Thermal Food Processing

62 Thermal Food Processing: New Technologies and Quality Issues
than one of these mechanisms is involved. In some processes, both heat and mass
transfers occur simultaneously. It is important to understand the heat and mass
transfer mechanism for the improvement of existing food thermal processes and
for the development of new and better processes.
The physical laws of heat and mass transfer have widely been used to describe
food thermal processes, producing a large number of mathematical models. Some
assumptions, such as simplified geometrical shape, constant thermal physical properties,
constant surface heat, and mass transfer coefficients, and no volume change
during processing, were widely used in modeling. However, more research should
be conducted to justify the acceptability of those assumptions and to improve the
accuracy of models by finding more information on surface heat and mass transfer
coefficients, food properties, and shrinkage during processing. Before heat and mass
transfer models can become a quantitative tool for correctly analyzing thermal
processes, determination of thermal physical propensities and surface mass and
heat transfer coefficients remains an important area to be studied.
NOMENCLATURE
A Area (m2 )
C Concentration (kg/m3 or kmol/m3 )
c Specific heat capacity (kJ/kg K)
C * Concentration in equilibrium with the bulk gas partial
pressure (kg/m3 or kmol/m3 )
d Diameter (m)
D Mass diffusivity (m2 /sec)
D0 Preexponential factor in Arrhenius equation (m2 /sec)
Dg Mass diffusivity in the gas phase (kmol/N m sec)
Dl Mass diffusivity in the liquid phase (m2 /sec)
E Electrical field strength (V/m)
e Emissivity (–)
Ea Activation energy (J/kg mol)
F12 View factor, fraction of radiation leaving surface 1 and
arriving at surface 2 (–)
g Acceleration due to gravity (m/sec2 )
H Enthalpy (J/kg) or Henry’s constant (J/kmol)
h Heat transfer coefficient (W/m2 K)
hb Boiling heat transfer coefficient (W/m2 K)
hc Convection heat transfer coefficient (W/m2 K)
hcd Condensation heat transfer coefficient (W/m2 K)
hr Radiation heat transfer coefficient (W/m2 K)
J Diffusive flow rate (kg/m2 sec or kmol/m2 sec)
k Thermal conductivity (W/mK)
Kg Gas mass transfer coefficient (kmol/N sec)