Finding the Right Film for Modified Atmosphere Packaging

By Jeff Wooster, Product Development Specialist, Dow Plastics

In recent years, the plastics industry has developed new catalyst technologies to manufacture resins used in breathable films for modified atmosphere packaging (MAP) systems, giving processors more options for keeping fresh-cut produce fresh.

The premise of MAP is fairly simple. After produce is harvested, it continues to live and breathe, consuming oxygen and giving off carbon dioxide in the process of converting glucose and oxygen to water and carbon dioxide. MAP extends the shelf life of fresh-cut produce by reducing its respiration rate and the aging associated with respiration.

Designing a successful MAP system is a somewhat complex task, involving multiple variables. First, it is important to consider the produce being packaged. Lettuce, spinach and cabbage, for example, all have different respiration rates and reactions. Dimensions of the package (volume and surface area) and weight of produce per package are also important, as are the control of storage conditions and refrigerated temperatures from field to table. It is best to take into consideration a worst-case scenario to protect the produce. How long and at what temperature will it be stored?

Determine the Optimum Atmosphere in the Package
A properly designed MAP system should reduce produce respiration, but not completely stop it. A fine line exists between extending shelf life and creating an atmosphere in which produce spoils. Care must be taken to maintain enough oxygen in the package to allow limited aerobic respiration. If very little or no oxygen is present, anaerobic respiration takes place, followed by rapid spoilage. For this reason, high-barrier packages, which prevent most transmission of oxygen and other gases, are generally not suitable for long-term packaging of fresh-cut produce.

Packages designed with selective barrier properties that provide a controlled oxygen transmission rate (OTR) and effectively control oxygen concentration inside the package are key to successful MAP applications. Figure 1 provides an estimation of the transmission of multilayer film structures.

Click here to see Figure 1.

It is also important to understand chemical reactions for individual produce items, as in the following equation: C₆H₁₂O₆ + 60₂ ==> 6H₂O + 6CO₂ + 686 kcal. In this case, one molecule of carbon dioxide is generated for each molecule of oxygen consumed. The ratio of CO₂ produced to O₂ consumed is called the respiratory quotient (RQ). Here, the RQ is 1.0. For other aging reactions the RQ may be less than one or greater than one. Each type of produce undergoes different types of reactions. It is important to understand the RQ for each type of produce being packaged. The RQ is often assumed to be 1.0. This is a reasonable assumption for many types of produce when stored under typical refrigerated conditions.

The RQ is important because in addition to slowing respiration, increased carbon dioxide concentration can inhibit growth of microorganisms and act as a fungicide in others. Some types of produce are sensitive to high concentrations of carbon dioxide, however, so an optimum level must be determined for each type of produce.

While oxygen and carbon dioxide concentrations are important, temperature can be the most important factor in determining produce respiration rate. When produce is stored at room temperature it ages rapidly. For this reason, many types of produce are refrigerated at or below 40°F. However, most produce undergoes irreversible damage at temperatures below 30°-34°F. This temperature can vary depending on produce type, and care should be taken to keep temperatures within tolerance limits.

Material Selection
Once produce requirements have been determined, it is important to select a film structure that meets them. Oxygen transmission rates, optical properties, hot-tack initiation temperatures and strengths and heat-seal initiation temperatures and strengths should be determined. Each should be appropriate for the product. Printability, machinability and toughness are also considerations.

Some specific factors to consider when designing the proper film structure include:

• resin selection - AFFINITY* polyolefin plastomers, ATTANE* ultra low-density polyethylene, DOWLEX* linear low-density polyethylene, low-density polyethylene, polypropylene, styrene-butadiene, ethylene-vinyl acetate, etc.).

• film construction - monolayer, coextruded or laminated.

• film thickness - total thickness and layer ratios.

• additives - slip, antiblock, antifog.

• processing conditions - extrusion temperature, post treatments.

Historically, products such as ethylene-vinyl acetate (EVA) have been used for packaging fresh-cut produce. EVAs, however, have disadvantages compared to polyolefin plastomers (POPs) and ultra low-density polyethylene (ULDPE) resins, which provide better optics, better seal performance, higher hot-tack strength and much lower water vapor transmission rates (WVTR) at comparable oxygen transmission rates.

POPs are a new category of polymer material that has found widespread use in fresh-cut produce packaging and other high-performance applications. Because they offer a unique combination of high oxygen transmission, relatively low WVTR, excellent seal performance, excellent optics and low off-taste and off-odor contribution, POPs are a preferred product for use as the sealant layer and high OTR structural layers in fresh-cut produce packaging.

AFFINITY POPs, manufactured by Dow Plastics using its proprietary INSITE* Technology, are designed to contain a specific amount of long chain branching. The incorporation of low levels of long chain branching improves the extrusion processability. This is an important quality for film producers, because it allows easier extrusion and provides greater flexibility in the conditions used to manufacture film.

Figure 2 shows OTRs for two AFFINITY POPs and two ATTANE ULDPE resins compared to two typical EVA resins. Films made with POP and ULDPE resins provide excellent OTRs, up to about 1,500cc (at STP)-mil/100 sq.in.-day-atm at 25 C. Note that as is customary, OTR values are reported in cubic centimeters at standard temperature and pressure (STP = 0 C and 1 atmosphere).

Click here to see Figure 2.

Figure 3 shows the excellent hot-tack strength and low hot-tack initiation temperatures for films made with AFFINITY POPs and ATTANE ULDPEs. Films made with EVAs, by comparison, have very poor hot-tack strength. Figure 4 shows the heat-seal strength at various seal temperatures; POP films have the best seal performance, followed by the ULDPE and EVA films.

Click here to see Figure 3.

Click here to see Figure 4.

Minimum Requirements for Film
Once film selection has been made, it is important to ascertain that all minimum requirements of the film have been met. Can the selected film thickness be easily fabricated and formed into bags on high-speed vertical form-fill-seal equipment? How easy is it to manufacture the film? Will the CO₂ concentration in the package ever exceed maximum acceptable CO₂ concentration? Does the package meet FDA requirements for materials and labeling? Will the package meet the requirements of the grocer? And ultimately, will the consumer's requirements be met?

There are a number of variables that impact the design process and overall success of a MAP system. Material suppliers that have an extensive understanding of resins, films and packaging design, and can assist packaging developers in the selection process.

For more information: The Dow Plastics Customer Information Group, 690 Building, Midland, MI 48640, Tel: (800) 258-2436, Fax: (517) 832-1465, E-mail: dowcig@dow.com, Web: http://www.dow.com/plastics