Inactivation of Shiga toxin-producing Escherichia coli O157: H7 and mesophilic background microbiota of meat homogenate using elevated hydrostatic pressure, mild heat, and thymol.

A six-strain mixture of E. coli O157:H7 was exposed to 0 to 9 min of six treatments: (i) hydrostatic pressure (400 MPa) at 4 °C; (ii) hydrostatic pressure and thymol at 4 °C; (iii) thymol at 4 °C; (iv) heat at 40 °C; (v) hydrostatic pressure at 40 °C; and (vi) hydrostatic pressure and thymol at 40 °C. Pressure intensity level of 400 MPa and thymol concentration of 0.15% (w/v) were used for the experiments of inoculated pathogen (4.0 to 5.0 log CFU/mL) in a homogenate (10% nonsterilized beef in 90% sterilized distilled water). Temperature was precisely monitored by stainless steel water jacket surrounding pressure chamber (16 mL volume), mechanically linked to a refrigerated circulating water bath. Analyses of variance were conducted followed by Tukey- and Dunnett's-adjusted mean separations. Pathogen counts before treatment were 4.08 ± 0.7 log CFU/mL and were reduced (P < 0.05) to 0.67 ± 0.2 log CFU/mL after 6 min of pressure treatment. Thymol and mild heat (40 °C) further augmented decontamination efficacy of pressure treatments where in their presence, the mesophilic background microbiota counts of pressure-treated samples after 3, 6, and 9 min were reduced (P < 0.05) by 2.1, 2.5, and 3.1 log CFU/mL, respectively. Results of the current study indicate that thymol and mild heat could enhance decontamination efficacy of elevated hydrostatic pressure for pasteurization of food commodities. This could be of great significance for industry practitioners to assure microbiological safety of a product and cost optimization by benefiting from synergism of antimicrobials, mild heat, and elevated hydrostatic pressure. PRACTICAL APPLICATION: Thymol and mild heat could enhance decontamination efficacy of pressure-based pasteurizer that could be of great significance for practitioners. Application of pressure coupled with antimicrobial and mild heat could assure microbiological safety of a product, lead to cost optimization, and assist in meeting regulatory requirements of food commerce such as Hazard Analysis and Critical Control Point and Preventive Control for Human Food rule of Food Safety Modernization Act. Addition of an antimicrobial could have further co-benefits for the product due to residual protective effects during shelf-life and minimizing potential undesirable organoleptic changes associated with pressure treatments of >400 MPa.