Harvested vegetables are alive, and that's why MAP and EMAP are the keys to keeping vegetables, fruits, and salad fresh.
Indeed, vegetables remain alive once they're harvested. In other words, they still breathe or respire. Fortunately, I have found a good article about their respiration.
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This article explains why respiration is essential for vegetables to continue living. And for keeping produce good, it is important they stay alive. Once respiration stops, decay will quickly set in.
The trick to storing vegetables, salads, and fruits 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.
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This is why EMAP is vital for keeping vegetables, salads, and fruits fresh in both whole products and cut products. EMAP, or Equilibrium Modified Atmosphere Packaging, brings the packaging in balance with the packed product. EMAP Packaging is perforated with micro holes, typically from 70 to 120 microns. This is the same size as the diameter of a human hair!!! Just one hole of this size can totally change your packaging from one that helps and extends the shelf life of your product to one that actually reduces the quality and shelf life of your product.
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The key is the knowledge of measuring respiration and calculating permeability and, of course, some knowledge of the product you pack.
Optimizing oxygen & carbon dioxide contents
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One way to slow down respiration is by controlling the levels of oxygen and carbon dioxide.
For some products like iceberg and romaine, the ideal oxygen level is very low, typically below 0,5 % O2. If you have a higher level of O2, you will risk that the white parts of the product change their colour to pink. A higher level of O2 will also increase browning on the cut surfaces.
For other products like rocket salad, spinach, and other types of baby leaf, it is beneficial to have a level of O2 around 10 - 15 %. With this level of O2, you might not get the longest possible shelf life, but if you expose these products to lower levels of O2, the consumer who opens the packaging will have a negative experience due to off odors (bad smell).
The reduced oxygen concentration will slow down the respiration rate, which will again extend the shelf life of the packed product.
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 the 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.
The difference between MAP and EMAP
MAP is an abbreviation of Modified Atmosphere Packaging. MAP is typically made with a packaging film with barrier properties (like mono PET or laminates with sealing layers and/or different film layers with barrier properties). MAP films typically have a low OTR (Oxygen Transmission Rate) of < 100 ml/m2/24h/atm. The transmission rate of the film is also called permeability of the film. At the moment of packing the product into the packaging (typically a bag or a tray with a sealed lid), the packs are flushed with gas, which is typically N2 or a mix of gas types. When you flush with, for example, N2, the O2 around the product in the package will be removed (flushed away) and replaced with N2. N2 is, in most cases, harmless for the product.
EMAP is an abbreviation of Equilibration Modified Atmosphere Packaging. EMAP is typically made with packaging film without barrier properties like OPP or PE. The packaging films for EMAP typically have OTR values ranging from 1000 ml/m2/24h/atm and up to about 4000 ml/m2/24h/atm. Most vegetables, fruits, and salads require OTR values between 4000 and up to 40000 ml/m2/24h/atm for creating the best possible environment inside the packed packaging. Laser perforation is commonly used to modify the OTR of EMAP packing films. The holes made with the laser typically have a size of 70 to 120 microns; this means that they are almost invisible to the human eye. The size of the laser hole and the number of holes per package determine the total OTR for the package.
When to use MAP or EMAP
For products without respiration like meat, cheese, etc., these products are all packed in MAP. This means that the level of O2 and CO2 can be 100 % controlled from the moment of packing, and the level of O2 and CO2 stays the same through the lifetime of the product.
Products with respiration like vegetables, fruit, and salads will have great benefits from using a packaging film specially made for EMAP.
In the moment of packing, the product is packed with the surrounding air (20.9% O2 and 0.04 % CO2). Due to the respiration, the products will start to use the O2 in the package and produce CO2. As the level of O2 in the package decreases, air from the outside of the packaging will start to enter the packaging via the laser perforated holes. At the same time, due to the respiration, the CO2 increases inside the package, so the CO2 will also flow through the laser perforated holes. After hours and days, the product and the EMAP package will enter a balance at its equilibrium. Depending on the respiration of the product and the number and size of laser perforated holes, the equilibrium level can be controlled.
Use of EMAP in combination with gas flushing
The optimum conditions for prolonged shelf life are first met when the packed product reaches its equilibrium. Depending on the product respiration, this can have a duration of hours and up to days. In this time where the product is not under optimum O2 and CO2 conditions, the product reduces its quality and shelf life. Therefore, it is important that the product reaches its equilibrium as fast as possible.
The way to achieve this is with gas flushing. This means that the packed product is at the optimum level of O2 and CO2 at the moment of packing. Because of the laser-perforated holes, the level of O2 and CO2 will remain at the correct level throughout the shelf life of the product.
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This can be seen as the ideal packing process for many types of vegetables, fruit, and salads.