Key Environmental Factors Affecting Modified and Controlled Atmosphere Storages

Modified Atmosphere (MA) and Controlled Atmosphere (CA) storage systems have transformed how we preserve perishable produce. By adjusting the levels of oxygen and carbon dioxide around fruits and vegetables, these technologies slow down metabolic processes and extend shelf life far beyond what refrigeration alone can achieve. But here’s the catch – the success of MA and CA storage doesn’t depend solely on getting the gas mix right. Several environmental factors play a decisive role in whether these systems work effectively or fall short. Temperature, relative humidity, light exposure, and sanitation each influence the outcome in significant ways. Let’s break down how each of these factors affects MA and CA storage performance.

Temperature: the most critical variable 🔗

Temperature is the single most important environmental factor in MA and CA storage systems. It has a dual effect – it directly influences both the respiration rate of stored produce and the gas permeability of packaging films used in MA systems. Higher temperatures accelerate the metabolic activity of fruits and vegetables, causing them to consume oxygen faster and produce more carbon dioxide. This can quickly throw off the carefully designed gas balance inside a storage environment or package.

In MA packaging, the system works by balancing the produce’s respiration rate with the gas transmission properties of the packaging film. When conditions are warmer than what the package was designed for, the produce’s respiration increases much faster than the film’s permeability to gases. This means oxygen inside the package gets depleted rapidly, and carbon dioxide builds up beyond safe levels. The result can be anaerobic respiration (fermentation), which produces off-flavours and damages the produce.

Why temperature fluctuations are dangerous 🔗

Even brief temperature fluctuations can be problematic. Research on MAP systems shows that if the temperature changes by more than a few degrees, the package atmosphere can shift and become unsuitable – or even harmful – for the product inside. The respiration rate of horticultural products responds to temperature changes far more dramatically than the gas permeability of packaging films does. This mismatch is a core challenge in MA storage design.

For CA storage, where gas levels are mechanically monitored and adjusted, temperature control is equally vital. Even with optimal gas compositions, produce stored at incorrect temperatures will still deteriorate rapidly. Most MA-packaged perishable products are stored within the 0-5°C range, though specific produce types have their own requirements. Apples, for instance, are best held between -1°C and 4°C, while bananas need warmer conditions around 13-14°C to avoid chilling injury.

Chilling injury: when cold is too cold 🔗

While low temperatures are generally beneficial for slowing metabolism, not all produce responds well to very cold storage. Tropical and subtropical fruits can suffer chilling injury at temperatures below 12-13°C. This condition causes membrane breakdown, releasing metabolites like amino acids and sugars that create a favourable environment for harmful microorganisms. The severity of chilling injury depends on both the duration of exposure and how low the temperature drops – it doesn’t have to reach freezing to cause damage. This is why precise temperature management must account for the specific needs of each commodity.

Relative humidity: balancing moisture retention and microbial risk 🔗

Relative humidity (RH) is the second major environmental factor governing MA and CA storage outcomes. It determines how much moisture the stored produce retains – and getting it wrong can lead to problems on both ends of the spectrum.

Why high humidity is usually necessary 🔗

Fruits and vegetables are largely made of water. After harvest, they continue to lose moisture through transpiration – the evaporation of water from their tissues. If the surrounding air is too dry, this moisture loss accelerates, causing the produce to shrivel, wilt, and lose its texture, colour, flavour, and nutritional quality. For most fresh produce stored in CA rooms, the relative humidity is kept high – typically around 90-95% – to minimise transpiration. Leafy greens like spinach and lettuce require even higher humidity levels of 95-100% to stay fresh and crisp.

In MA packaging, the situation is similar. Most polymeric films used in MAP have relatively low water vapour permeability compared to the moisture loss rate of fresh produce. This means that high relative humidity naturally builds up inside the package – which is generally desirable for preventing dehydration of the product.

The condensation problem 🔗

However, excessively high humidity creates its own set of issues. When relative humidity reaches saturation levels, water vapour condensation occurs on the produce surface and inside the package. This condensation creates an ideal environment for microbial growth – bacteria, yeasts, and moulds thrive on wet surfaces. According to research on humidity and bacterial growth, elevated humidity levels significantly promote microbial proliferation, especially in low-ventilation environments like sealed storage systems.

Temperature fluctuations make the condensation problem worse. When the temperature inside a storage room or package drops, the air’s capacity to hold moisture decreases, and water droplets form on produce surfaces. This is particularly problematic in the low-oxygen, high-CO₂ environments of MA and CA storage, where standard aerobic spoilage organisms may be suppressed, but anaerobic or facultative pathogens can still thrive.

Commodity-specific humidity requirements 🔗

Different types of produce have different humidity needs. Apples and pears perform well at 90-95% RH, while commodities like onions and garlic need lower humidity to prevent sprouting and decay. Root vegetables generally tolerate moderate humidity, while berries and stone fruits require careful humidity management to avoid rapid mould development. Getting the RH right for each specific product is essential for maximising the benefits of atmosphere modification.

Light exposure: an overlooked factor 🔗

Light is often underestimated as an environmental factor in MA and CA storage, but it can significantly affect certain types of produce. The key reason is that harvested plant tissues retain the ability to respond to light – including continuing photosynthesis at low levels.

How light affects green vegetables 🔗

Green leafy vegetables like lettuce, spinach, broccoli, and pak choi contain chlorophyll, the pigment responsible for capturing light energy during photosynthesis. Even after harvest, these vegetables can continue to carry out photosynthetic activity when exposed to light. This continued photosynthesis affects gas exchange dynamics inside a sealed MA package or CA room – the produce consumes carbon dioxide and produces oxygen under light conditions, potentially disrupting the carefully maintained atmospheric balance.

