Microwave Inactivation of Bacteria under Dynamic Heating ...€¦ · Q abs E rms p f 0 r E rms 2 local rms E E Q abs: volumetric heating -rate (W.m 3) = Electrical conductivity (S/m)

Microwave Inactivation of Bacteria under Dynamic Heating Conditions

in Solid Media S. Curet, M. Mazen Hamoud-Agha

LUNAM Université, ONIRIS, CNRS, GEPEA, UMR 6144, site de la Géraudière, Nantes, France

Excerpt from the Proceedings of the 2012 COMSOL Conference in Milan

Context Model Design

Microbial inactivation

Thermal modelling

Conclusion/ Perspectives

2

The microwave pasteurization process

Application for prepacked food (yoghurt, pouch-packed meals, milk…)

Difficulty in monitoring and predicting the microwave heating pattern during processing

Few literature reviews concerning microwave inactivation process within a solid food products

Objective: predicting microbial inactivation during microwave pasteurization



Thermal modelling


3

Microwave processing into a TE10 rectangular waveguide

Pin = 130 W



Thermal modelling


4

Governing equations

• Heat transfer equation (PDE from COMSOL®)

absp QTkdivt

TC

).(

Thermophysical properties of the Ca-alginate gel*

Microwave absorbed power (W.m-3)

Maxwell’s equations (kg.m-3) 1010

Cp (J.kg-1.K-1) 4120

k (W.m-1.K-1) 0.84

* Lin, Y. E., R. C. Anantheswaran, et al. (1995). "Finite element analysis of microwave heating of solid foods." Journal of Food Engineering 25(1): 85-112.



Thermal modelling


5

Governing equations

• Electric field propagation (RF module)

Maxwell’s equations for a TE10 rectangular waveguide (sinusoidal time-varying fields with w = 2p f)

²...2² ''

0 rmsrrmsabs EfEQ p

2

localrms

EE

Qabs : volumetric heating rate (W.m-3)

= Electrical conductivity (S/m)

f : frequency of microwaves (2.45×109 Hz)

0 : permittivity of free space (F.m-1)

r’’ : relative dielectric loss factor

Erms: root-mean-square average value of electric field at a location (V.m-1)

01'

'2

2

2

2

2

y

yyEj

z

E

x

E

w

w



Thermal modelling


6

Governing equations

• Dielectric properties of the sample (2.45 GHz)

r’ = 81.79-0.299×T(°C)

r’’ = 22.6-0.378×T+2.93×10-3×T2

* Lin, Y. E., R. C. Anantheswaran, et al. (1995). "Finite element analysis of microwave heating of solid foods." Journal of Food Engineering 25(1): 85-112.



Thermal modelling


7

Governing equations

• Boundary conditions

rLzandzRratHHn

zyxatHn

zyaxatandzxbyyatEE

ab

PZEyxzat

a

xExE

zyxtatE

sampleair

y

inTEy

,2/,0

,00

,2/,2/,00

4;,cos

00

tan

00

p

rLzandzRratTThTk

zyxtatTT

,2/,

,00



Thermal modelling


8

Governing equations

• Microbial inactivation of E. Coli K12 (2 ODE equations)

Dynamic model from Geeraerd et al.* (2000)

Inactivation kinetics during dynamic heating (Bigelow, 1921)

Dref is estimated at Tref within the lethal temperature range

cc

c

Ckdt

dC

NC

kdt

dN

.

.1

1.

max

max

).(

10ln

max

10ln)(

refTTz

ref

eD

Tk

N = microbial population (CFU/g) Cc = physiological state of the cells (-)

* Geeraerd, A. H., C. H. Herremans, et al. (2000). "Structural model requirements to describe microbial inactivation during a mild heat treatment." International Journal of Food Microbiology 59(3): 185-209.



Thermal modelling


9

Mesh generation

• 26750 tetrahedral elements

sample

Air surrounding medium



Thermal modelling


10

Dynamic temperature profile

Dref = 234 s at 57°C, z = 6.28°C tprocess = 270 s

Estimation of D and z values from water bath experiments (9°C/min)



Thermal modelling


11

Local inactivation Global inactivation

(3D numerical integration)

Microbial inactivation during microwave heating

t = 270 s

H =

10

mm

r = 4 mm waves

water bath heating with 9°C/min



Thermal modelling


12

thermal heterogeneities at the end of microwave processing (DTmax = 5°C between the hot and cold spots)

local electric field concentrations around sample edges

Temperature and electric field distribution

t = 270 s



Thermal modelling


13

Highlights

• Modelling microwave pasteurization process: Non-uniform temperature distribution into a 0.5 mL cylindrical sample

(prediction of the cold point),

Lower bacteria inactivation compared to conventional water bath thermal treatment,

The global inactivation of bacteria under microwaves is successfully predicted from D and z values obtained from water bath experiments,

Cells death during microwave heating is mainly due to a thermal effect.



Thermal modelling


14

Future prospects

• Validation of the numerical model with a time-temperature controlled loop:

In order to insure better cells inactivation during microwave heating:

Study on the inactivation efficiency by maintaining the temperature within the lethal range (T > 55 °C)



Thermal modelling


15

Thank you for your attention,

any questions ??

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Microwave Inactivation of Bacteria under Dynamic Heating ...€¦ · Q abs E rms p f 0 r E rms 2 local rms E E Q abs: volumetric heating -rate (W.m 3) = Electrical conductivity (S/m)