Effect of Cooling Methods, pH and Temperature on Salmonella Survival on Inoculated Intact and Pureed Strawberries and Pureed Tomatoes

Introduction: Postharvest strawberries are chilled to 3-4°C, usually by forced-air-cooling. Hydrocooling could ensure faster cooling, although the risk of cross-contamination for strawberries has not been evaluated. Salmonella survives on wounded berries, but is unable to multiply. Limited information is available on whether the inability to multiply is due to the low pH or other intrinsic factors associated with strawberries.

Purpose: This study evaluated Salmonella survival a) on the surface of intact hydrocooled and forced-air-cooled strawberries; b) as affected by pH, temperature and food matrix, in strawberry and tomato puree.

Methods: Intact strawberries inoculated with Salmonella (10⁷ CFU/berry) and packed in plastic ‘clamshells’ (8 berries/shell) were subjected to a) forced-air-cooling at 2°C for 90 min or; b) hydrocooling in water (2-5°C) containing a sanitizer (100 or 200 ppm active chlorine) for 12 minutes. Salmonella population was enumerated on tryptic soy agar at 0, 7 and 8 days post-treatment. Strawberry and tomato puree (pH 3.7 and 4.6) were spiked with Salmonella (10⁵ CFU/25 g), and incubated at 4, 10 or 25°C. Salmonella survival was evaluated 0, 1 and 3 days post-inoculation. Each experiment was performed in triplicate (n = 9).

Results: On day 0, Salmonella survival on intact, hydrocooled strawberries was reduced by > 4.0 log CFU/berry, compared to forced-air-cooled berries. Salmonella was below enumerable levels (1.5 log CFU/berry) on hydrocooled strawberries on days 7 and 8. When incubated at 4 and 10°C, Salmonella survived for 3 days, but did not multiply in either strawberry or tomato puree, regardless of the pH. When incubated at 25°C, Salmonella was capable of growth in both strawberry and tomato puree, when the pH was 4.7.

Significance: Salmonella survival on inoculated intact strawberries was reduced when subjected to hydrocooling, compared to forced-air-cooling. Food matrix did not significantly affect Salmonella survival or growth, whereas temperature and pH significantly affected growth.