Behavior of Listeria monocytogenes inoculated into raw tomatoes and processed tomato products

Abstract

Rates of death and growth of Listeria monocytogenes inoculated onto raw whole and into chopped tomatoes stored at 10 and 21°C were not influenced by prior treatment of tomatoes with chlorine or packaging under an atmosphere of 3% O2 and 97% N2. Growth of the pathogen occurred in whole tomatoes held at 21°C but not at 10°C, while death occurred in chopped tomatoes stored at these temperatures. Likewise, growth patterns of mesophilic aerobic microorganism, psychrotrophic microorganisms, and yeasts and molds on whole and chopped tomatoes were essentially unaffected by chlorine and modified atmosphere packaging treatments. Populations of L. monocytogenes inoculated into commercially processed tomato juice and sauce and held at 5°C remained constant for 14 days. A gradual decrease in the number of viable L. monocytogenes cells was observed in juice and sauce held at 21°C. In contrast, the organism died rapidly when suspended in commercial tomato ketchup at 5 and 21°C. Unlike low-acid raw salad vegetables such as lettuce, broccoli, asparagus, and cauliflower on which we have observed L. monocytogenes grow at refrigeration temperatures, tomatoes are not a good growth substrate for the organism. Nevertheless, L. monocytogenes can remain viable on raw whole and chopped tomatoes and in commercial tomato juice and sauce for periods extending beyond their normal shelf-life expectancy.