Exploring the Use of Gaseous Preservatives in Food Safety

From the dried apricots in your pantry to the fresh salad greens in your refrigerator, gaseous preservatives are quietly working behind the scenes to keep your food safe and fresh. Unlike solid or liquid preservatives that are mixed directly into food, gaseous chemical preservatives work by surrounding food with protective atmospheres that slow down spoilage, inhibit microbial growth, and preserve natural colour and flavour. The most widely used gaseous preservatives in the food industry are sulfur dioxide (SO₂), carbon dioxide (CO₂), ozone (O₃), and ethylene oxide. Each serves a distinct purpose depending on the type of food being preserved and the conditions required for safe storage.

What are gaseous chemical food preservatives? 🔗

Gaseous chemical food preservatives are compounds applied in their gas phase to protect food from microbial contamination, enzymatic reactions, and oxidative deterioration. They work by altering the environment around food – either by directly attacking microorganisms, by removing oxygen, or by creating atmospheric conditions hostile to bacterial and fungal growth. Their advantage over many liquid or solid preservatives is that they can penetrate packaging, reach food surfaces evenly, and in many cases decompose into harmless by-products without leaving significant residues.

Sulfur dioxide (SO₂): the versatile defender 🔗

Sulfur dioxide has one of the longest histories of any food preservative. It was used by ancient Romans to preserve wine, and today it remains indispensable in the processing of dried fruits, fruit juices, wines, and pickled vegetables. Its effectiveness comes from a combination of antimicrobial and antioxidant properties that make it uniquely suited for a wide range of food products.

How sulfur dioxide inhibits microbial growth 🔗

Sulfur dioxide works by penetrating microbial cell walls and disrupting the normal cellular functions of yeasts, bacteria, and moulds. Once inside the cell, SO₂ interferes with essential enzyme activity – specifically, it reduces disulfide linkages in critical enzymes, effectively shutting down microbial metabolism and reproduction. This makes it difficult for spoilage organisms to survive and multiply on treated food.

An important factor in SO₂’s effectiveness is pH. At lower pH values (more acidic conditions), sulfur dioxide exists primarily as molecular SO₂ gas and sulfurous acid (H₂SO₃), both of which are the most active antimicrobial forms. As pH rises above 4.0, SO₂ shifts into less effective bisulfite and sulfite ion forms. This is why sulfur dioxide performs best in naturally acidic foods like wine and fruit juices.

Preventing enzymatic browning 🔗

One of the most visible benefits of SO₂ is its ability to prevent enzymatic browning in fruits and vegetables. When fruits like apricots, apples, or potatoes are cut and exposed to air, enzymes such as polyphenol oxidase catalyse reactions that produce brown pigments. Sulfur dioxide inhibits these enzymes and also binds to the carbonyl intermediate compounds that drive the browning reaction. Without SO₂ treatment, light-coloured dried fruits darken rapidly upon exposure to air, making them less appealing to consumers.

Applications in winemaking 🔗

In winemaking, sulfur dioxide serves a dual purpose. It kills wild yeasts and unwanted bacteria before fermentation begins, giving winemakers control over which yeast strains drive the process. After fermentation, SO₂ is added again to prevent oxidation and microbial spoilage during ageing and storage. Wines produced without any sulfur dioxide typically have a shelf life of only about six months and require perfect storage conditions – a standard that is rarely achievable in commercial distribution.

Safety considerations and regulation 🔗

While sulfur dioxide is considered safe for general consumption when used within regulatory limits, it can trigger adverse reactions in sensitive individuals – particularly those with asthma. Symptoms may include breathing difficulties, headaches, and nausea. Regulatory agencies such as the U.S. Food and Drug Administration (FDA) and the European Food Safety Authority (EFSA) have established maximum permissible sulfite levels in food. The FDA mandates that any food containing more than 10 parts per million (ppm) of sulfites must clearly declare this on the product label. The international food safety authority JECFA has evaluated sulfur dioxide and confirmed it is safe when used according to Good Manufacturing Practice (GMP).

Carbon dioxide (CO₂) and modified atmosphere packaging 🔗

Carbon dioxide plays a fundamentally different role from sulfur dioxide. Rather than being applied directly as a chemical treatment, CO₂ is most commonly used as a component of modified atmosphere packaging (MAP) – a technology that replaces the air inside food packages with carefully controlled gas mixtures to slow down spoilage.

How modified atmosphere packaging works 🔗

Normal atmospheric air contains approximately 21% oxygen, 78% nitrogen, and just 0.04% carbon dioxide. In MAP, this composition is altered – typically by reducing oxygen levels while increasing nitrogen or carbon dioxide concentrations. The specific gas ratio depends on the type of food product being packaged. For example, fresh red meat is often packaged with around 70-80% oxygen to maintain its bright red colour, while fresh produce and bakery items benefit from higher CO₂ and lower O₂ levels.

The antimicrobial action of carbon dioxide 🔗

Carbon dioxide inhibits microbial growth through several mechanisms. When CO₂ dissolves in the moisture present on food surfaces, it forms carbonic acid, which lowers the pH of the food’s surface environment and creates conditions unfavourable for many spoilage organisms. Research suggests that CO₂ also directly inhibits certain enzyme systems in microbial cells, alters cell membrane function and nutrient uptake, and decreases intracellular pH through gas penetration of bacterial membranes. Importantly, the antimicrobial activity of CO₂ increases at lower temperatures, which is why MAP works best in combination with refrigeration.

