Submitted to: Journal of the Science of Food and Agriculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/17/2004
Publication Date: 3/18/2005
Citation: Kim, J., Luo, Y., Tao, Y., Saftner, R.A., Gross, K.C. 2005. Effect of initial oxygen concentration and film oxygen transmission rate on the quality of fresh-cut romaine lettuce. Journal of the Science of Food and Agriculture. 85:1622-1630 Interpretive Summary: Packaged fresh-cut vegetables are becoming more and more popular because they are convenient and ready-to-eat. The quality of packaged fresh-cut lettuce and other salad vegetables is best maintained by selecting packaging films that match the oxygen transmission rate (OTR) to the respiration rate of the product being packaged. This practice faces several challenges among which is the large variation in respiration rates from one harvest of lettuce to another, and the difficulty in changing films in a commercial practice quickly enough to correct for changing respiration rates of the produce being packaged. In this study, we found that initial headspace oxygen plays an important role in maintaining quality and shelf-life of fresh-cut romaine lettuce, and it interacts with film OTR significantly. Adjusting initial oxygen can provide a practical means to processors for maintaining quality of packaged fresh-cut lettuce in situations where the packaging film is sub-optimal and the quality of the fresh-cut product would otherwise be compromised.
Technical Abstract: Modified atmosphere packaging (MAP) is widely used to maintain the quality of fresh-cut produce by matching the oxygen transmission rate (OTR) of the packaging film to the respiration rate of the packaged product. The effect of the interaction between film OTR and the initial headspace oxygen pertaining to quality of fresh-cut vegetables has not been reported. Romaine lettuce leaves were sliced, washed, and dried according to standard commercial practice. Lettuce pieces were packaged with film OTRs of 8.0 and 16.6 pmol s-1 m-2 Pa-1, and flushed with 0, 1, 2.5, 10 and 21 kPa oxygen partial pressures. Packages were hermetically sealed and stored at 5 °C for up to 14 days. With 16.6 OTR-packaged samples, increasing the initial headspace oxygen concentration delayed oxgyen depletion within the packages, hastened the onset and increased the intensity of discoloration, and inhibited the development of carbon dioxide injury, acetaldehyde and ethanol accumulation, off-odors, and electrolyte leakage. With 8.0 OTR-packaged lettuce pieces, '1 kPa initial headspace oxygen treatments induced an essentially anaerobic environment within the packages and increased acetaldehyde and ethanol accumulation and off-odor development. Increasing the initial oxygen concentration above 1 kPa in 8.0 OTR packages transiently increased oxygen concentrations, and reduced fermentative volatile production, off-odors, electrolyte leakage, and carbon dioxide injury. Regardless of the initial headspace oxygen concentration, the overall quality score of 16.6 OTR-packaged lettuce pieces after 14 days storage was low due to severe discoloration. With 8.0 OTR-packaged samples, the overall quality score increased with increasing initial headspace oxygen concentrations. A 21 kPa initial oxygen treatment of 8.0 OTR-packaged lettuce pieces maintained good quality with the highest overall quality score (6 using a 1 to 9 hedonic scale) at the end of 14 days storage. Since respiration rates of lettuce pieces change with growing season, the OTR of the packaging film used, and other factors, the practice of packaging lettuce pieces at low initial headspace oxygen concentrations may not always be optimal for maintaining shelf stability. Adjusting the initial headspace oxygen concentration can provide a practical means for processors to minimize quality loss of lettuce processed under sub-optimal OTR packaging conditions.