Modified atmosphere packaging and what the film has to do
Modified atmosphere packaging (MAP) replaces the air inside a pack with a chosen gas mixture. Air is roughly a fifth oxygen. Oxygen supports aerobic spoilage organisms and drives oxidative reactions: fats oxidise in it and pigments change colour in it. Change the balance of oxygen, carbon dioxide and nitrogen to suit the product, and several of those clocks run slower.
There are two routes to that atmosphere, and the distinction runs through everything below. In active MAP the air is evacuated or displaced by gas flushing before the pack is sealed, so the mixture is there from the start. In passive MAP, which is how most fresh produce is packed, no gas is injected at all. The atmosphere develops after sealing, out of what the product and the film do to each other.
Either way, the gas is the easy part. The difficult part is what happens next, because the pack starts changing the moment it is sealed. That is a question about the film and the seal, not about the gas supply. Modified atmosphere packaging is therefore a material decision before it is a process decision.
This page covers what the atmosphere does and what it demands of the film. Films that deliberately let gases through, for produce that is still breathing, are covered on perforated film, and the barrier polymer itself on EVOH.
MAP gases: carbon dioxide, nitrogen and oxygen
Almost every commercial mixture is built from three gases. They are not interchangeable, and each of them is there for a different reason.
| Gas | What it does | What limits it |
| Carbon dioxide | Usually the main antimicrobial component. It has a bacteriostatic effect and slows the respiration of many products | It is soluble in both water and fat, so it leaves the headspace and enters the product |
| Nitrogen | Inert, tasteless and poorly soluble. It displaces oxygen and helps hold the pack’s volume | It has no antimicrobial activity of its own |
| Oxygen | Usually removed, but deliberately retained for red meat colour and for products that must not go anaerobic | It inhibits anaerobic organisms while feeding aerobic ones |
Nitrogen is the one most often dismissed as filler. It is a filler, but that is a job. Because it is poorly soluble, it stays in the headspace while carbon dioxide does not, so in a mixture carrying substantial carbon dioxide it holds volume that would otherwise be lost. Oxygen is misread in the other direction. It is not simply the enemy. In some products it is in the mixture on purpose.
Carbon dioxide absorption, headspace loss and pack collapse
This is the part that surprises people, and the single most useful thing to understand about the technique. Carbon dioxide dissolves into the water and the fat of the product itself. Its solubility rises as temperature falls, so a chilled pack absorbs more than a warm one.
Two consequences follow, and both are visible on a shelf. The pack loses headspace volume and can collapse. In some chilled meat and poultry systems, that collapse has been linked to the product being compressed, with an effect on drip loss.
Headspace and product are one system, not two alternatives. The antimicrobial effect depends heavily on carbon dioxide dissolved in the water phase, and the concentration in the headspace is what drives and sustains that dissolution. Published work identifies two main factors deciding how much dissolves: the starting proportion of carbon dioxide, and the gas-to-product ratio. That ratio is a pack design decision, which is why headspace is specified rather than left to whatever fits.
The mechanism is chemical rather than physical, and it is not a single mechanism. Dissolved carbon dioxide lowers pH inside the microbial cell, but work on spoilage organisms also reports membrane depolarisation, inhibition of enzymes in the respiratory chain and a general weakening of energy metabolism. The result is that growth slows. It is not sterilisation.
Respiring and non-respiring products, and equilibrium MAP
This split decides the whole structure, and getting it wrong is the most common specification error on the subject.
- A non-respiring product, meat, fish, cheese, bakery, pasta, has no plant respiration consuming oxygen and generating carbon dioxide. Its atmosphere still changes, through gas dissolving into the product, microbial activity and permeation through the film, but nothing is actively driving it. The mixture that goes in should still be roughly there at the end, so the film has to hold it.
- A respiring product, fresh fruit and vegetables, keeps consuming oxygen and producing carbon dioxide for as long as it is alive. Seal that in a true barrier and it suffocates and spoils faster than it would in air.
