1. 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

2. 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

3. 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

4. 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

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

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

7. 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

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

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

10. 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

11. 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

12. Validation of the Weibull model – low inoculum
R ; slope
Introduction
Objectives
Materials & Methods
Results
Conclusions

13. Validation of the Weibull model – high inoculum
R ; slope
but for 130oC R2 0.17; slope 0.15
Introduction
Objectives
Materials & Methods
Results
Conclusions

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

15. Hi again,
Do you have questions?