Influence of Temperature Differential between Tomatoes and Postharvest Water on Salmonella Internalization

Salmonella bacteria may internalize into tomato pulp when warm tomatoes from the field are submerged into colder water. Several washing steps may follow the initial washing and packing of tomatoes at the packinghouses; the potential for internalization into tomatoes in subsequent washing steps when tomatoes have a cooler pulp temperature is unknown. Our objective was to evaluate Salmonella internalization into mature green and red tomatoes with ambient (21°C) and refrigeration (4°C) pulp temperatures when they were submerged into water at various temperature differentials, simulating repacking and fresh-cut operations. Red (4°C and 21°C) and mature green (21°C) tomatoes were submerged (6 cm) into a six-strain Salmonella cocktail (6 log CFU/ml) and maintained at ±5 and 0°C temperature differentials for varying time intervals, ranging from 30 s to 5 min. Following submersion, tomatoes were surface sterilized using 70% ethanol, the stem abscission zone and blossom end epidermis were removed, and cores were recovered, separated into three segments, and analyzed. Salmonella populations in the segments were enumerated by most probable number (MPN). The effects of temperature differential and maturity on Salmonella populations were analyzed; results were considered significant at a P value of ≥0.5. Internalized populations were not significantly different (P ≥0.5) across temperature differentials. Salmonella internalization was seen in tomatoes under all treatment conditions and was highest in the segment immediately below the stem abscission zone. However, populations were low (typically >1 log MPN per segment) and varied greatly across temperature differentials. This suggests that the temperature differential between tomatoes and water beyond the initial packinghouse may be less important than submersion time in Salmonella internalization.