Effect of pH and Water Activity on Inactivation of Non-O157 Shiga Toxin-Producing Escherichia coli (STECs) in Acidulated Beef Sausage by High Pressure Processing

Introduction: High pressure processing has gained use in the meat industry to achieve pathogen reduction in products with minimal impact on product quality. The USDA-FSIS has declared six additional Shiga Toxin-producing E. coli (STEC) serogroups as adulterants in non-intact raw beef products in response to increasing human illness from these organisms. 

Purpose: The objective of this study was to evaluate the effect of pH and water activity (a_w) on the efficacy of high pressure processing (HPP) for inactivation of non-O157 STEC in acidulated beef sausages.

Methods: A six-strain cocktail of non-O157 STECs (E. coli O26, O45, O103, O111, O121 and O145; ca. 7 log CFU/ml) was inoculated into irradiated, acidulated beef sausages of different pH (4.0 - 6.0) and a_w (0.950 - 0.980) values. Samples were pressure-treated at 586 MPa for 1 s, 1 min, 3 min and 5 min. STEC populations were enumerated on Petrifilm™ E.coli/Coliform Count Plates and Aerobic Count Plates.

Results: An STEC inoculation level of 6.96 ± 0.17 log CFU/g was achieved in sausages. Achieving the target pressure of 586 MPa (1 s) resulted in 0.90, 0.20, 0.39 and 0.44 log CFU/g reductions in STEC populations in beef (pH 6.0) with a_w values of 0.980, 0.970, 0.960 and 0.950, respectively. Increasing the pressure to 586 MPa (1 s) resulted in 0.90, 0.22, 0.46, 1.70 and 1.41 log CFU/g reductions in STEC populations in beef (a_w 0.980) with pH values of 6.0, 5.5, 5.0, 4.5 and 4.0, respectively. Greater STEC reductions were observed with increasing HPP processing time (586 MPa), decreasing pH and increasing a_w.

Significance:  High pressure processing can be used as an effective method to achieve STEC reductions in beef sausages of various pH and a_w values.