Protecting Chilled Foods with Packaging Gases
Packaging is a key component in food safety programs, and modified-atmosphere packaging (MAP) systems can keep perishable products fresh longer. In general, using MAP, the shelf life of meat and poultry products can be extended two to three times, and the shelf life of baked goods can be extended three to four times.
Modified-atmosphere packaging is widely used for protecting chilled meat and poultry, refrigerated foods, ready meals and fresh pasta and produce. Although the process is not new technology, it finds wide use in emerging food and packaging applications. Modified-atmosphere packaging for food products has become more prevalent as companies strive to deliver more high margin products that satisfy consumer tastes and extend their distribution range. Consumers tend to associate fresh foods with health, and many will pay extra for premium products. Innovative products may be processed to enhance color, texture or flavor, or to include special blends or select ingredients. Keeping foods fresh longer, of course, also improves profitability.
Overview of Modified-Atmosphere Packaging
MAP equipment fills each package with a protective atmosphere before it is sealed. Modified-atmosphere packaging gases include food-grade carbon dioxide (CO2), nitrogen (N2), oxygen (O2) and sometimes argon. At times, a combination of gases is used. The protective atmosphere can be used with any type of sealed-packaging machine designed to accept them. These include tray sealers, deep-drawing machines, bag-sealing machines and horizontal- and vertical-flow packs.
For the food manufacturer, goods protected by modified-atmosphere packaging offer extended shelf life, and they can be delivered less often and across a wider region. This allows food manufacturers to extend market reach, simplify distribution logistics, and reduce labor and transportation costs, including fuel and maintenance. Extended shelf life also can help lower the number and cost of returns.
On the revenue side, consistently delivering high quality, fresh products at the point of sale contributes to customer satisfaction — and repeat sales. Maintaining consistent quality is especially important as new products gain traction and word-of-mouth spreads.
Ensuring Food Safety
Improvements in modified-atmosphere packaging systems can contribute to compliance requirements under the Food Safety Modernization Act (FSMA). As part of an overall food safety and hazard prevention program, MAP provides an extra blanket of protection that works together with other elements such as safety training, operating procedures and temperature control during production.
The specific gas or blend for the protective atmosphere will depend on the type of food product as well as both internal and external control factors. Internal factors include the type of microorganisms, water activity, pH value and, for produce, cell respiration. How well a modified-atmosphere package protects the food from spoilage also will depend on the composition of the protective atmosphere, food temperature, hygienic conditions and processing methods. The latter includes the quality of the film and seal, without which package integrity is lost.
New package testing technology can help processors address this issue, improve food safety and maximize opportunities for modified-atmosphere packaging products.
Modified-atmosphere packaging systems can extend the shelf life of chilled meat and poultry products two to three times compared to traditional air-packaging systems.
While modified-atmosphere packaging equipment is generally reliable, packages can fail for number of reasons:
- Adhesive failure.
- Undetected pinholes in the packaging film.
- Problems with the seal or another part of the package.
- Maintenance issues.
Over the years, food processors have relied on a number of methods to test the integrity of modified-atmosphere packaging. The simplest is a water submersion test. There also are handheld and portable metering devices and inline systems that can test for gas leaks or specific gas ratios inside the package.
The time between samples can be a function of line speed or the test method itself. When a problem with a particular package is detected, it could impact hundreds of packages that may need to be inspected and reworked. The packaging line stops while the source of the leak is identified and repaired. Questionable packages must be removed before the line is again restarted and tested. In some cases, both the product and the packaging may need to be discarded.
Needle and immersion testing often relies on statistical sampling. This means that only a single package is checked for each batch or test period.
One drawback of spot-metering systems is they may overlook fine pinholes in the packaging film. The package may pass, but the protective atmosphere will be lost prematurely.
Inline testing offers an advantage over other leak-detection methods. Inline leak-detection systems can deliver high speed testing of flexible modified-atmosphere packaging as well as trays and other MAP containers that are film-sealed at the top. The packages pass into a test chamber on a conveyor belt, and a sensor checks for the presence of a gas.
Figure 1. The bacterial growth on pork in different atmospheres at 40°F (4°C) is shown.
MAP Gases Help Extend Shelf Life
Figure 1 shows the relative impact on shelf life for fresh pork when comparing three modified-atmosphere packaging gases — CO2, N2, O2 — versus air. In practice, different foods require different atmospheres, and two or more gases often are used in combination.
Oxygen. Foods spoil due to a variety of biological, thermal and enzymatic processes, including oxidation. Air is about 21 percent oxygen. By limiting oxygen inside the package, the modified atmosphere inhibits bacterial growth and helps foods retain their natural color, texture and appearance.
For fresh fruits and vegetables, a low level of O2 — typically 1 to 5 percent — is necessary for respiration. For meats, oxygen helps preserve the oxygenated form of myoglobin, which gives meat its red color. This is especially important for case-ready packages.
A low level of oxygen can be beneficial for another reason. Aerobic microorganisms such as Pseudomonas and Acinetobacter will produce byproducts during decomposition reactions that will give foods a bad taste and odor. (Anaerobic bacteria such as Clostridium spp. also can produce odiferous reactions.) Yet, not all bacterial decay processes proceed at the same rate, and not all pathogenic species are as easily detected. Controlled aerobic decomposition may serve as a type of early-warning system for other spoilage vectors.
Carbon Dioxide. A water-soluable inert gas, CO2 is the most widely used and important modified-atmosphere packaging gas for controlling molds and aerobic bacteria. Many foodborne pathogens prefer an alkaline environment. CO2 can inhibit microbial activity at atmosphere packaging levels of 20 percent or more.
The gas diffuses into the liquid and fat phases of food and reduces pH levels, especially at the surface where bacteria tend to spread most rapidly. The CO2 gas also works by penetrating the membrane wall of the bacteria and causing changes in permeability and cell function.
Nitrogen. Another inert gas, N2 often is used to flush the modified-atmosphere package of air. It can be used alone or in combination with CO2.
N2 is typically used as a filler gas to maintain package shape, especially for packing high fat or high moisture foods that tend to absorb CO2. It can fill the headspace between a high viscosity chilled food such as hummus and the modified-atmosphere packaging film, or it can help fill out an entire package. Potato chips and other dried snacks are typically packed in 100 percent N2 to prevent rancidity due to oxidation.
Nitrogen also can slow plant metabolic reactions. It often is used to maintain the appearance of leafy vegetables, specialty salads and other cut or uncut fruit and vegetable products.
Argon. Argon is twice as soluble as nitrogen and can be used in similar modified-atmosphere packaging applications. Because it is costly, however, it generally is reserved for special applications and used at very low rates. It can slow respiration and other metabolic reactions better than nitrogen under certain conditions, making it an option for some premium produce products.
In conclusion, modified-atmosphere packaging can be used to help enhance color, texture or flavor.