Ong, Law - Hygrothermal Properties of FVFs
2.3.2. Thermal Properties
The thermal properties of food product, include the specific heat capacity and ther-
mal conductivity, are intrinsic properties of the materials and represent the ability of the
materials to accumulate and to transport heat (Lewicki and Jakubczyk, 2004). These
properties are important parameters in the modelling and evaluation of food processing
operations that involve heat transfer, energy utilization and food product quality. Gen-
erally, the heat transfer characteristics within a food product during drying can be de-
scribed using the Fourier’s heat conduction model (Equation 2.6).
(^)
=∇ ∇
∂
∂ T
t
T
ρcp
. k^ (2.6)^
where k is the thermal conductivity (W m-1 K-1), ρ is the density (kg m-3) and cp is the
specific heat capacity (kJ kg-1 K-1), T is the temperature (K) and t is the time (s).
The term thermal diffusivity (Equation 2.7) is usually defined for the above equation. It
has the similar unit as that of moisture diffusivity (m^2 s-1).
cpk
ρα=^ (2.7)^The thermal properties of food materials depend on the temperature and composi-
tion of the individual constituent. The constituents that commonly present are such as
water, protein, fat, carbohydrate and ash. By knowing the composition of these constitu-
ents the thermal properties of the food material can be determined empirically. Various
equations are available in literatures that relate the thermal properties of these consti-
tuents with temperature. The individual values are then added up by the additive prin-
ciple to obtain an overall value. However, by using such equations, the values obtained
should serve as a preliminary estimation only because thermal resistance of each food
material is somehow depends on the structural arrangement of the constituents in food.
Experimental verification is recommended for better estimation. For porous food mate-
rials, the porosity or volume fraction of the air is included into the estimation. Air has a
low value of thermal conductivity and thus porous foods are poor heat conductors in
general. The various methods of estimation have been extensively reviewed and readers
are advised to refer to literatures published elsewhere (Chen, 2008; Geankoplis, 2003;
Marinos-Kouris and Maroulis, 2006).
Some selected examples of the variation of thermal properties with moisture content
and temperature from recent published literatures are illustrated in Figure 2.12 to Error!
Reference source not found.. In most cases, the thermal conductivity and heat capacity of
food material increase with increasing moisture content above freezing temperature.
This is because water has a much higher specific heat and thermal conductivity than the
other major food constituents such as the protein, fat, and carbohydrates (Nesvadba,
2005 ). Thus moisture content can greatly influence the thermal properties of foods. In
some instances, temperature may cause changes in the physical properties and composi-
tion of the food material hence affects the thermal properties as well (Kouchakzadeh
and Tavakoli, 2009; Rahman et al., 1997; Sweat, 1995).