Thermal Food Processing

Thermal Physical Properties of Foods 7
1.2.4 THERMAL DIFFUSIVITY
Thermal diffusivity (α) determines how rapidly a heat front moves or diffuses
through a material and can be defined as
(1.5)
where r is the density (kg/m 3 ), c p is the specific heat at constant pressure (J/kg K),
and k is the thermal conductivity (W/m K) of the material. The SI unit of α
is square meters per second (m 2 /sec). Thermal diffusivity measures the ability
of a material to conduct thermal energy relative to its ability to store thermal
energy; products of large α values will respond quickly to changes in their
thermal environment, while materials of small α values will respond more
slowly.
The thermal diffusivity of unfrozen foods ranges from about 1.0 × 10 –7 to
1.5 × 10 –7 m 2 /sec and does not change substantially with moisture and temperature
because any changes of k are compensated by changes of the density of the
material. 21 In microwave heating, for example, the fact that thermal diffusivities
of unfrozen foods are similar means that foods heat similarly for equivalent energy
deposition. 5
The measurement of thermal diffusivity can be divided into two groups: direct
measurement and indirect prediction. 10 Indirect prediction, that is, the estimation
from experimentally measured values of thermal conductivity, specific heat, and
density, is the recommended method to determine α. 9 Since specific heat can be
estimated with sufficient accuracy from the product composition, the experimental
determinations are the thermal conductivity and the mass density. 9
1.2.5 DENSITY
α =
ρ
k
,
cp Density (ρ) is a physical property widely used in process calculations. Density
of food materials depends on temperature and composition, and it is the unit mass
per unit volume:
ρ = m
.
Vs (1.6)
The SI unit of density is kilograms per cubic meter (kg/m 3 ). Different ways of
density definition, measurement, and usage in process calculations are well discussed
in the literature. 10 In some cases, the apparent density is used as bulk
density. The apparent density is the density of a substance, including all pores
remaining in the material, while bulk density is the density of a material when
packed or stacked in bulk. 10 Hence, it is necessary to mention the definition of
density when presenting or using data in process calculations. 10