Biomedical Engineering Reference
In-Depth Information
7.6.2.10
Thermal Properties and Prediction Models
The thermophysical properties of foods depends on the heat and mass transfer
mechanisms, phase changes involved, and chemical reactions during chilling, freezing
or heating. These characteristics include temperature and velocity, fl uid properties
such as viscosity, density, conductivity, and heat capacity, product surface proper-
ties such as geometry and dimension, and internal structure or arrangements. The
dielectric properties of foods are very important for microwave heating. Due to the
interaction of these parameters, the boundary layers can be very complex, therefore
predicting the cooling time by considering all the above factors can be a diffi cult
task (Hu and Sun
1999
,
2000
).
Convection heat transfer is the major mode of heat transfer between the surface
of a solid material and the surrounding fl uid. The rate of convective heat transfer
depends on the properties of the fl uid and the fl uid fl ow characteristics. Originally
as suggested by Prandtl, the resistance to heat transfer may be considered to be
localized in a boundary layer within the fl uid present at the surface of the solid
material. Although this concept is for ideal situations, it has been widely used in
studying convective heat transfer. Commercial numerical techniques, such as com-
putational fl uid dynamics (CFD) using CFX software have been successfully
adapted to the food industry to simulate the thermal process. This can provide a
better understanding of the mechanism and aid in the improvement of design and
operation (Anon
1997
). Recently, a number of models have been developed for
predicting the food chilling process and the infl uence of its thermal properties
(Chuntranuluck et al.
1998
; Kuitche and Daudin
1996
; Davey and Pham
1997
;
Maroulis et al.
1995
; Hu and Sun
1999
,
2000
; Xia and Sun
2002
; Kaushal and
Sharma
2012
).
Computational fl uid dynamics can be used for other equipment designs, for
detailed product development and for scale-up of the process. The microbiological
stability and safety of sous-vide products is mainly determined by the different ther-
mal treatments including heating, cooling and reheating. The theoretical aspects of
the different modes of heat transfer relevant to 'sous-vide' cooking can be opti-
mized using computational techniques such as the fi nite element method and
CFD. These techniques can provide a powerful means to investigate the effect of
modifi cations in the processing conditions on the internal temperature of the food
(Baerdemaeker and Nicolaï
1995
).
A CFD model to study heat and mass transfer of a cylindrical shaped cooked
meat within an air-blast chiller was carried out to predict its cooling rate and weight
loss during chilling (Hu and Sun
2000
). The investigation was based on a mathe-
matical analytical model of unsteady heat and mass transfer with the assumption of
a homogeneous heat transfer coeffi cient (i.e. which takes into account the effects of
forced convection, radiation and moisture evaporation on the surface of the cooked
meat joint). This method allows the simultaneous CFD prediction of both tempera-
ture distribution and weight loss in the meat throughout the chilling process. Cooling
time and weight loss from 75 to 3.5 °C were approximately 530 min and 4.25 %,
