Heat Transfer Coefficient and Physical Properties

Abstract

No matter how sophisticated the calculation method, the results can be reliable only if accurate information is supplied. The main inputs for a freezing problem, apart from the product geometry, are the heat transfer coefficient and the food’s thermal properties: freezing point, calorimetric properties, thermal conductivity and density. The freezing behaviour of foods resembles that of aqueous solutions, which start to change phase at a temperature below 0 °C (the initial freezing point) and continue to gradually freeze as temperature falls. Raoult’s law of ideal solutions is used to predict freezing point and ice fraction at various temperatures, then combined with empirical data and the bound (unfreezable) water model to develop predictive equations for the above properties at various temperatures. Recommendations are given on the choice of model for calculating thermal conductivity. Experimental properties of the Tylose gel food analogue, often used in freezing experiments, are given.

CAUTION

• In Choi and Okos (1986) there was an error in a parameter for fat thermal conductivity, which has been corrected in Table 2.3.

• For water density, Eq. 2.5 is recommended over Choi and Okos’s equation.

• For water specific heat, Eq. 2.21 is recommended over Choi and Okos’s equation.

• Many papers use obsolete thermal properties for Tylose gel. Those presented in Sect. 2.6 are recommended provided the composition is as described in that section.

• Specific heat may mean sensible heat only or may include latent heat depending on context. Make sure that the correct interpretation is applied.

• The prediction method given above for thermal conductivity may not work well for foods with anisotropic microstructures, such as muscle meat.