This report outlines thermal property measurement techniques and thermal property modelling of foods. The discussion on measurement technique focuses on thermal conductivity and thermal diffusivity only. Since thermal conductivity and thermal diffusivity measurements are based on the heat transfer mechanism, measurement apparatus is specifically designed for certain materials and environmental conditions. The probe technique designed for simultaneous measurement of thermal conductivity and thermal diffusivity has been one of the most widely used because it can be easily operated, and commercially produced. A PC-based probe apparatus is presented. Published models on thermal conductivity prediction are grouped according to their appropriate food system. It is proposed in this study that the thermal conductivity model of foods be made up of its components and a structural model that accounts for the component arrangement in the system. The Keey model was found the best structural model for porous foods while the parallel-perpendicular model worked well for frozen nonporous system. This model was also appropriate for meat measured along the fibers at above freezing temperatures. The rest of nonporous foods was found best predicted with the parallel model.
Measurement TechniquesFood materials in general, have irregular shapes, non-homogeneous composition and are small. Because of these constraints measurement of thermal properties demands as much ingenuity as basic knowledge in heat transfer. Experimental measurements that strictly conform to theoretical requirements are impossible to do in most cases. So it is common practice to make simplifying assumptions with regards to sample homogeneity, heat losses, contacts at heat transfer area, similarity to a standard geometry and so on. Difficulty in measurement is more severe in thermal conductivity and thermal diffusivity since they require data on temperature history and profile. Take the case of measuring thermal conductivity and thermal diffusivity of a com grain. A grain of com is made of several distinct parts with each one having different properties from the others. Recognizing the differences among the parts, one must measure the properties of each. But with com, most of the parts would be too small for any measurement techniques. Therefore, one can only study the major parts (in terms of sizes) and assume that the others are too small to make significant difference.There is no standard technique for measuring thermal properties of foods but there are several methods available for measuring them. Method selection depends on the sample and its intended application and apparatus and experimental set up are usually custom-made. Several measurement techniques for thermal conductivity and thennal diffusivity are briefly discussed. Due to space limitations, the review is limited to the basics. Instead the readers are referred to the designers and the users.
MEASUREMENT TECHNIQUES FOR K AND IXSeveral teclmiques for thennal conductivity and thennal diffusivity are pres...
