ABSTRACT
Nomenclature ................................................................................................................................ 576 References ..................................................................................................................................... 577
17.1.1 Thermal Conductivity
Transport phenomena in food processing involve momentum, heat, and mass transfer and need thermophysical properties to solve heat transfer problems. A review on the transport phenomena in food engineering is given by Welti-Chanes et al. (2005). The thermal conductivity of a food is an important thermophysical property that is commonly used in calculations involving rate of conductive heat transfer. The rate of heat flow through a material by conduction can be predicted by Fourier’s law as
Q ¼ kA @T @x
(17:1)
where Q is the rate of heat flow (J=s) A is the area of heat transfer normal to heat flow (m2) (@T=@x) is the temperature gradient along the x-direction k is the proportionality constant of thermal conductivity (W=m K)
Fourier’s law indicates that thermal conductivity is independent of the temperature gradient but varies with temperature; however, it is not always in the same direction. Thermal conductivity can be expressed at a wider temperature range by an empirical equation:
k ¼ aþ bT (17:2)
where a and b are empirical constants. Most foods contain high moisture and, therefore, thermal conductivity values are close to that of water. Water has k values in the range of 0.5 to 0.7 W=(m 8C), and k increases with an increase in temperature.
