Quantification of the Survival of Pathogenic Escherichia coli during Meat Preparation

Introduction:  Meat is, based on epidemiological data, considered an important transmission route for pathogenic bacteria. Although inactivation through heating has been a subject of investigation, with respect to evaluation of possible health risks, quantification of inactivation is often lacking.

Purpose: The goal of this project was to quantify survival of pathogenic bacteria during meat preparation, for risk assessment calculations, with a special interest on the side of the steak. The methods for preparation of the steaks were those most commonly recommended for the various types of readiness.

Methods: Steaks were contaminated with Escherichia coli O111 (cured, i.e. lacking the genes for Shiga Toxins) and fried in butter according to a standardized protocol. Frying times were 2 minutes per side (rare), 4 minutes per side (medium) and 6.5 per side minutes (done). After frying, steaks were left for three minutes and, subsequently, the number of surviving bacteria was determined by culture method. During frying, the temperature at the top and side of the steak, was determined with an Infrared Thermal Imager. Appropriate controls were part of the investigations.

Results: Increase in frying time led to a fast decrease in survival of E.coli. Decrease in survival on top/bottom of the steaks was two to three Log₁₀ units higher than on the side of the steaks. Overall, a decrease in numbers of E.coli ranged from two to three Log₁₀ units. Two samples from the 2 minutes per side frying time and two samples from the 6.5 frying time per side samples showed seven to eight Log₁₀ unit reductions.

Significance:  The temperature profile, in combination with the inactivation data, allowed for the calculation of D/z-values. This is important for risk assessment calculations, for any meat type and preparation method.