Title: THE PROBABILITY OF GROWTH OF LISTERIA MONOCYTOGENES IN SIMULATED SMOKED SALMON AND TYPTIC SOY BROTH AS AFFECTED BY SALT, SMOKE COMPOUND AND STORAGE TEMPERATURES Author
Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 1, 2008
Publication Date: February 1, 2009
Citation: Hwang, C. 2009. THE PROBABILITY OF GROWTH OF LISTERIA MONOCYTOGENES IN SIMULATED SMOKED SALMON AND TYPTIC SOY BROTH AS AFFECTED BY SALT, SMOKE COMPOUND AND STORAGE TEMPERATURES. International Journal of Food Microbiology. 129:244-252. Interpretive Summary: Smoked fish products, if not processed properly, can be contaminated with Listeria monocytogenes (LM), a foodborne pathogen that has been implicated in causing outbreaks of foodborne illness. This study examined the growth probability of LM in cooked salmon and tryptic soy broth (TSB), a nutrient broth used for growing bacteria, that contained various levels of salt and smoke compounds and stored at refrigerated (4 degree C) and abuse temperatures up to 25 degree C. The purpose was to develop mathematical equations to describe the growth probability of LM as affected by salt, phenol and storage temperature, and to examine if TSB can be used to substitute salmon for testing. The resulted models indicate that the growth probability of LM in both salmon and TSB decrease as salt and phenol concentrations increase, and at lower storage temperatures. The salt and storage temperature have more effect in affecting the growth probabilities of L. monocytogenes than phenol. Comparing the probability models for salmon and TSB showed that the growth probabilities of LM estimated by TSB model were close to the estimations by the salmon model at higher storage temperatures (e.g., >12 degree C). Therefore, it is not suitable to use TSB model for predicting LM growth in salmon when storage temperatures are at refrigerated or mild abuse temperatures. The salmon model may be used by smoke seafood manufacturers to estimate the growth probabilities of LM in their products or to estimate the concentrations of salt, phenol and storage temperature for their products to minimize the probability of growth of LM.
Technical Abstract: The objective of this study were to examine and model the probability of growth of L. monocytogenes in cooked salmon containing salt and smoke (phenol) compounds and stored at various temperatures. A growth probability model was developed, and the model was compared to another model developed from tryptic soy broth (TSB) to assess the suitability of using TSB as a substitute for salmon. A 6-strain mixture of L. monocytogenes was inoculated into minced cooked salmon and TSB containing 0-10% NaCl and 0-34 ppm phenol to levels of 2-3 log 10 cfu/g, and the samples were vacuum-packed and stored at 0-25 degree C for up to 42 days. A total 32 treatments, each with 16 samples, tested were selected by 2 central composite designs. A logistic regression was used to model the probability of growth of L. monocytogenes (with a 1.5-log10 increase during storage) as a function of concentrations of salt and phenol and storage temperature. Resulted models showed that the probabilities of growth of L. monocytogenes in both salmon and TSB decreased when the salt and/or phenol concentrations increased, and when the samples were stored at lower temperatures. In general the growth probabilities of L. monocytogenes were affected more profoundly by salt and storage temperature than by phenol. The growth probabilities of L. monocytogenes estimated by the TSB model are higher than those by the salmon model at same salt/phenol concentrations and storage temperatures. The differences in the predicted growth probabilities between salmon and TSB models were greater at lower storage temperatures, indicating the potential use of TSB as a model system to substitute salmon in studying the growth behavior of L. monocytogenes may only be suitable when the temperatures of interest are in higher storage temperatures (e.g., >12 degree C). The model for salmon demonstrated the effect of salt, phenol and storage temperature and their interactions on the growth probabilities of L. monocytogenes, and may be used for determining the growth probability of L. monocytogenes in smoked seafood.