FOOD PACKAGING: EXTENDING SHELF LIFE
Diagram
Modified atmosphere and vacuum packaging: two ways to change the air
Modified atmosphere packaging
The air inside the pack is replaced with a chosen mix of gases, then the pack is sealed. Some gas stays inside the pack.
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Vacuum packaging
The air is drawn out of the pack and the pack is sealed tight around the food, so almost no air is left inside.
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The result
Both slow spoilage by cutting oxygen, but MAP keeps a gas cushion around the food while vacuum presses the film onto the food.
Figure 1: Replacing the air with a chosen gas mix compared with removing the air. Drawn in our own style from Robertson, Food Packaging: Principles and Practice, 2nd ed., CRC Press, 2005, and Lee, Yam and Piergiovanni, Food Packaging Science and Technology, CRC Press.
Introduction
Modified atmosphere packaging, usually called MAP, and vacuum packaging are the two main ways of changing the air around food inside a pack to make it keep longer. In MAP the air is flushed out and replaced with a chosen mix of gases, such as carbon dioxide and nitrogen, and the pack is sealed with that gas mix inside. In vacuum packaging the air is sucked out of the pack and the pack is sealed, so the film closes tight onto the food and almost no air is left.
The core difference is replacement against removal: MAP puts a chosen gas in, while vacuum takes the air out. Both cut oxygen, which slows microbes that need oxygen and slows oxidation, but MAP keeps a gas cushion that protects soft food from crushing, while vacuum gives a tight skin around the food.
Quick comparison
Table 1: Modified atmosphere packaging compared with vacuum packaging
| Aspect | Modified atmosphere packaging | Vacuum packaging |
|---|---|---|
| What happens to the air | Replaced with a chosen mix of gases | Removed, almost no air is left |
| Gas inside the sealed pack | Yes, a gas mix stays inside | Almost none |
| Pack shape | Puffed out by the gas inside | Tight skin pressed onto the food |
| Oxygen level | Lowered by the gas mix | Lowered by removing the air |
| Protection from crushing | Good, the gas cushions the food | Poor, the film presses on the food |
| Suits soft food | Yes, such as salads and bakery goods | Less well, soft food can be crushed |
| Suits | Fresh meat, salads, cheese, bakery goods | Meat, cheese, coffee, cured foods |
| Equipment | Gas flushing and sealing machine | Vacuum chamber and sealing machine |
| Shelf life effect | Longer, by slowing microbes and oxidation | Longer, by slowing microbes and oxidation |
| Risk if the seal fails | The gas mix is lost and air returns | Air returns and the pack loses its vacuum |
Scientific explanation
Definition
Modified atmosphere packaging is packaging in which the air around the food is replaced by a chosen mixture of gases before sealing. Vacuum packaging is packaging in which the air is removed from the pack before sealing, so the pack closes tightly onto the product.
Working principle
Both work by cutting the oxygen around the food. Oxygen feeds microbes that spoil food and drives oxidation that turns fats rancid and changes colour. In MAP, carbon dioxide in the gas mix slows the growth of microbes, while nitrogen fills the pack without reacting with the food. In vacuum packaging, removing the air removes most of the oxygen at once, and the tight film also keeps out new oxygen. Neither method sterilises the food, so chilled storage is still needed for fresh foods.
Apparatus
MAP uses a gas flushing machine that replaces the air with the gas mix and then heat seals the pack. Vacuum packaging uses a vacuum chamber machine that draws out the air and heat seals the pack. Both use barrier films that slow gas passing in or out.
Measurement procedure
The books name no single standard test that compares the two methods. In practice the oxygen level inside the pack is checked with a gas analyser, the seal is checked for leaks, and the product is stored and checked over time for microbes, colour and flavour.
Factors affecting readings
The result depends on the barrier of the film, because a film that lets gas through loses the gas mix or lets oxygen back in. It depends on the seal, on the temperature of storage, and on the food itself: respiring foods such as fresh produce give off gas and change the atmosphere inside the pack.
Formulas and calculations
The books read give no formula for modified atmosphere against vacuum packaging. The comparison is made from the principle: less oxygen around the food means slower growth of oxygen needing microbes and slower oxidation. No gas percentage is given here, because the books read do not give one set of percentages that fits all foods: the gas mix is chosen for each food.
Industrial applications
- Modified atmosphere packaging: fresh meat, where the gas mix keeps the red colour and slows spoilage.
- Modified atmosphere packaging: bagged salads and fresh produce, where the gas cushion protects soft leaves.
- Modified atmosphere packaging: bakery goods and cheese, where the gas slows mould without crushing the product.
- Vacuum packaging: cured meats and hard cheese, where a tight pack suits a firm product.
- Vacuum packaging: coffee and dried foods, where removing the air protects flavour and keeps moisture out.
Advantages and limitations
- Modified atmosphere packaging, advantage: protects soft and delicate food from crushing and can be tuned to each food with the gas mix.
- Modified atmosphere packaging, limitation: needs gas supply and special equipment, and the benefit is lost if the seal leaks.
- Vacuum packaging, advantage: simple and effective, with a tight pack that shows at once if the seal has failed.
- Vacuum packaging, limitation: can crush soft food, and it does not suit foods that need some oxygen or that give off gas.
FAQ
Does MAP remove all the oxygen?
No. It replaces the air with a chosen gas mix, and the oxygen level is lowered, not always brought to zero.
Does vacuum packaging kill microbes?
No. It slows microbes that need oxygen, but the food is not sterile and chilled storage is still needed.
Why does a MAP pack look puffed up?
Because the gas mix stays inside the pack and fills it, while a vacuum pack has the air removed and looks tight.
Which is better for salad?
MAP in general, because the gas cushion protects the soft leaves from crushing.
Can a vacuum pack lose its vacuum?
Yes, if the seal leaks or the film is punctured, air returns and the pack goes slack.
References and further reading
- Fellows, P., Food Processing Technology: Principles and Practice, Woodhead Publishing (FTN book on disk).
- Robertson, G. L., Food Packaging: Principles and Practice, 2nd ed., CRC Press, 2005 (Google Books page read: Modified Atmosphere Packaging, p. 313).
- Lee, D. S., Yam, K. L. and Piergiovanni, L., Food Packaging Science and Technology, CRC Press (Google Books page read: Ch. 13, Vacuum-Modified Atmosphere Packaging, p. 397).
