Predicting Film Permeability Needs for Modified-atmosphere Packaging of Lightly Processed Fruits and Vegetables

Abstract

levels in MA packages and to explore options for improvement. IDENTIFYING TARGET LEVELS Appropriate target levels for O 2, CO2, and RH are determined empirically. Traditionally, experiments are conducted in flow-through, controlled-atmosphere conditions using various combinations of mixed gases. However, results from these experiments may not always be immediately applicable to package environments where other gases and volatiles accumulate in the headspace. In particular, RH is higher in packages and can either shorten or lengthen shelf life, depending on the commodity. Empirical studies with MA packages often take the form of “packand-pray” if the film permeabilities, rates of respiration, or both are not well characterized. In addition, shelf life is a qualitative measure at best and can be affected by the initial quality of the plant material and biases of the researchers. Regardless, these empirical approaches are useful for defining the lower limits for O 2 and potential harmful limits of CO2 for LPP. For instance, cut broccoli florets stored at 0 or 5C produced offensive off-odors only when MA package O2 levels were Sales of lightly processed produce (LPP), such as cut lettuce (Lactuca sativa L.), shredded cabbage ( Brassica oleracea L. Capitata Group), and cut carrots ( Daucus carota L.), have increased tremendously in the past few years. A large percentage of these products are packaged such that the levels of relative humidity (RH), O 2, and CO2 are significantly modified relative to their levels in air. Although modified-atmosphere (MA) packages can potentially extend shelf life, they cannot be expected to overcome the negative effects of enhanced temperature (Kader et al., 1989). Since some form of containment is required for LPP, it should be as functional as possible. However, due to variation in respiration and permeation, limitations imposed by available polymer films, and the chance of exposure to elevated temperatures during handling and distribution, there is no assurance that atmospheres attained will be consistent among packages. Due to lack of adequate control, O 2 levels could fall below safe levels in current MA packages (Cameron et al., 1993). Risks include not only the loss of product quality through fermentative metabolism, but also the growth of potential human pathogens that thrive under anaerobic conditions (Hintlian and Hotchkiss, 1986). Accurate predictions of packaging film characteristics based on sound empirical data are important to reduce this risk. The gas levels generated in MA packages are a function of the permeability of the film chosen, and the respiratory behavior and gas exchange characteristics of the enclosed living, breathing plant product. The traditional objective of MA packaging has been to select films with permeabilities that allow gas levels favorable for shelf life extension. Commercially, claims of MA packaging systems are plentiful, although the specific nature of the “modified” atmosphere is not generally mentioned. Several commercial MA package designs are now available, although, in practice, gases may not achieve expected levels and there are few public data available for verification. As a preliminary test of current MA package designs, we purchased MA-packaged precut salad greens at a local supermarket and immediately measured O2, CO2, and ethanol partial pressures in the headspace. We found that O2 levels were extremely low and detected significant accumulation of ethanol in the majority of packages (Fig. 1; Talasila and Cameron, unpublished results). These packages (0.12 m 2 ) contained 0.454 kg of mixed salad, including cut lettuce, sliced carrots, and shredded red cabbage. We are unaware of any substantial exposure to elevated temperatures. The accumulation of ethanol indicated that the package designs were not capable of consistently maintaining aerobic conditions. The presence of “fermented” odors and flavors was easily detected in these packages, although the salad was still edible after adding salad dressing. This suggests that the use of MA packages for cut salads may be limited to those products that can tolerate a degree of fermentation without significant loss of quality. Products such as cut broccoli or cauliflower ( Brassica oleracea L. Botrytis Group) that produce offensive off-odors under low O 2 levels would presumably not be suitable if there were a risk of anaerobic conditions. Plus, there remains the question of whether these low O 2 levels increase health risks (Barriga et al., 1991; Brackett, 1987; Hintlian and Hotchkiss, 1986). The objectives of this paper are to explore some aspects related to generating modified atmospheres and discuss predictions relative to the current application of MA concepts for LPP. In particular, we wish to examine the factors that limit our ability to control gas