Controlled and Modified Atmosphere Storage Systems for Foodgrains

Post-harvest losses remain one of the most persistent challenges in food security. Estimates suggest that 30-50% of grain is lost during storage and transport, particularly in developing countries where conventional storage methods fall short. Controlled and modified atmosphere storage systems offer a science-backed, chemical-free solution – one that works by changing the very air surrounding stored grain to make it hostile to pests and fungi.

What is controlled atmosphere storage? 🔗

Controlled atmosphere (CA) storage is an agricultural method in which the concentrations of oxygen, carbon dioxide, and nitrogen, as well as temperature and humidity inside a storage room, are continuously regulated. Rather than introducing toxic chemicals into the grain, it manipulates the composition of the storage environment itself. The approach works on a straightforward principle: most storage pests and mold-causing fungi depend on oxygen to survive. Reduce it sufficiently, or flood the space with carbon dioxide or nitrogen, and they cannot.

In CA storage, gas concentrations are precisely controlled – usually within ±1% of the desired value – and continuously corrected throughout the storage period. This precision requires hermetically sealed structures, gas analyzers, and injection or removal mechanisms, making CA the more technically rigorous option.

Modified atmosphere storage: the simpler cousin 🔗

Modified atmosphere (MA) storage follows the same gas-manipulation principle but with an important difference in how the atmosphere is managed. In MA storage, the initial gas composition is adjusted in an airtight room and then allowed to evolve naturally over time due to the respiratory activity of the grain, insects, and microorganisms present. There is no continuous gas correction once the initial conditions are set.

This makes MA storage less equipment-intensive than CA. A sealed silo or structure, a gas source, and periodic monitoring can be sufficient. Hermetic storage – the organic form of modified atmosphere storage – relies entirely on the metabolic activities of insects, microflora, and the stored grain itself to generate a low-oxygen, carbon dioxide-enriched atmosphere that asphyxiates living organisms within the sealed bulk.

The gases used and how they work 🔗

Three gases are central to controlled and modified atmosphere storage: oxygen (O₂), carbon dioxide (CO₂), and nitrogen (N₂). Their roles differ, but the objective is the same – create conditions that kill or suppress insects and prevent fungal growth.

Low-oxygen atmospheres using nitrogen 🔗

Nitrogen is used to displace oxygen from the storage space. Oxygen-deficient atmospheres are produced by flushing storage structures with nitrogen from liquid nitrogen tanks or inert atmosphere generators, reducing oxygen to below 1%. Most insect species found in stored grain will die when oxygen concentration falls below 2%.

However, nitrogen-based treatments demand extremely high purity levels to be effective. Nitrogen concentrations must be maintained above 97%, because insect mortality remains low below this threshold – and even at 95% nitrogen, adult grain weevils exposed for 20 days at 25°C showed a mortality rate of only about 82%. This places very stringent demands on the airtightness of the storage structure.

Carbon dioxide as an active fumigant 🔗

Carbon dioxide acts more directly than nitrogen and is generally more effective for rapid insect kill. Insects are killed more rapidly by CO₂ than by oxygen deprivation alone – a concentration of 60% CO₂ can achieve over 95% control of most stored grain insects within four days at 27°C or higher.

High CO₂ stress suppresses the production of NADPH in insects and subsequently reduces glutathione – compounds involved in cellular protection – which disrupts their metabolic function and leads to death. CO₂ is applied using cylinders of liquefied gas or by adding dry ice to silos before sealing. Research has shown that maintaining CO₂ above 35% for 15 days can completely kill adults and inhibit larval production of corn weevils and other pests in soybeans, without significantly affecting grain quality parameters such as protein solubility or crude fatty acid values.

Mold and fungi: a separate challenge 🔗

Insects are not the only threat. Fungi can cause severe quality loss, produce mycotoxins, and reduce grain viability. While most insect species perish at oxygen levels below 2%, fungi can still grow at oxygen concentrations as low as 0.2% – but only if grain moisture content exceeds 16%. This makes moisture management before and during storage critical for CA systems to effectively prevent fungal growth as well.

Grain must be dried to below 12% moisture content before being placed in a CA enclosure, and during storage, aeration and fans are used to regulate temperature and humidity alongside gas concentrations.

Key requirements for an effective controlled atmosphere structure 🔗

The effectiveness of any CA or MA system hinges almost entirely on airtightness. Structures used for controlled atmosphere treatments must have a high degree of gas tightness; changes in temperature, atmospheric pressure, and wind forces can significantly increase gas loss from the storage structure. Any leakage allows oxygen back in, undermining the treatment and wasting expensive gases.

