What Modified Atmosphere Packaging
Can and Can't Do for You
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.
Benefits of MAP
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 by 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. A summary of what MAP can and cannot do is shown in Table 1.
Table 1. What MAP can and cannot do.
|MAP Can||MAP Cannot|
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% to 7% oxygen [O2]) can help maintain firmness of cherries, while 10% to 15% carbon dioxide [CO2] can maintain green stems. Therefore, MAP can help with these specific problems. How can MAP create the desired atmosphere?
Types of MAP
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 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 equilibrium that is different.
Properties of MAP Films
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, which may be desirable in some cases. However, the atmosphere that will occur inside a MAP is a function only 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 permeability ratio is referred to as b (PCO2/PO2), and is one of the most useful descriptive parameters of a plastic film. Films with a high b value will allow CO2 to escape the package relatively easily, resulting in an atmosphere with low CO2. Films with lower b values will allow greater buildup of CO2 in the package (Figure 1). The most commonly used MAP for the produce industry are low-density polyethylene films b~2 to 4. The b value of a film is the determinant of the possible combinations of gas concentrations within a 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. Because fruits and vegetables vary in their tolerance to CO2 and in their ability to benefit from high CO2, the b 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 b values of 1, 3, or 6. The b=3 and b=6 lines represent the possible combinations of O2 and CO2 concentration that could evolve within sealed packages made from films with those b values. Films with b values of 3 or 6 will provide atmospheres with low O2 and low CO2 because they will allow CO2 to exit the package 3 or 6 times faster than O2 enters the package. A film with a b=1, which is characteristic of micro-perforated and micro-porous films, will allow atmospheres to evolve with low O2 and high CO2, which would be suitable for cherries.
Figure 1. Effect of b value on O2 and CO2 concentrations.
Figure 1 shows that polyethylene films with b values greater than 1 will not be suitable for cherries because they allow CO2 to exit the package too rapidly for the CO2 concentration to accumulate to 10% to 15%, the desirable amount for cherries. This knowledge makes it possible to determine which bags might be appropriate without testing the majority of packaging materials. Apples and pears, which are expected to benefit from low O2 and low CO2, would require films with b values of 3 to 6 or more.
Having determined that a film with a b=1 would be appropriate for cherries, how can we determine what the oxygen transmission rate (OTR) should be? Figure 2 shows how to calculate the necessary OTR for any produce commodity. The OTR needs to be higher for: 1) produce with a higher respiration rate (RR), 2) thicker films (T), and 3) if there is more product in the package (W). The OTR can be lower for packages with greater surface area (A). The desired equilibrium in the package (O2pkg) is subtracted from the O2 concentration in the air (O2atm).
Figure 2. Determining the OTR.
Determining the Appropriate Packaging
What do you need to know to identify appropriate packaging materials for fruits or vegetables?
- What problem do I want to address through packaging?
- What are the symptoms of the problem?
- Can modified atmospheres help with that problem?
- What atmosphere will help with the problem (percentages of O2 and CO2)?
- What atmosphere might injure my product?
- What is the b value of a film that will give me the desired atmosphere?
- What is the respiration rate of my product at the temperature at which it is handled?
- How much product will go in the package?
- What will be the size of the package?
Armed with the answers to the above questions it will be possible to determine the characteristics of the plastic film or package that will provide the optimal conditions for your product. Having done so, it will then be necessary to find such a package and to test it under real world conditions to confirm that it actually solves the problem. However, you will have to test very few packages because you will have already determined what will not work for you. This may be the chief benefit of the analytical approach described here.
Devon Zagory, Ph.D.
Davis Fresh Technologies
129 C Street, #4, Davis, CA 95616
16th Annual Postharvest Conference, Yakima, WA
March 14-15, 2000