Applications of CO₂ in food preservation 🔗

Fresh fruits and vegetables: After harvest, fresh produce continues to respire – consuming oxygen and releasing carbon dioxide and ethylene gas, which accelerates ripening and eventual spoilage. MAP with controlled CO₂ levels slows this respiration rate, delays ripening, and can extend shelf life by 50 to 200 percent compared to standard packaging. However, CO₂ levels above 10% can be harmful to certain fruits and vegetables, so the concentration must be carefully calibrated for each product.

Meat and poultry: For poultry and processed meats where colour retention from oxygen is not required, MAP typically uses higher CO₂ concentrations (25-30%) to suppress bacterial growth and extend chilled shelf life. The combination of reduced oxygen and elevated carbon dioxide slows oxidative reactions and inhibits the growth of aerobic spoilage microorganisms.

Bakery products and ready meals: Breads, cakes, and prepared meals packaged under modified atmospheres using combinations of nitrogen and CO₂ can achieve shelf life extensions from days to weeks. This has been a key enabler for the growth of the refrigerated ready-meal market, allowing weekly rather than daily production and distribution schedules.

Grain storage: Carbon dioxide is also used in grain storage silos. At appropriate concentrations, it prevents insect infestation and, depending on the level, can also inhibit mould growth and oxidation. Grain stored under CO₂-enriched atmospheres can remain viable for several years.

The role of nitrogen in MAP 🔗

While nitrogen (N₂) is not itself antimicrobial, it plays a critical supporting role in MAP. As an inert filler gas, nitrogen displaces oxygen to prevent oxidation and stop the growth of aerobic microbes. It also prevents package collapse that can occur when CO₂ is absorbed into the food’s moisture and fat content, maintaining the structural integrity of the package.

Ozone (O₃): the emerging green preservative 🔗

Ozone is gaining significant attention as an environmentally friendly gaseous preservative. It is a powerful oxidising agent – significantly stronger than chlorine – and has been recognised as Generally Recognised as Safe (GRAS) by the U.S. FDA for direct contact with food in both gaseous and aqueous forms.

How ozone works against microorganisms 🔗

Ozone destroys microorganisms by attacking their cell walls and membranes through oxidation. This mechanism is effective against a broad spectrum of bacteria, moulds, yeasts, parasites, and viruses – even at relatively low concentrations and short contact times. A key advantage of ozone is that it rapidly decomposes back into oxygen (O₂), leaving no chemical residues on treated food. This makes it an attractive alternative to chlorine-based sanitisers, which can produce potentially harmful by-products.

Applications of ozone in food preservation 🔗

Ozone is used across multiple food sectors. In fresh produce processing, ozonated water is used to wash fruits and vegetables, reducing microbial loads and extending shelf life. In grain and spice storage, gaseous ozone is applied to control pests, fungi, and mycotoxins within silos and warehouses. In the meat industry, ozone is used in storage facilities and freezing chambers to reduce bacterial levels and extend the durability of fresh, refrigerated, and frozen products. Ozone can be generated on-site using UV radiation, corona discharge, or electrolysis, which eliminates the need for transporting and storing chemical sanitisers.

Ethylene oxide: effective but controversial 🔗

Ethylene oxide (EtO) is a highly reactive gaseous fumigant that has been used to sterilise spices, herbs, dried vegetables, and certain packaged foods. It is effective at eliminating a broad range of microorganisms – including bacteria like E. coli and Salmonella – without requiring heat treatment, making it suitable for heat-sensitive products.

Regulatory restrictions 🔗

Despite its effectiveness, ethylene oxide is classified as a human carcinogen and a germ cell mutagen. The European Union banned the use of ethylene oxide as a pesticide in 1991 due to its highly toxic nature, and strict maximum residue limits (MRLs) have been set for food products. However, countries like the United States, Canada, and India still permit its use under regulated conditions. The U.S. EPA has set tolerances of 7 ppm for herbs and spices and 50 ppm for walnuts. The contrast in regulations between regions has led to ongoing food safety challenges, particularly with imported spices and food additives where ethylene oxide contamination has been repeatedly detected in recent years.

Comparing gaseous preservatives: strengths and limitations 🔗

Each gaseous preservative occupies a specific niche in the food industry. Sulfur dioxide excels in acidic food products like wines and dried fruits where it provides both antimicrobial and anti-browning protection, but it poses risks to sulfite-sensitive individuals. Carbon dioxide is the backbone of modified atmosphere packaging and works best in combination with refrigeration for fresh produce, meats, and convenience foods, though it requires careful calibration to avoid phytotoxic effects on certain produce. Ozone is the most environmentally benign option, leaving no residues, but it can alter the texture, colour, and flavour of certain foods at high concentrations. Ethylene oxide is extremely effective for sterilisation but carries significant health risks, leading to bans or severe restrictions in many regions.

The future of gaseous food preservation 🔗

The food industry is increasingly moving toward hurdle technology – combining multiple preservation methods to achieve better results with milder individual treatments. For example, ozone treatments combined with MAP and refrigeration can provide stronger microbial control while minimising any single treatment’s negative effects on food quality. Advances in packaging materials, including bio-based films with tailored gas permeability, are also enhancing the effectiveness of CO₂-based MAP systems. Meanwhile, ongoing research aims to find safer alternatives to sulfur dioxide and ethylene oxide, driven by growing consumer demand for cleaner-label food products with fewer synthetic additives.

What do you think? Given the health concerns around sulfur dioxide sensitivity and ethylene oxide toxicity, should the food industry accelerate its shift toward residue-free preservatives like ozone – or do traditional gaseous preservatives still have an irreplaceable role to play in global food safety?

References

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3. https://www.ams.usda.gov/sites/default/files/media/Sulfur%20dioxide%20report%202011.pdf
4. https://www.co2meter.com/blogs/news/7425044-what-is-modified-atmosphere-processing
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