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United States Department of Agriculture

Agricultural Research Service

Research Project: MICROBIAL FOOD SAFETY OF FRESH AND FRESH-CUT PRODUCE Title: Effect of storage temperature and duration on the behavior of Escherichia coli O157:H7 on packaged fresh-cut salad containing Romaine and Iceberg lettuce

Authors
item Luo, Yaguang
item He, Qiang -
item McEvoy, James

Submitted to: Journal of Food Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 25, 2010
Publication Date: October 1, 2010
Citation: Luo, Y., He, Q., Mcevoy, J.L. 2010. Effect of storage temperature and duration on the behavior of Escherichia coli O157:H7 on packaged fresh-cut salad containing Romaine and Iceberg lettuce. Journal of Food Science. 75(7):M390-M397.

Interpretive Summary: An increasing number of food-borne illness outbreaks have been associated with the consumption of packaged fresh-cut products. Storage temperature is a critical factor for maintaining produce quality, and shelf life; yet its impact on food safety of fresh-cut leafy green vegetables is unclear. This study examined the effect of temperature on the growth of disease-causing microorganisms on commercially packaged fresh-cut Iceberg and Romaine lettuce. We demonstrated that elevated temperatures at 12 deg C or above significantly promoted the growth of disease-causing microorganisms while cold storage at 5 deg C or below curtailed the proliferation of these microorganisms. This suggests that proper refrigeration is important to limit the growth of disease-causing microorganisms and to maintain food safety. The outcome of this research would provide avenues of intervention and methods for mitigating food-borne illnesses on fresh and fresh-cut produce.

Technical Abstract: This study investigated the impact of storage temperature and time on the survival and growth of Escherichia coli O157:H7 on commercially packaged lettuce salads, and the changes in product quality. Fresh-cut Romaine and Iceberg lettuce salads of different commercial brands were obtained from both retail and wholesale stores. The packages were cut open at one end; lettuce salad inoculated with E. coli O157:H7 via a fine mist spray; then re-sealed with or without an initial N2 flush to match the original package atmospheric levels. The products were then stored at 5 and 12 deg C until their labeled "Best If Used By" dates. Microbial enumeration and product quality evaluation were conducted periodically during storage. Results indicate that storage at 5 deg C allowed the survival of E. coli O157:H7 on the packaged lettuce, but limited its growth, whereas storage at 12 deg C facilitated the proliferation of E. coli O157:H7. Over 2.0 log CFU/g increase in E. coli O157:H7 population was noted on lettuce salads held at 12 deg C for 3 days, followed by additional growth during the remainder of the storage. Although there was eventually a significant decline in visual quality of lettuce held at 12 deg C, the quality of this lettuce was still fully acceptable when E.coli O157:H7 growth reached a significant level. This observation suggests that the visual quality of packaged fresh-cut products is a poor indicator of their food safety status. Maintaining fresh-cut products at 5 deg C or below is critical for reducing the food safety risks as E. coli O157:H7 grows at a rapid, temperature-dependent rate prior to significant quality deterioration.

Last Modified: 10/22/2014
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