Ultra high pressure increasing thermal conductivity of food materials

Abstract: During high pressure processing, the adiabatic compression increase of food materials would cause non-uniform temperature distribution and heat transfer, which would also influence the inactivation of bacteria, spore, enzyme and the quality of foods. Thermal conductivity of food materials at high pressure is a very important parameter for understanding heat transfer and temperature variation during high pressure processing. Available data and measuring method of thermal conductivity of food materials under high pressure are still scarce. In this study, a thermal conductivity probe developed based on the line heat source theory was installed in a high pressure chamber to measure thermal conductivity of food materials under high pressure. The thermal conductivity probe was calibrated using 1.5% agar gel whose thermal conductivity was very close to that of pure water but the convective effect during the measurement was eliminated due to the gel network at pressure from 0.1 to 400 MPa with a pressure increment of 50 MPa. The results of calibration experiment indicated that thermal conductivity values of 1.5% agar gel measured under different pressures using the thermal conductivity probe were very close to that of reference data of pure water. Calibration factors defined as the ratio of measured and reference values of the thermal conductivity of 1.5% agar gel were found almost no effect by pressure. A general calibration coefficient value of 0.9944 (R square was 0.9997, observation number was 30) was obtained by linear regression analysis (zero intercept) of measured thermal conductivity values of 1.5% agar gel against reference values of pure water. The coefficient value was used for the correction of all experimental results in the following. Thermal conductivities of egg white, egg yolk, ham sausages and cream were measured at pressure from 0.1 to 400 MPa with a pressure increment of 50 MPa. The results demonstrated that the thermal conductivities of these selected food materials at high pressure conditions were higher (up to 28%) than that of the sample at the atmospheric pressure conditions and had a tendency to increase with increasing pressure. Results at atmospheric pressure in this study were compared to estimate values using empirical equation based on water content. The measured thermal conductivities of egg yolk (0.43 W/(m·℃)), ham sausages (0.45 W/(m·℃)) and cream (0.49 W/(m·℃)) were very close to the estimated values (egg yolk 0.40 W/(m·℃), ham sausages 0.43 W/(m·℃), cream 0.41 W/(m·℃)) , while the measured value of egg white (0.65 W/(m·℃)) was significantly higher than that of the estimated value (0.53 W/(m·℃)) as well as that of pure water. The reason probably was that the flowability of egg white caused the generation of convective heat transfer. Measured thermal conductivities of egg white decreased at pressure higher than 300 MPa which was different from other three materials. The reason probably was that the pressure processing more than 300 MPa induced coagulation of egg white resulting in decrease of flowability as well as convective heat transfer. The water content of food materials had a significant effect on thermal conductivity. In general, the higher the water content, the higher the thermal conductivity. This rule was also confirmed at high pressure as the thermal conductivity from highest to lowest in order being egg white (83.1% water) > ham sausages (63.5%) > cream (57.5%) > egg yolk (50.7%). An empirical equation was established for prediction of the thermal conductivity of food materials at high pressure. The fourth-order polynomial was used to fit the thermal conductivity values of egg white, egg yolk, ham sausages and cream at pressure range from 0.1 to 400 MPa with temperature of 25 ℃. The regression coefficients of these equations were all above 0.91. This study could provide basic scientific datas for high pressure processing of food materials.