According to the FAO Manual of Fumigation, four basic requirements must be met for CA treatment to succeed: a storage structure capable of containing the gas; a source of suitable gas or a means of producing the required atmosphere; a method of maintaining that atmosphere for the required period; and a method of aerating the structure to remove the altered atmosphere after treatment. Beyond these technical requirements, safety is also a serious concern. Nitrogen, while non-toxic in itself, creates an oxygen-deficient environment that is lethal to humans who enter without protection. High CO₂ concentrations are directly toxic. Portable oxygen monitors and breathing apparatus are essential wherever CA systems are in use.

CA vs. MA: understanding the difference in practice 🔗

Controlled atmosphere storage involves continuous, active regulation of gas concentrations throughout the storage period. Sensors measure CO₂ and O₂ levels in real time, and gases are injected or removed to keep concentrations within a target range. This demands investment in gas analyzers, injection systems, and skilled operators. Modified atmosphere storage, by contrast, adjusts the initial gas composition and lets respiration processes naturally sustain it. It is less precise but more practical for many farm-level or cooperative-level operations.

The distinction also matters in terms of cost. The main disadvantages of both MA and CA are economic – crops with insufficient sales value may not justify the higher investment, and the equipment cannot always be used year-round. For staple grains like wheat, rice, and maize stored in large volumes, however, the calculation is often favorable, given the scale of losses that CA and MA can prevent.

Advantages over conventional chemical fumigation 🔗

Chemical fumigants such as methyl bromide and phosphine have long been the default for controlling storage pests. But they come with growing drawbacks. Excessive use of chemical fumigants can lead to pesticide residues in treated grain, create human health and environmental risks, and promote resistance development in insect populations.

CA and MA storage sidestep all of these issues. The gases used – nitrogen, oxygen, and carbon dioxide – are natural components of air and leave no harmful residues in the commodity. The major advantage of using CA storage is the reduction of pesticides needed for controlling insects. For organic grain production or export markets with strict residue limits, this is a decisive advantage.

Limitations and practical challenges 🔗

Despite clear benefits, CA and MA storage systems face real-world barriers. The most significant is infrastructure. Existing grain storage structures – conventional silos, warehouses, and bins – are often not airtight enough for CA treatment to be effective. Key challenges in practical application include poor airtightness of silos, difficulties in obtaining gas sources, low automation levels, and complex pest conditions – all of which have restricted the large-scale industrialization of modified atmosphere and ozone pest control technologies.

The cost of gases is also a recurring constraint. Currently, tanker-delivered liquefied nitrogen or CO₂ is typically used to create these atmospheres, though alternatives such as pressure-swing absorption systems, propane burners, and fermentation-based gas generation are being explored as on-site options that reduce dependency on external supply chains. For smaller-scale operations, low-cost hermetic bags and sealed PVC structures offer a practical entry point. Research on low-cost hermetic storage structures in developing countries has shown that grain stored in sealed PVC bag-supported structures results in lower weevil populations, less grain damage, reduced weight loss, and better retention of crude protein and fat content compared to traditional open storage.

Applications in paddy and cereal grain storage 🔗

For paddy (rice) storage specifically, airtightness trials in the Philippines showed that bulk storage of dry paddy in hermetic plastic silos did not significantly change moisture content or grain temperature over the storage period, indicating that sealed storage under the right conditions preserves grain quality effectively. CO₂ treatment in shallow round bin tests has been shown to completely eliminate adult rice weevils and mixed pest populations without significant effects on fatty acid values or tasting scores of the rice – meaning grain quality is maintained along with pest control.

The technology is also increasingly being applied to wheat, maize, and sorghum at cooperative and commercial scales. Where temperature control is added alongside gas management, the combination is particularly effective: CA is most effective when combined with temperature control.

What do you think? Given the clear advantages of controlled and modified atmosphere storage over chemical fumigants, what do you think are the most realistic barriers to its adoption by small-scale grain farmers in countries like India – and what kinds of support or policy changes would make the biggest difference? With post-harvest grain losses still reaching 40% in some developing regions, is investing in airtight storage infrastructure at the cooperative level a more impactful priority than improving field-level crop yields?

References

1. https://www.co2meter.com/blogs/news/6077164-controlled-atmosphere-storage-using-carbon-dioxide
2. https://en.wikipedia.org/wiki/Controlled_atmosphere
3. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/controlled-atmosphere
4. https://www.sciencedirect.com/topics/food-science/modified-atmosphere-storage
5. https://www.ijert.org/hermetic-storage-technology-the-way-forward-in-solving-numerous-cereal-grains-storage-challenges-in-developing-countries
6. https://www.fao.org/4/x5042e/x5042E0o.htm
7. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/controlled-atmosphere-storage
8. https://link.springer.com/article/10.1007/s44462-025-00017-5
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC6422892/
10. https://sciencedirect.com/science/article/abs/pii/B9780444424174500487
11. https://scijournals.onlinelibrary.wiley.com/doi/10.1002/jsfa.10122