Dagmara Head and Stefan Cenkowski Weibull Model for Inactivation of Geobacillus stearothermophilus Spores in Superheated Steam Dagmara Head and Stefan Cenkowski Department of Biosystems Engineering, University of Manitoba, Winnipeg, MB CSBE-SCGAB 2009 Annual Conference, Rodd’s Brudenell River Resort, Prince Edward Island, 12 – 15 July 2009

Superheated steam (SS) SS – water vapour heated to temperature above boiling point at given pressure SS processing: rapid transfer of heat air-free environment energy savings – condensate reuse Introduction Objectives Materials & Methods Results Conclusions

Geobacillus stearothermophilus Gram positive, thermophilic bacterium Forms highly heat-resistant spores Hi, I’m G. stearothermophilus spore, and I’m TOUGH!!! Introduction Objectives Materials & Methods Results Conclusions

Thermal inactivation of microorganisms Presumed to follow the first order kinetics model: Where: No - initial number of microorganisms (cfu/g), N - number of microorganisms at time t (cfu/g), t - treatment time (min), k – a rate constant (1/min) Non-linearity of survivor curves (shoulders, tails, upward or downward concavity) Introduction Objectives Materials & Methods Results Conclusions

Survivor curves for G. stearothermophilus spores exposed to SS Introduction Objectives Materials & Methods Results Conclusions

The Weibull model Fit of the model Introduction Objectives Materials & Methods Results Conclusions

The Weibull model Where: No - initial number of microorganisms (cfu/g), N - number of microorganisms at time t (cfu/g), t - treatment time (min), α - scale parameter (min), β - shape parameter (non-dimensional) Introduction Objectives Materials & Methods Results Conclusions

Superheated Steam Processing System 6.3 cm Introduction Objectives Materials & Methods Results Conclusions

Processing conditions SS at 0.35 m/s and 130-175oC (15oC increments) Geobacillus stearothermophilus ATCC 10149 + sand: @ low inoculum: 3 log cfu/g @ high inoculum: 6 log cfu/g Introduction Objectives Materials & Methods Results Conclusions

α and β parameters of the Weibull model Temperature of SS (C) Inoculum Level (log cfu/g) α β 130 145 160 175 3 6 1.90 5.88 3.82 13.25 1.59 1.56 0.005 0.35 0.21 0.39 1.08 0.61 1.29 0.18 0.72 Introduction Objectives Materials & Methods Results Conclusions

The shape and scale parameters as functions of SS temperature Where: a1, a2, b1, b2 are coefficients, T is SS temperature (oC) The expanded Weibull model Introduction Objectives Materials & Methods Results Conclusions

Validation of the Weibull model – low inoculum R2 0.82-0.97; slope 0.78-1.09 Introduction Objectives Materials & Methods Results Conclusions

Validation of the Weibull model – high inoculum R2 0.7-1; slope 0.82-1.28 but for 130oC R2 0.17; slope 0.15 Introduction Objectives Materials & Methods Results Conclusions

The Weibull model  to predict survival of Geobacillus stearothermophilus spores exposed to SS at 145-175oC Survival predicted based on SS temperature and treatment time Introduction Objectives Materials & Methods Results Conclusions

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