Research has shown that light exposure during postharvest storage can have both positive and negative effects on vegetable quality. On the positive side, studies on leafy greens found that maintaining daily light/dark cycles during storage helped preserve tissue integrity, chlorophyll content, and phytochemical levels – in some cases performing comparably to refrigeration in maintaining quality. Light exposure can slow the rate of chlorophyll degradation, keeping vegetables looking green and fresh for longer.

The complications of light in storage 🔗

On the other hand, light can also cause problems. Certain produce items, such as potatoes, develop undesirable greening (due to chlorophyll and solanine accumulation) when exposed to light. Onions can start sprouting. For these commodities, dark storage conditions are essential.

Light exposure can also induce stomatal opening in leafy vegetables, which increases water loss through transpiration. This means that while light may help maintain chlorophyll and nutritional quality, it can simultaneously promote dehydration – a trade-off that storage managers need to navigate carefully.

In MA packaging systems, the interaction between light and the produce’s physiology becomes particularly complex. Since the atmospheric balance inside an MA pack depends on the equilibrium between respiration and film permeability, any change in the produce’s gas exchange behaviour – such as a shift caused by photosynthetic activity under light – can alter the package atmosphere in unintended ways.

Sanitation: preventing pathogen growth in low-oxygen environments 🔗

Sanitation is a factor that often gets less attention than temperature or humidity, but it is absolutely critical in MA and CA storage. The unique atmospheric conditions in these systems – high humidity, low oxygen, and elevated carbon dioxide – create an environment that can be surprisingly favourable for certain types of pathogens if sanitation is neglected.

Why MA and CA environments need extra vigilance 🔗

While the reduced oxygen and elevated CO₂ levels in MA and CA storage inhibit many aerobic spoilage organisms and can even suppress fungal growth (high CO₂ concentrations above 15% have fungistatic effects on organisms like Botrytis cinerea), they don’t eliminate all microbial threats. In fact, the high-humidity conditions necessary for preventing produce dehydration also provide an ideal growth medium for bacteria and moulds that can tolerate low-oxygen conditions.

Listeria monocytogenes, for example, is a pathogenic bacterium that can survive and even grow under the cold, low-oxygen conditions typical of many CA storage environments. Similarly, certain moulds and yeasts are facultative – they can adapt to both aerobic and anaerobic conditions. The combination of moisture on produce surfaces (from condensation) and reduced competition from aerobic organisms (suppressed by the low-oxygen atmosphere) can give these pathogens a competitive advantage.

Essential sanitation practices 🔗

Effective sanitation in MA and CA storage requires a multi-pronged approach. Regular cleaning and disinfection of storage rooms, packaging equipment, and containers is fundamental. Food-grade sanitisers should be used on all surfaces that come into contact with produce. Storage bins, pallet covers, and MA packaging equipment must be cleaned on a scheduled basis to prevent the build-up of microbial biofilms.

Proper handling of produce before it enters storage is equally important. Produce should be inspected for physical damage, as wounds and bruises provide entry points for pathogens. Pre-storage treatments like washing with sanitising solutions can help reduce the initial microbial load. It’s also important to ensure that storage facilities have adequate ventilation and airflow management, as stagnant air pockets combined with high humidity accelerate microbial proliferation.

For MA-packaged products intended for consumption without cooking (such as fresh-cut salads), sanitation is even more critical. Since the microbiological safety of MAP food partly depends on whether it will be cooked before eating, ready-to-eat products carry a higher risk because there is no kill step between the package and the consumer’s plate.

Monitoring and quality assurance 🔗

Modern CA storage facilities increasingly rely on automated monitoring systems that track not just gas concentrations and temperature, but also humidity levels and microbial indicators. Regular microbiological testing of storage environments, combined with stringent sanitation protocols, helps ensure that the benefits of atmosphere modification aren’t undermined by pathogen growth. An integrated approach that combines proper gas management, temperature control, humidity regulation, and sanitation delivers the best results.

How these factors interact 🔗

It’s important to recognise that temperature, humidity, light, and sanitation don’t operate in isolation – they interact with each other in complex ways. Higher temperatures increase both respiration rates and microbial growth rates while also raising the air’s capacity to hold moisture. Humidity levels affect both produce quality and the likelihood of microbial contamination. Light exposure can change gas exchange dynamics, which in turn affects the atmospheric composition that temperature and humidity are working to maintain.

Effective MA and CA storage management requires an integrated understanding of all these factors. A state-of-the-art CA facility with perfect gas concentrations will still fail if temperature fluctuates wildly, if humidity is uncontrolled, if light exposure is mismanaged, or if sanitation is inadequate. The most successful storage operations take a holistic approach, carefully calibrating every environmental variable to suit the specific needs of the commodity being stored.

What do you think? As MA and CA storage systems become more advanced with AI-driven monitoring and smart packaging materials, which of these environmental factors do you believe presents the greatest challenge for the industry to manage effectively? And could innovations in packaging film technology eventually eliminate the temperature sensitivity problem in MAP systems?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC7089433/
2. https://www.postharvest.net.au/postharvest-fundamentals/atmosphere/modifying-the-atmosphere/
3. https://www.sensitech.com/en/blog/blog-articles/blog-modified-atmosphere.html
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC4554610/
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC9691097/
6. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0311100
7. https://pubmed.ncbi.nlm.nih.gov/25879637/
8. https://ebooks.inflibnet.ac.in/ftp1/chapter/253/