Growth kinetics of Escherichia coli O157:H7 in mechanically-tenderized beef

A kinetic study was conducted to investigate the growth of Escherichia coli O157:H7 in mechanically-tenderized beef meat (MTBM) inoculated and internalized with a cocktail of 5 rifampicin-resistant (Rif(r)) or 3 arbitrarily selected wild-type strains of the bacteria. The storage was conducted at 5, 10, 15, 20, 25, and 37 degrees C. No growth was observed at 5 degrees C and the growth was minimal at 10 degrees C. Above 15 degrees C, a sigmoid trend was observed for all growth curves. Three primary growth models (modified Gompertz, Huang, and Baranyi) were used to fit the growth curves. A new Belehdradek-type secondary model was found more suitable than the traditional Ratkowsky model for describing the temperature dependence of growth rate. The statistical analysis suggested that both bacterial strains and primary growth model affect the determination of growth rates (at alpha=0.05), with Rif(r) strains growing 10-20% slower than the wild-type strains. While there was no significant difference between the growth rates estimated by the modified Gompertz and Baranyi models, and between those of Huang and Baranyi models, the rates estimated from the Gompertz model were significantly higher than those estimated from the Huang model. The temperature dependence of growth rate of E. coli O157:H7 in MTBM was described by both a new Belehdradek-type rate model and the Ratkowsky square-root model. While the theoretical minimum growth temperatures determined by the Ratkowsky square-root model ranged from 1.5 to 4.7 degrees C, more realistic values, varying from 6.64 to 8.76 degrees C, were estimated by the new rate model. For the Baranyi model, the average h(0) value was 2.06+/-0.74 and 2.15+/-1.14 for Rif(r) and wild-type strains of E. coli O157:H7, respectively. For the Huang and modified Gompertz models, the inverse of square-roots of lag phases was found proportional to temperature, making it possible to estimate the lag phase duration from the growth temperature. The results of this work can be used to assess the microbial safety of MTBM during refrigerated and temperature-abused storage conditions.