Effects of Tomato Variety, Temperature Differential, and Post–Stem Removal Time on Internalization of Salmonella Enterica Serovar Thompson in Tomatoes†

Abstract

Tomatoes have been implicated in salmonellosis outbreaks due to possible contamination through bacterial internalization during postharvest handling. This study was conducted to determine the effects of tomato variety, temperature differential between tomato pulp and bacterial suspension, and the time delay between stem removal and immersion in bacterial suspension on internalization of Salmonella enterica serovar Thompson in tomato fruit. Mature green tomatoes at 32.2°C were immersed in water containing approximately 106 CFU/ml S. enterica bacteria. Different tomato varieties (Mountain Spring, Applause, and BHN961), temperature differentials (−10, 0, and 10°F, or −5.6, 0, and 5.6°C, respectively), and post–stem removal times (0, 2, and 16 h) were evaluated for their effects on S. enterica internalization. The incidence and density of internalized cells were determined by culture enrichment and most-probable-number methods, respectively. Overall, variety and post–stem removal time by variety interaction significantly affected the incidence of S. enterica internalization (P < 0.0001), while temperature differential had no significant effect (P = 0.36). Mountain Spring tomatoes were less susceptible to S. enterica internalization than were Applause and BHN961. Increasing the time interval between stem removal and immersion greatly reduced pathogen internalization in BHN961 and Applause, while it had no effect in Mountain Spring tomatoes. The variety and interactions between varieties and post–stem removal times (P = 0.0363) and between temperature differentials and post–stem removal times (P = 0.0257) had significant effects on the populations of internalized S. enterica. Furthermore, all internalized S. enterica cells were found within the core tissue segments immediately underneath the stem scars.