For comprehensive understanding of the effective thermal conductity data of heterogeneous foodstuffs, both the thermal conductivities of individual food components and the spatial distribution(s) of the components have to be known. Among the major components of foodstuffs, proteins and carbohydrates are not known of their thermal conductivities yet, perhaps because they themselves are the heterogeneous materials from the view point of heat conduction. Also, the complicated spatial distribution of food components is hardly mathematically describable is their original state.
The author, examining the applicability of four simple spatial distribution models-series, parallel, Maxwell-Eucken´s, and Kunii´s packed bed models -to the effective thermal conductivity data of the coagulated defatted soy protein of various water contents, found that only the series model could explain the whole experimental results, and based on the series model, determined the thermal conductivity of soy protein. By using this value and the series model, the effective thermal conductivity of heterogeneous soy protein foodstuffs, with or without fat, was almost completely predicted.
The published thermal conductivity data for animal as well as fish meats of known composition were analyzed with the series model to obtain the thermal conductivity value for meat protein. Then,. the effective thermal conductivities of meats, for which two out of three contents of water, protein and fat were given, heat flow not parallel to meat grain, were well predicted with the standard deviations about 5% for the unfrozen, while about 12% for the frozen meats. Since the published data gave about 30% different values for the same composition of frozen meats, the deviation of prediction will be rather small.
Gels of various proteins and carbohydrates were subjected to the same analysis as that for the coagulated soy protein. Again, only the series model could explain the whole experimental results, although any model could describe the gelatin gel´s result because the effect of water content was not significant. The obtained thermal conductivities of proteins were in the order of water＞gelatin＞meat protein＞soy protein＞egg albumin＞wheat gluten＞milk casein＞fat. The order of proteins was in reverse order of their hydrophobicities, suggesting that the thermal conductivity values would involve the effect of bound water. The thermal conductivities of agar and potato starch were close to the value of egg albumin. These results will be useful not only for prediction of the effective thermal conductivities of heterogeneous foodstuffs but also for designing the composition(s) to give a desired effective thermal conductivity.
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