Storage Temperature Affects E. coli Growth

WASHINGTONA new study published in the Journal of Food Science suggests proper refrigeration is important to limit the growth of disease-causing microorganisms and to maintain food safety. The findings revealed that elevated temperatures at 12°C or above significantly promoted the growth of disease-causing microorganisms while cold storage at 5°C or below curtailed the proliferation of these microorganisms.

The researchers 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 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°C and 12°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°C allowed the survival of E. coli on the packaged lettuce, but limited its growth, whereas storage at 12°C facilitated the proliferation of E. coli. Over 2.0 log CFU/g increase in E. coli population was noted on lettuce salads held at 12°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°C, the quality of this lettuce was still fully acceptable when E.coli 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°C or below is critical for reducing the food safety risks as E. coli grows at a rapid, temperature-dependent rate prior to significant quality deterioration.