storing grains and seeds in hermetically sealed containers or enclosures, with or without inerting, appears to be an economical solution that can be adapted to production levels ranging from a few tonnes to several hundred tonnes.