The pre-packaged, pre-cut produce market is a multi-billion dollar growth market fuelled by consumers’ interest in natural products, convenience, and nutrition. Since fresh-cut produce is a non-processed, living product, preserving freshness and minimizing quality degradation associated with temperature challenges during transit is crucial. And there enters Modified Atmosphere Packaging.
Modified atmosphere packaging is a technique where either the atmosphere within the package is removed entirely, referred to as vacuum packaging, or the atmosphere is altered, referred to as controlled atmosphere or gas flushed packaging. In each case, the objective is to extend the shelf life of perishable foods, while at the same time the quality of the product is maintained.
The composition of gas mixtures used for this purpose depends on the food and the nature of deterioration mechanisms. Deteriorative reactions of food can be of biochemical, physiological, physical and microbiological origin. In MAP systems O2 concentration is usually kept at low levels in order to reduce the rate of oxygen to avoid undesirable (oxidative) reactions and to reduce the growth of aerobic microorganisms. Elevated CO2 is mostly used due to its bacteriostatic effects and nitrogen is used as balance gas.
This is the technique mainly used for meat, chicken, bakery and similar products. However MAP of fresh fruits and vegetables is a more laborious issue and in contrast to the above O2 has to enter and CO2 to exit as fresh-cut produces need to “breathe” to stay fresh. The reason is that horticultural produces are living organisms and consequently continue to respire even after harvest in order to produce energy for vital biological reactions. As a result when fresh fruits and vegetables are put in packs they naturally modify surrounding atmosphere by consuming O2 and producing CO2 (passive MAP). MAP slows the ongoing life processes not by changing the product but by adjusting its environment. Under the influence of MAP, the product is held in a state similar to that of animal hibernation, during which deterioration is effectively stopped.
Since anoxic conditions lead to fermentation, complete consumption of oxygen should be avoided by selecting packaging materials permitting O2 to enter and CO2 to leave the package. In MAP of fresh produces another concern is the accumulation of ethylene in packs. Ethylene is a natural plant hormone which regulates numerous physiological reactions. Its physiological impact is more prominent on climacteric produce such as avocado, melon, tomatoes, broccoli; and apricot, because exposure of these commodities to exogenous ethylene accelerates their ripening thus diminishes their shelf-life. Therefore keeping ethylene concentration at low levels is mandatory in order to prolong shelf-life of ethylene sensitive produces. For this purpose, generally atmospheres which are poor in O2 or/and rich in CO2 are used to diminish ethylene production.
Active Modified Atmosphere Packaging
Sometimes, certain additives are incorporated into the polymeric packaging film or within packaging containers to modify the headspace atmosphere. This is referred to as Active Modified Atmosphere Packaging. The concept of active MAP has been developed to rectify the deficiencies in passive MAP. For instance, when a film is a good barrier to moisture, but not to oxygen, the film can still be used along with an oxygen scavenger to exclude oxygen from the pack. Similarly, carbon dioxide absorbents/emitters, ethanol emitters and ethylene absorbents can be used to control oxygen levels inside MAP. The appropriate absorbent materials are placed alongside with the food. By their activity, they modify the headspace of the package and thereby contribute to the extension of the shelf-life of the contents.
Thus, the Modified Atmosphere Package system is an active one where respiration of the packaged product and gas permeation through the packaging film takes place simultaneously. Hence, oxygen consumed during respiration is replaced simultaneously by the ingress of oxygen. Likewise, an equal amount of carbon dioxide that is evolved by the packaged produce permeates out of the package. As a result, the air composition remains constant. This state is known as equilibrium or steady state.
The attainment of the Equilibrium State depends on proper designing of MAP. But some are even going one step further, as the Equilibrium State isn’t always the optimal answer to the packaged fresh produce (see table taken from a booklet, I will refer to at the end of this article).
Dynamic Modified Atmosphere Packaging
In contrast to the above the Breatheway membrane technology from APIO Inc, is not relying on scavengers and/or the permeability of the packaging film. Ergo, the technology requires a film, in the format of a fully enclosed bag, of any type of material as long as it is impermeable to gases. The system is capable of establishing, inside the package, O2 and CO2 levels independently of each other, creating the correct combination of oxygen, carbon dioxide, relative humidity and other factors for maximum shelf life.
The BreatheWay membrane technology incorporates a designed selectivity ratio, which means that the permeability of carbon dioxide across the membrane occurs at a rate up to 3.8 times faster than the rate of oxygen (CO2TR/OTR perm ratio 3.8). Using this feature the correct membrane size and ratio for virtually any produce item in virtually any package configuration can be selected to maintain the ideal oxygen and carbon dioxide ratios.
Furthermore BreatheWay technology features a crystallisable polymer that has been designed to behave as a temperature switch at an identified temperature. While the change is invisible to the naked eye it dramatically shifts the permeability of the membrane – becoming more permeable at high temperatures and less at lower temperatures.
The membranes are reversible, meaning they will move from a crystalline state to amorphous and back as many times as needed. The temperature switch allows membrane permeability to increase up to 1.8 times greater at 50ºF (+10ºC) than at 32ºF (0ºC) thereby compensating for the increase in respiration rate of produce over that same temperature range. The result is that the system can maintain almost any combination of O2 and CO2 inside the package and tolerate moderate temperature fluctuations and product respiration changes that normally occur during commercial transport and transhipments.
Because the permeability does not depend on the material of the bag itself, the bag can be designed to exact application needs whether that be to eliminate condensation, retain moisture, improve transparency or any other feature. In general the bags are designed to be used as “liner bags” that is to be used inside a plastic or cardboard shipping box.
Note: There are 2 informative little booklets (in pdf) about MAP, you can download free of charge. Click: “Handbook for Modified Atmosphere” of Food Packaging Free Press and “Guide Packing Fresh Fruit and Vegetables” of the Danish Technological Institute. Both are worth a read.