Equilibrium MAP, and why temperature breaks it
Produce is therefore packed for an equilibrium modified atmosphere packaging (EMAP), in which the film’s gas transmission is matched to the product’s respiration rate so that a workable atmosphere settles by itself and then stays put. It is designed, not injected.
There is a trap in that design, and it is worth knowing before it costs a load. Respiration rate and film permeability both rise with temperature, but not at the same rate. Published work reports that produce respiration climbs faster than film permeability does, so a warm spell in transport can push a correctly designed pack into oxygen starvation.
The films for this are set out on perforated film. A perforated structure is built for controlled gas exchange rather than for retaining an injected mixture, so the two design objectives pull in opposite directions.
What a MAP film has to deliver: OTR, CO₂ permeability and seal integrity
For a non-respiring product a MAP film has three jobs, and a failure in any one of them ends the shelf life early.
It has to keep oxygen out. That is the barrier layer’s work, and it is normally specified as an oxygen transmission rate (OTR) at a stated temperature and humidity, because both change the answer. For most products with a long chilled shelf life a polyolefin on its own will not reach it.
It has to keep carbon dioxide in, which is a separate question with a separate number. Common packaging polymers usually transmit carbon dioxide faster than oxygen, and the ratio of the two is called permselectivity. Work on commercial films puts that ratio above three for most of what is available. A structure can therefore be tight to oxygen and still leak the gas doing the preserving.
The third job is seal integrity, and it is a common practical reason an otherwise adequate structure loses its atmosphere. A seal contaminated with fat, brine or powder leaks, and a leak makes the barrier irrelevant. Sealing through contamination is part of the atmosphere specification, not a separate quality topic.
What modified atmosphere packaging does not do
Three limits are worth stating plainly, because briefs regularly assume otherwise.
- It is bacteriostatic, not sterilising. Carbon dioxide delays microbial growth. It does not remove the organisms already there.
- It cannot compensate for what went into the pack. A product with a high initial microbial load simply reaches the same end point later, and the atmosphere is a supplement to hygiene and temperature control rather than a replacement for either.
- Changing the atmosphere does not by itself protect against light or moisture. Those are separate requirements, and they have to be designed into the structure deliberately.
Which polymer carries the oxygen barrier, and what each option costs in recyclability, is set out under barrier options and PPWR recyclability.
Frequently asked questions
Does modified atmosphere packaging use preservatives?
Not in the sense most people mean. No preservative ingredient is added to the food, and the shelf life comes from changing the gas around it. The legal framing is more precise than that, though. In the EU these are packaging gases, a functional class of food additive under Regulation (EC) No 1333/2008, defined as gases other than air introduced into a container. A food whose durability has been extended this way also has to carry the words “packaged in a protective atmosphere” under Regulation (EU) No 1169/2011. Whether the product may separately claim no added preservatives depends on its recipe and on the label rules where it is sold, not on the packaging technique.
Is modified atmosphere packaging the same as vacuum packing?
No, and the difference is not only technical. Vacuum packing removes the atmosphere and lets the film collapse onto the product. Modified atmosphere packaging replaces it and keeps a headspace. That protects delicate products from being crushed, and it lets a gas mixture do work a vacuum cannot. Both usually rely on a gas barrier and a sound seal, although how much barrier depends on the product and the shelf life being asked for. The choice often follows the product’s appearance requirement as much as its shelf life.
Matching the structure to the atmosphere the product needs
Send the product, the gas mixture you run and the shelf life you have to reach. We come back with a structure and the documentation that goes with it.
Related resources
EVOH, the oxygen barrier
What creates the barrier that holds the atmosphere in, how ethylene content scales it, and why humidity reverses it.
Barrier options and PPWR recyclability
EVOH set against a metallised layer and a ceramic coating, and what each option costs in the recycling stream.
Perforated film
The opposite problem: produce that is still breathing needs a film that lets gases through rather than one that holds them.
Polyethylene as a polymer
Why the sealing layer decides whether the atmosphere stays in, and what controls how it closes over contamination.