The Effects of pH on Pathogen Death Rate (Escherichia coli O157:H7, Salmonella and Listeria monocytogenes) in Minimally Processed, Shelf-Stable, Non-dairy Dressings

E. coli 0157:H7, Salmonella spp. and Listeria monocytogenes are human pathogens that have been associated with numerous food-borne illness outbreaks. These pathogens are a major threat to children, elderly, pregnant women and people with a weakened immune system. Some non-dairy based, salad dressings are marketed as shelf-stable without being thermally processed. Thermal processing will destroy the integrity of these products. Shelf-stable salad dressings rely on low pH to prevent pathogen growth. The low pH of salad dressing could also result in the rapid death of E. coli 0157:H7, Listeria monocytogenes and Salmonella. Two experiments for each pathogen were conducted to determine the effect of pH on pathogen survival in minimally processed, acidified salad dressings. Two types of dressings, Roasted Garlic Vinaigrette and Cranberry Ginger, were inoculated with (a high (5-log CFU/g) and low (2-log CFU/g) inoculation level of pathogens) three strain cocktails of, L. monocytogenes, E. coli O157:H7 and Salmonella at pH levels of 3.3 and 3.9. Inoculated dressing samples were spread plated onto selective media with recovery overlays, 14 mL of Trypticase Soy Agar on days 0, 1, 3, 7, 10 and 14 following inoculation. If pathogens were not detected in the inoculated samples, they were enriched using selective enrichment broths. A multi-way analysis of variance was used to determine significance in death rate between pH levels. Of the three pathogens tested, E. coli O157:H7 survived the longest from direct plating. E. coli O157:H7 in dressings at pH 3.3 were detected up to day 7 and day 10 for pH 3.9 samples. Listeria monocytogenes was not detected in samples at pH 3.3 after day 1 and after day 7 for samples at pH 3.9; most samples had no detectable Listeria monocytogenes by day 3. Listeria monocytogenes was detected 10 days after inoculation by enrichment. Salmonella was not detected in the pH 3.3 dressings after day 1, and only until day 3 in dressings at pH 3.9. The pathogens survived longer in the Cranberry Ginger Dressing compared with Roasted Garlic Vinaigrette; this could be from differences in water activity, sugar and salt concentration. The survival rate was prolonged in the high level inoculation (5-log CFU/g) compared with the low level inoculations (2-log CFU/g)