Microbiological Safety of Salad Dressing Challenged with Foodborne Pathogens

Introduction: Acidified foods (pH ≤ 4.6, a_w  ≥ 0.85) traditionally have been considered safe against foodborne pathogens. However, several recent outbreaks caused by acid-resistant foodborne pathogens in some acid foods have been reported. FDA requests scientific support for cold fill and hold products through challenge study to demonstrate a 5 log reduction of pathogens (Draft Guidance for Industry: Acidified Foods).

Purpose: The objective of this study was to determine the inactivation rate of Listeria monocytogenes, Salmonella spp. and Escherichia coli O157:H7 in five formulations of salad dressing when stored at 73-77°F for 5 days. 

Methods: Products were inoculated separately with 4 strain composite of each acid-adapted pathogen to achieve 10^6-7CFU/g.  After inoculation, product was sub-sampled and stored in sterile conical tubes (30 g each) at 73-77°F.  Three replicate samples were enumerated for each pathogen daily for 5 days.  Once the counts were below the detection limit (10 CFU/g), samples were qualitatively analyzed for presence of each pathogen.

Results: The average initial counts of L. monocytogenes, Salmonella spp. and E. coli O157:H7 in 5 formulations ranged 5.23-5.38, 5.69-5.86 and 5.56-5.68 log CFU/g, respectively.  The counts of all pathogens were < 10 CFU/g after 2 days, regardless of product formulation. In addition, pathogens were not detected for 3 consecutive analyses in all 5 formulations.

Significance: Salad dressing formulations successfully demonstrated a 5- log reduction of L. monocytogenes, Salmonella spp. and E. coli O157:H7 within 2 days when stored at ambient temperature (73-77°F). This data suggest that the product needs to be held at ambient temperature for at least 2 days before shipping to ensure safety against foodborne pathogens to meet the draft guidance for Industry for acidified foods.