Weibull Model for Inactivation of Geobacillus Stearothermophilus Spores in Superheated Steam

Spores of Geobacillus stearothermophilus at two inoculum levels (3 and 6 log colony forming units per gram, cfu/g) were challenged with superheated steam (SS) at a velocity of 0.35 m/s and temperature of 130, 145, 160, and 175C. Tailing was observed in the survivor curves (plots of survival ratios, log( o N N ) against treatment time) as a monotonic upward concavity. Therefore, a non-linear regression model, the Weibull model, instead of the traditional first order kinetics model was used to describe the survivor curves. The Weibull model was successfully fitted to all survivor curves obtained in this study. Good fit of the Weibull model to spore survivor curves and high correlation between the predicted and the experimental values of spore survival ratios were observed for SS treatment at 145-175C at both inoculum levels, and for SS at 130C at low inoculum level. The Weibull model was expanded by describing its parameters (α, β) as a function of SS temperature for the range of 130-175C. This allowed accurate predictions of the spore survival ratios based on the SS temperature and processing time only (with the exception of predictions for spores at high inoculum level challenged with SS at 130C). The Weibull model is recommended to describe and to model the inactivation of G. stearothermophilus spores in SS at high temperatures (145-175C).