Weibull Model for Inactivation Weibull Model for Inactivation of of Geobacillus Geobacillus

stearothermophilus stearothermophilus Spores in Spores in Superheated SteamSuperheated Steam

Dagmara Head and Stefan CenkowskiDagmara Head and Stefan Cenkowski

Department of Biosystems Engineering, University of Department of Biosystems Engineering, University of Manitoba, Winnipeg, MBManitoba, Winnipeg, MB

CSBE-SCGAB 2009 Annual Conference, Rodd’s Brudenell River CSBE-SCGAB 2009 Annual Conference, Rodd’s Brudenell River ResortResort,, Prince Edward Island, Prince Edward Island, 12 – 15 July 200912 – 15 July 2009

Introduction Materials & MethodsObjectives Results Conclusions

Superheated steam (SS)Superheated steam (SS)

SS – water vapour heated to SS – water vapour heated to temperature above boiling point at temperature above boiling point at given pressuregiven pressure

SS processing:SS processing:►rapid transfer of heatrapid transfer of heat►air-free environmentair-free environment►energy savings – condensate reuseenergy savings – condensate reuse

Introduction Materials & MethodsObjectives Results Conclusions

Geobacillus stearothermophilusGeobacillus stearothermophilus

► Gram positive, thermophilic bacteriumGram positive, thermophilic bacterium

► Forms highly heat-resistant sporesForms highly heat-resistant spores

Hi, I’m G. stearothermophilus spore, and I’m TOUGH!!!

Introduction Materials & MethodsObjectives Results Conclusions

Thermal inactivation of Thermal inactivation of microorganismsmicroorganisms► Presumed to follow the first order kinetics model: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, Non-linearity of survivor curves (shoulders, tails, upward or downward concavity)tails, upward or downward concavity)

t)(oN

Nlog k

Introduction Materials & MethodsObjectives Results Conclusions

Survivor curves for Survivor curves for G. stearothermophilusG. stearothermophilus spores exposed to SSspores exposed to SS

-5

-4

-3

-2

-1

0

1

0 20 40 60 80 100 120

Treatment Time (min)

Sur

vivo

rs,

log

(N/N

o)

105°C 130°C 145°C 160°C 175°C

Introduction Materials & MethodsObjectives Results Conclusions

►The Weibull model The Weibull model

►Fit of the modelFit of the model

Introduction Materials & MethodsObjectives Results Conclusions

The Weibull modelThe 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)

β

α

t

2.303

1

N

Nlog )()(

o

Introduction Materials & MethodsObjectives Results Conclusions

Superheated Steam Processing Superheated Steam Processing SystemSystem

condensate steam out water

processing chamber

condenser

steam generator superheater

sup

erh

eat

ed

st

ea

m

sample tray

6.3 cm

Introduction Materials & MethodsObjectives Results Conclusions

► SS at 0.35 m/s and 130-175SS at 0.35 m/s and 130-175ooC (15C (15ooC C increments)increments)

► Geobacillus Geobacillus stearothermophilusstearothermophilus ATCC 10149 ATCC 10149 + sand: + sand:

@ low inoculum: 3 log cfu/g@ low inoculum: 3 log cfu/g

@ high inoculum: 6 log cfu/g@ high inoculum: 6 log cfu/g

Processing conditionsProcessing conditions

Introduction Materials & MethodsObjectives Results Conclusions

αα and and ββ parameters of the Weibull modelparameters of the Weibull model

Temperature of SS (C)

Inoculum Level (log cfu/g) α β

130

145

160

175

36

36

36

36

1.905.88

3.8213.25

1.591.56

0.0050.35

0.350.21

0.391.08

0.611.29

0.180.72

Where: a1, a2, b1, b2 are coefficients, T is SS temperature (oC)

The shape and scale parameters as functions The shape and scale parameters as functions of SS temperatureof SS temperature

Taaα 12 Tbbβ 12

T)1

b2

(b

Taa

t

2.303

1

N

Nlog )()(

12o

The expanded Weibull modelThe expanded Weibull model

Introduction Materials & MethodsObjectives Results Conclusions

Validation of the Weibull model – low Validation of the Weibull model – low inoculuminoculum

Introduction Materials & MethodsObjectives Results Conclusions

-2.5

-2

-1.5

-1

-0.5

0

0 10 20 30 40 50 60 70 80 90Processing Time (min)

Sur

vivo

rs,

log(

N/N

o)

predicted at 130°C predicted at 145°C predicted at 160°C predicted at 175°C

measured at 130°C measured at 145°C measured at 160°C measured at 175°C

Low Inoculum

R2 0.82-0.97; slope 0.78-1.09

Validation of the Weibull model – high Validation of the Weibull model – high inoculuminoculum

Introduction Materials & MethodsObjectives Results Conclusions

-5.5

-4.5

-3.5

-2.5

-1.5

-0.5

0.5

0 20 40 60 80 100 120Processing Time (min)

Sur

vivo

rs,

log(

N/N

o)

predicted at 130°C predicted at 145°C predicted at 160°C predicted at 175°C

measured at 130°C measured at 145°C measured at 160°C measured at 175°C

High Inoculum

R2 0.7-1; slope 0.82-1.28 but for 130oC R2 0.17; slope 0.15

Introduction Materials & MethodsObjectives Results Conclusions

►The Weibull model The Weibull model to predict survival to predict survival ofof

Geobacillus stearothermophilus Geobacillus stearothermophilus spores spores exposed to SS at 145-175exposed to SS at 145-175ooC C

►Survival predicted based on SS Survival predicted based on SS temperature and treatment timetemperature and treatment time

Hi again,Do you have questions?