Determination of Thermal Processing Conditions for Acidified Foods with a pH of 4.6 or Below

Introduction: Acidified foods with a pH at or below 4.6 must be processed to achieve a 5-log reduction for vegetative bacterial pathogens. Published research does not exist to support FDA process filings for products with pH 4.1-4.6. This lack of documentation is challenging to all areas of the industry but especially for small processors.

Purpose: To develop models and data for the thermal destruction of acid resistant vegetative microbial pathogens in acidified foods with a pH above 4.1 (pH 4.1-4.6), and generate Z and F value for the safe processing of acidified foods.

Methods: Five strain cocktails (3 independent replications each) of Escherichia coli O157:H7, Salmonella enterica, and Listeria monocytogenes were incubated at 56°C to 66°C in a cucumber juice medium (CJ) at pH 4.6 containing acetic acid (100 mM), a non-inhibitory buffer. After heating, surviving viable cells were diluted at room temperature prior to plating on non-selective agar. Z and F values were then determined.

Results: Log CFU/ml vs time data exhibited non-linear kinetics. A Weibull model was used to generate 5-log reduction times. Unlike previous results for thermal processing at pH 4.1 or lower, we found that L. monocytogenes strains were significantly more heat resistant than E. coli O157:H7 (P < 0.05). Overall, S. enterica was the least heat resistant organism. The predicted Z value was 9.98°C with a standard error of 1.80. The corresponding F value at 71.1°C (F_71.1, or F₁₆₀) was 5.68 min.

Significance: With the advent of the Food Safety Modernization Act, producers of acidified foods will be required to determine if production processes assure a 5-log reduction in vegetative pathogens. These data may be used by process authorities to aid in determining safe production practices, although an additional safety margin, based on the standard error for the Z value, may be included.