A comparative study on growth limits of Listeria monocytogenes as affected by temperature, pH and aw when grown in suspension or on a solid surface Original Research Article

The growth/no growth response of a mixture of five strains of Listeria monocytogenes in tryptic soy broth and on tryptic soy agar was monitored in combination treatments of temperature (4–30°C), pH (4.24–6.58) and aw (0.900–0.993) for 30 days. A significant difference between the growth limits of the pathogen in broth and agar was observed. At 25°C the minimum pH values that permitted growth in broth and agar were 4.45 and 5.10, respectively, while the respective aw limits were 0.900 and 0.945. Although the growth limits increased as the temperature decreased in both systems, a relatively constant difference between broth and agar was observed at all temperatures tested. The growth/no growth data of the pathogen in broth and agar were modelled using logistic polynomial regression. The concordance indexes were 99.8% for the broth model and 99.9% for the agar model, indicating a good fit to the observed data in both cases. The present study points out the importance of substrate structure and type of microbial growth on the effect of food preservation parameters, and provides quantitative data in a functional form (models) that may be valuable in efforts to improve safety of liquid as well as solid foods by means of more accurate predictive models.