340 Chapter 19
A = area of transference
Δ T = temperature difference
L = thickness of the material
k = thermal conductivity coeffi cient
The coeffi cient k depends on the food -
intrinsic properties; it actually defi nes how
heating is facilitated or prevented by the
treated material. Comparing k for stainless
steel versus k for meats makes clear the low
conduction rate in foods. In stainless steel,
k = 45.872 kg cal/h m^2 ° C (Green and
Maloney 1997 ), whereas on the average,
k = 1.464 kg cal/h m^2 for meat (Mittal and
Usborne 1985 ). Conduction may differ even
in different cuts of the same animal, due to
the particular chemical composition. Siripon
et al. (2007) reported different average
thermal conductivity for white and dark
poultry meat.Convection
Convection is carried out in fl uids, due to
density differences. It is the mechanism
occurring in homogenous soups, brines,
sauces, and syrups. This mechanism is based
on Newtons ’ s law:
qhAT=Δ (19.3)
where:
A = area of transference
Δ T = temperature difference
h = heat transfer coeffi cient
The heat diffusion rate is higher if an
external force is applied; can rotation
decreases the temperature difference ( Δ T) to
a minimum (Welti - Chanes et al. 2003 ). The
coeffi cient h describes the heating potential
of a given medium; it depends on fl ow prop-
erties, surface type, and fl ow velocity of the
heating medium. For instance, for boiling
water, h = 1.464 kg cal/h m^2 ° C; for condens-
ing steam, h = 2.928 to 19.520 kg cal/h m^2 ° C
(Green and Maloney 1997 ). Therefore, to
obtain the maximum heating rate duringAs mentioned before, another heat treat-
ment objective is enzyme inactivation; meat
enzymes (endogenous and exogenous) play
an important role in meat spoilage, and it
is, therefore, necessary to inactivate them.
Enzyme inactivation depends on several
factors and practically the same affecting
microorganisms. However, in mixed - food
products, several heat - resistant isoenzymes
can be present, especially if plant material is
added to the formulation, as peroxidases
may be present; the process parameters are
calculated taking into account the most heat -
resistant enzyme (Braun et al. 1999 ).
Heat Transfer Mechanisms
In all thermal operations, the amount of
transferred heat is necessary for calculations.
Thermal processing is basically an operation
in which heat fl ows from a hot element — the
heating medium — to a cold element —
the food. As it is a dynamic process, the heat
fl ux is proportional to the driving force and
inverse to the fl ow resistance. Heat transfer
obeys one of the following mechanisms: con-
duction, convection, or radiation. In canning
operations, only conduction and convection
mechanisms take place. Radiation occurs in
heating systems such as microwaves and
infrared heating.
Conduction
In conduction, heat is transmitted by vibra-
tions of adjacent molecules; this mechanism
occurs in solids. This is the heating mecha-
nism that takes place in canned products such
as brines containing solid chunks (e.g., in
soups containing meat pieces, canned sausage
in brine, and products that gel during heating,
such as luncheon meats and pat é ; Mittal and
Blaisdell 1984 ). Conduction follows the
Fourier law:
qkATL= ()Δ (19.2)
where: