Prevalence of Listeria monocytogenes in ready-to-eat foods, and growth boundary modeling of the selected strains in broth as a function of temperature, salt and nisin

Abstract

The objectives of this study were to determine the prevalence of Listeria monocytogenes in ready-to-eat (RTE) foods, and the growth boundaries of two strains of L. monocytogenes isolated from RTE foods and one ATCC7644 strain under different temperature (4, 20, and 37°C), salt (0, 1, 2, 3, and 4% w/v), and nisin (0, 50, 100, 200, and 400 µg/mL) levels with different inoculum sizes (3, 4, 5, and 6 log10 CFU/mL). One hundred thirty-three retail RTE foods were examined and a total of 39 isolates were identified. The positive Listeria spp. and L. monocytogenes samples were 29.3% and 12.8%, respectively. Dairy and fruit-based RTE foods were L. monocytogenes negative whereas seafood-based (26.7%), meat-based (19.4%), poultry-based (16%), and vegetable-based (10%) RTE foods contained L. monocytogenes. It was shown that nisin was more effective than NaCl to avoid growth. Moreover, protective effect of NaCl on nisin activity was observed. As inoculum size increased probability of growth also increased. Suppressing the growth by using temperature, NaCl and nisin was more difficult in RTE-isolates strains (strains 120 and 137) than that of ATCC7644 strain. A mathematical model based on logistic regression could be used to determine growth limits of L. monocytogenes strains with a concordance of > 91%.