Effectiveness of the Thermal Treatments Used for Curd Stretching in the Inactivation of Shiga Toxin-Producing O157 and O26 Escherichia coli

The kneading treatment of the fresh curd in hot water is a critical control point in the manufacturing of mozzarella. Factors such as the ratio between hot water and curd mass, the rheological properties, and the mixing and kneading activity affect the processing time and the internal temperature of the curd. The aim of this study was to investigate the effect of thermal treatments on the fate of Shiga toxin-producing Escherichia coli (STEC). Nine curd samples (weight 160-270 g) were artificially contaminated with O157 or O26 STEC and stretched in hot water (90-95 degrees C) for 5-10 min. Depending on the heating process and spinning, different nonisothermal profiles were recorded. Observed reductions of O157 and O26 STEC varied between 1.01 and more than 5.38 log MPN (Most Probable Number)/g at the end of the temperature treatments. Further, nonisothermal log-linear tail models were developed to compare observed reductions for O157 and O26 VTEC under variable temperature conditions. Results obtained showed that the comparison of predictions provided by the dynamic model with observations described well the linear inactivation pattern since nonsignificant differences were denoted at all profiles tested. The dynamic model developed can be useful to evaluate the effectiveness of the thermal treatments used in the manufacturing of mozzarella in the inactivation of STEC.