Submitted to: Journal of Environmental Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/22/2004
Publication Date: 8/1/2005
Citation: Guenter, A., Linton, R., Cousin, M., Luchansky, J.B. Behavior of Listeria monocytogenes in pH-modified chicken salad during refrigerated storage and temperature abuse. Journal of Environmental Health. v. 68. p. 31-37. Interpretive Summary: There have been several large outbreaks and numerous sporadic cases of listeriosis in the United States in recent years, most of which have been caused by contaminated ready-to-eat (RTE) foods. A recent risk assessment by U.S. Federal agencies has identified several foods as having a higher risk of listeriosis. Included on this list are deli-style, prepared salads such as chicken salad. Such foods may contain Listeria monocytogenes through contaminated ingredients and/or post-process contamination. We obtained chicken salad from a commercial manufacturer to evaluate how well it would support the growth of this pathogen at both refrigeration (5 and 7C) and abuse temperatures (21C). In addition, we modified the acidity of the product to pH 4.0, 4.6, and 5.2 to determine if such conditions might also affect the viability of the pathogen during storage. The results revealed that the 3-strain mixture of Listeria monocytogenes that was purposefully added to the chicken salad actually decreased in numbers throughout the 120 day shelf life regardless of temperature or pH. Additional studies should be conducted in other formulations/brands of chicken salad and in other RTE foods of a similar nature to determine if the findings of our study are universal or unique to the product we evaluated.
Technical Abstract: Ready-to-eat (RTE) foods including prepared deli salads may contain Listeria monocytogenes through contaminated ingredients and/or post-process contamination. The combined preservation strategy of using temperature and acidulants can be used to control the survival and growth of L. monocytogenes in RTE foods. The growth and inactivation kinetics of L. monocytogenes was evaluated in this study at three pH levels (pH 4.0, 4.6, and 5.2) during storage at three temperatures (5.0, 7.2, and 21.1C). Commercially-produced pasteurized, chicken salad was adjusted to the targeted pH levels using 2N acetic acid or 2M sodium acetate solutions. Next, 25 g samples of the pH-modified salad were inoculated to a level of ~1 x 10(6) cells/g with a three-strain mixture of L. monocytogenes (Scott A, LCDC 81-861, and F2365) and stored at the three temperatures for up to 119 days. Samples were enumerated for L. monocytogenes using the FDA modified most-probable number (MPN) procedure and log MPN was plotted against time. Inactivation was seen at all pH levels and at all temperatures. At 21.1C, a 6-log reduction was seen after 14 days at pH 4.0, 52 days at pH 4.6 and 38 days at pH 5.2. Inactivation began very quickly at pH 4.0, and after a lag phase of 10-12 days at pH 4.6 and 5.2. Inactivation rates were slower in cold storage, with decreases of 1.1 log (pH 5.2) and >3 log (pH 4.0 and 4.6) at 119 days. The data generated in this study may be used to develop predictive models that could specifically address the interactions of pH and storage temperature on viability of L. monocytogenes in prepared salads. Such predictive models may be helpful in developing formulation and storage guidelines for protein-based deli salads.