Washington State University
16th Annual Postharvest Conference & Trade Show
March 14 & 15, 2000, Yakima Convention Center
Devon Zagory, Ph.D.
Senior Vice President for Food Safety & Quality Programs Food Safety & Quality Programs, Davis Fresh Technologies, LLC
The search for a modified atmosphere package (MAP) often starts with the question What bag do I use? While the answer to this question is certainly important, this is not the first question that needs answering. While it may be possible to find an appropriate MAP through simply testing miscellaneous packages presented by suppliers, the optimal package is only likely to be found through taking a systems approach to packaging. A systems approach requires asking and answering several questions before it is possible to know what bag to use.
Perhaps the first question that needs to be answered is Can MAP help me? While MAP can deliver many benefits, there are many things MAP cannot do. Selecting an appropriate package starts with understanding if MAP can solve your problem. MAP can help extend shelf life by slowing respiration, maintain appearance by slowing color development, maintain texture through slowing softening, maintain quality by slowing the growth of some microorganisms and preserve flavor by slowing use of sugars during respiration. MAP will not improve quality but can slow loss of quality. Nor will MAP contribute to product safety, improve flavor or make the product more nutritious.
To know if MAP can help, start by being very specific about what you want from the package. If longer shelf life is the goal, define shelf life. What do you see that tells you that shelf life has ended? Does the product turn brown, get slimy, soften, begin to decay, lose flavor, shrivel, or smell bad? By being specific about the symptoms of short shelf life, it will become clear if MAP can address that symptom and help extend shelf life.
As an example, suppose that cherries are arriving at their destination market with soft fruit and brown stems, thereby reducing their value and shortening their shelf life. Can MAP address these problems, and if so, how can we determine what package to use? The scientific literature reports that reduced oxygen (3-7% O2) can help maintain firmness of cherries while 10-15% CO2 can maintain green stems. So, MAP can help with these specific problems. How can MAP create the desired atmosphere?
Passive MAP relies on the respiration of the commodity to consume the O2 in a sealed bag and replace it with CO2, a byproduct of normal aerobic respiration. The bag itself restricts the movement of gases in and out of the sealed package due to its selective permeabilities to O2 and to CO2. Over time, the system achieves an equilibrium modified atmosphere with the O2 lower than that found in air (20.9%) and the CO2 concentration higher than that in air (0.03%). Active MAP introduces a desired gas mixture into the bag prior to sealing, thereby accelerating the process of achieving an equilibrium atmosphere. Vacuum packaging draws a slight vacuum prior to sealing the bag, thereby reducing the headspace in the bag and accelerating the process of achieving an equilibrium atmosphere. In all three cases the result is the same. It is only the time to reach an equilibrium atmosphere that is different.
The films used in MAP include various kinds of plastic polymers that provide protection, strength, sealability, clarity, and a printable surface. However, their unique function is to restrict the movement of O2 and CO2 through the bag and allow the establishment of a modified atmosphere. They maintain a gradient between the gas concentrations in air and those inside the bag. It is the interaction of the respiration of the product and the gas gradient formed by the bag that results in the formation of the modified atmosphere. The gradient that results is not dependent on the initial gas concentrations inside the bag but rather on the respiration rate of the product and the gas permeabilities of the bag. It is important to recognize that by adding gas mixtures to the bag (active MAP) or by drawing a vacuum before sealing (vacuum packaging), the equilibrium atmosphere is not affected. These measures may allow the desired atmosphere to evolve more rapidly and this may be desirable in some cases. But the atmosphere that will occur inside a MAP is a function of the film and the product. For this reason it is essential to know the respiratory requirements of the product (how much O2 the product will consume under specified conditions) and the permeability properties of the plastic bag that will be used.
The determinant of the relative proportions of CO2 and O2 in the package is the ratio of film permeabilities to CO2 and O2. This ratio is referred to as (PCO2/PO2), and is one of the most useful descriptive parameters of a plastic film. Films with a high value will allow CO2 to escape the package relatively easily, resulting in an atmosphere with low CO2. Films with lower values will allow greater buildup of CO2 in the package (Figure 1). For most low density polyethylene films (the most common in use for MAP in the produce industry) ~2-4. It will be different for different kinds of film. The permeability ratio () determines the possible combinations of O2 and CO2 concentrations inside the package. Gas flushing, vacuum packing, changing the size of the bag, or changing the amount of the product in the bag will not affect these possible gas concentrations. The value of a film is the determinant of the possible combinations of gas concentrations within a package. Since fruits and vegetables vary in their tolerance to CO2 and in their ability to benefit from high CO2, the value of a film is very important as a predictor of the relative amounts of O2 and CO2 that will accumulate in the package. The absolute amounts of O2 and CO2 will be determined by the absolute permeability values of the film. Figure 1 shows the possible gas concentrations in a MAP made from films with values of 1, 3 or 6. The boxes labeled broccoli and tomato represent the zone of desirable gas concentrations for those two commodities. The =3 and =6 lines represent the possible combinations of O2 and CO2 concentration that could evolve within sealedpackages made from films with those b values. A film with a b=3 would be suitable for broccoli, but not for tomatoes. The opposite would be true for a film with b=6. Broccoli is benefited by high CO2 and low O2 concentrations. Tomato is less tolerant of high CO2 but will benefit from low O2. Tomatoes, therefore, should be packaged in a film that will allow the escape of CO2 (high), while broccoli should be packaged in a film that allows accumulation of CO2 (low ).