Heat and High Hydrostatic Pressure Resistance of Escherichia coli Isolated from a Beef Processing Facility

Introduction: High hydrostatic pressure processing is a promising non-thermal food preservation technology and is used commercially for preservation of meat products. Escherichia coli AW 1.7, which was isolated from a beef carcass, is an exceptionally heat resistant organism. Little is known about the heat and pressure resistance of E. coli isolated from meat or meat processing facilities.

Purpose: To determine the resistance of E. coli AW1.7 and other isolates from beef to heat or pressure. The impact of NaCl on resistance was also determined.   

Methods: E. coli were grown and heated to 60°C in LB broth with addition of 0 to 4% NaCl.  Cultures were treated in LB broth at 400 MPa at 40°C.  

Results: Of the 11 strains of E. coli, 4 showed a resistance to heat that was comparable to E. coli AW1.7, corresponding to a reduction of cell counts of less than 5 log (CFU/ml) after 20 min at 60°C. All 11 strains were as pressure resistant as E. coli AW1.7, corresponding to a reduction of cell counts of less than 6 log (CFU/ml) after 15 min at 400 MPa and 40°C. When 2 or 4% NaCl was added to the growth- and treatment medium, after treatment at 60°C for 40 min cell counts of E. coli AW1.7 were higher by 3.5 log (CFU/ml) compared to strains grown and treated with no NaCl added. The heat resistance of all other strains responded similarly with the addition of NaCl. Remarkably, the resistance to pressure was not affected by addition of NaCl. 

Significance: Heat resistance of E. coli is not correlated to pressure resistance. Strains of E. coli that resist 60°C for 5 min or 400 MPa at 40°C for 15 min are part of the natural population that can be isolated from beef processing facilities.