Ever tried to squeeze all the air out of a pack of pre-cut vegetables? Noticed that instead of collapsing, it would just let out all air and return to its original size once you stopped pressing? Ever had a closer look at your bag of fresh spinach and noticed a little hole with a diameter of a few centimeters? You’re looking at pure science here, modified atmosphere packaging of vegetables!
This technology, abbreviated as MAP can extend the shelf life of vegetables extensively without any (chemical) additions or fancy materials. All you need is plastic foil with the right amount and size of holes in it.
How and why it works is what we’ll be talking about today. Whether you’re a consumer, wondering how it works, or a product developer having to improve your product’s shelf life, I hope this article will be helpful for you.
Harvested vegetables are alive
Indeed, vegetables still live once they’re harvested. In other words they still breath or respire. Fortunately, I already wrote a whole post about their respiration.
In that post I explain that respiration is essential for vegetables to continue living. And for keeping vegetables good it is important they stay alive. Once respiration stops, decay will set in very quickly.
The trick to storing vegetables for a long period of time is thus to keep respiration going for as long as possible. This can be done by slowing down respiration as much as possible (without stopping it).
Optimizing oxygen & carbon dioxide contents
One way to slow own respiration (as you can also read in the other post) is by controlling the levels of oxygen and carbon dioxide.
For oxygen the level should generally be kept as low as possible. Oxygen is required for respiration to occur, so if there is none at all the plant will stop respiring. But once there is enough for respiration to occur, you’d generally (yes, there are always exceptions) like to keep the oxygen concentration as low as possible.
The low oxygen concentration will slow down the respiration rate, which will again extend the shelf life of vegetables.
Carbon dioxide unfortunately, is a slightly more complex story. Vegetables differ a lot in how they react to carbon dioxide. Though, generally saying you should increase carbon dioxide content around vegetables to decrease respiration rate.
This is indeed opposite of what should be done with oxygen. However, whereas oxygen is consumed during respiration, carbon dioxide is produced during respiration. This explains the difference.
Gas composition changes over time
So, to increase the shelf life of our vegetables we should lower oxygen content and increase carbon dioxide content, compared to the air. How should we do this?
Unlike with a product such as meat or dried cereals, the air composition inside a pack of vegetables changes over time. This is again due to respiration. Two things happen. Let’s take a bag of salad as an example.
This shows that gas concentrations can vary greatly in a closed bag which is the main challenge when packaging fresh vegetables.
Modified atmosphere packaging
That brings us to modified atmosphere packaging. This technology is used for a variety of food products, not only vegetables or fruits. With this technology products are packaged in a ‘modified atmosphere’, hence the name. This modified atmosphere has a different gas composition than that of regular air around is. It might be very low in oxygen, high in carbon dioxide, or vice versa or maybe have a very high nitrogen content.
The modified gas has been chosen in such a way that it is optimal for that specific food product. If the gas composition doesn’t change, that works great in extending the shelf life of a product.
But, as we know by now, that is not the case for fresh vegetables, so researchers had to come up with a new solution. They had to assure that gases could be exchanged with the outside world. Thus, in the case of our salad the following should be able to happen:
This type of modified atmosphere packaging (MAP) is also called equilibirum modified atmosphere packaging (EMAP). An equilibrium is reached between the two gas concentrations.
Designing MAP or EMAP
Ideally, a pack of vegetables should be designed in such a way that the optimal gas concentration is stable and exactly that which is best for the product. Unfortunately, it is not as easy as it sounds, the respiration rate of vegetables is not often very constant. A different temperature and the rate changes. As the vegetable gets older, or ripens, the respiratio nrate changes. When a product is cut, the respiration rate increases, but a few hours later and it will have dropped again.
If the rate of consumption of oxygen or rate of production of carbon dioxide changes, the rate of these gases flowing in and out of the bag should change as well!
This is the hardest part of designing a good EMAP package. A Dutch company has come up with an ingeniously simple piece of equipment to help companies with this though. The Perfotec measures the respiration rate of a product over a specific duration and at a specified temperature. Based on these rates, an estimate of the exchange rate of gases can be made. Of course, the rate will never be the exact same value over time and might differ per harvest or supplier, it will give a good baseline.
Controlling gas exchange rate
Once the respiration rate is known, the exchange rate of gases can be derived. This rate is then used to determine the optimal packaging material.
For EMAP the most commonly used packaging material are plastics. But not all plastics are equal. Through some it migt be very easy for oxygen and carbon dioxide to diffuse through. Their gas permeability is very high. Other plastics might prevent movement of gases all together, they have a very low gas permeability.
Often, the permeability of a certain plastic is not high enough to keep up with the respiration rate. In other words, oxygen cannot travel in fast enough and carbon dioxide cannot leave fast enough. In those cases it is often chosen to punch holes in the plastic. These holes may be as small as only a few micron or as large as half a centimeter!
The number and the size of holes determines how high the permeability of the punched plastic has become. This will decide whether the pack is a success for storing the chosen vegetable.
The tool I mentioned before, the Perfotec, can actually calculate the number and size of holes for a product which was put in their equipment.
Other ways of spoilage
Unfortunately, EMAP doesn’t solve all the problems. Even though it can greatly extend shelf life by limiting the respiration rate, there might be other processes at play which causse spoilage. A very common example is discolouration. Ice berg lettuce as well as several other white vegetables tend to turn brown/red when they are exposed to oxygen after cutting. It would be best to not put any oxygen in these packages to prevent discolouration. However, that would stop respiration. In other cases, vegetables might not be able to cope with high carbon dioxide concentrations due to other processes at play.
The next time you buy pre-packed (cut) vegetables, have a careful look at the pack. Most likely will you be able to see, or feel, little holes. This is modified atmosphere packaging a work!