Submitted to: Journal of Food Process Engineering
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/20/2005
Publication Date: 3/1/2006
Citation: Guentert, A., Mohtar, R., Luchansky, J.B., Tamplin, M.L., Linton, R. 2006. Modeling the behavior of listeria monocytogenes in ph-modified chicken salad during cold storage and temperature abuse conditions. Journal of Food Process Engineering. 29:1 Article 045. Interpretive Summary: Interpretive Summary: Listeria monocytogenes has been responsible for several large food recalls and numerous cases/outbreaks of food poisoning within the past 10 years in the United States, particularly from ready-to-eat (RTE) foods. Several RTE foods, including prepared salads such as chicken salad, were identified in a recent risk assessment by U.S. regulatory agencies as having a higher risk of listeriosis. We obtained chicken salad from a commercial manufacturer to evaluate how well it would support the growth of this pathogen at three different temperatures (5, 7, and 21C) and acidity levels (pH 4.0, 4.6, and 5.2). We demonstrated that levels of L. monocytogenes decreased throughout the 120 day shelf life regardless of temperature or pH. Next, we analysed the data mathematically to develop models that would predict the behavior of L. monocytogenes in pH-modified chicken salad at various cold and abuse storage temperatures. The objective of the modeling was to address the interactions of pH and storage time and temperature to better estimate pathogen viability and to compare these findings to estimates obtained from the United States Department of Agriculture (USDA) Pathogen Modeling Program (PMP). One of the models developed, a linear model, gave results that were not consistent with our laboratory observations, whereas a non-linear model that was developed gave improved accuracy over the linear model. Our results indicated that development of a fully-functional predictive model for L. monocytogenes in chicken salad will require further analyses of the existing data and possibly the collection and analyses of new data sets in order to cover the full range of anticipated parameters for pH and storage times and temperatures. Once validated, however, such models will be helpful in developing guidelines for formulating and storing chicken salad and other protein-based prepared salads to lower the risk of listeriosis.
Technical Abstract: Listeria monocytogenes grows at refrigeration temperatures less than and equal to 5ºC, and it tolerates various environmental stresses. The Food and Drug Administration specifies a "zero-tolerance" for this pathogen in certain ready-to-eat processed foods. Modeling its dynamic behavior during fluctuating temperatures at various pH values is important to the safety of food. This study presents linear and non-linear models to predict the behavior of L. monocytogenes in pH-modified chicken salad at various cold storage and temperature abuse conditions. A linear model of the kinetics accounting for simple and interactive effects of storage time, temperature, and pH was developed. Predictions of the linear model were inconsistent with laboratory observations. The linear model’s limitations were reflected in the poor correlation of model predictions to the observed values (R2 = 0.58). A non-linear model proposed by Geeraerd and colleagues, was, therefore, used to model the observed data. The four model parameters ((N(0), Cc (0), kmax and Nres,) were optimized for each of the nine treatments. Correlation coefficient (R2) values ranged from 0.70 (pH 5.2, 7.2C) to 0.99 (pH 4.0, 21.1C), indicating improved accuracy. Developing a functional and validated microbial predictive model requires further analyses and collection of data at additional pH and temperature values to determine a single set of parameters that represent the microbial behavior at the full range of pH and temperatures observed under storage conditions. Future experiments should address the adaptive nature of L. monocytogenes as the response to environmental stresses affects survival of the organism in food systems.