A rapid testing instrument to estimate thermal properties of food materials at elevated temperatures during nonisothermal heating

Modeling kinetics of thermal degradation of nutrients for food quality or kinetics of microbial reduction for food safety requires reliable estimates of the thermal properties. Thermophysical properties, especially thermal conductivity and specific heat, are important in establishing thermal processes for food manufacturing, especially at higher processing temperatures. Hence, in this study, a novel instrument (TPCell) was designed and developed using principles of intrinsic verification and inverse heat conduction. An intrinsic verification method was developed to ascertain the parameter identifiability in the model and to check the accuracy of the numerical codes used to solve the partial differential equation for heat conduction. The concept of intrinsic sum was introduced, which is sum of all the scaled sensitivity coefficients in the model. The intrinsic sum was derived using dimensionless derivation of scaled sensitivity coefficients. The design of the instrument was based on the insight gained from the dimensionless scaled sensitivity coefficients and the intrinsic sum. With the instrument, thermal conductivity can be measured from room temperature to higher processing temperature of 140 °C. Several food materials were tested using the instrument. Sweet potato puree thermal conductivity was measured to be 0.539 W/m°C at 20 °C and 0.574 W/m°C at 140 °C. The experimental time with TPCell is less than a minute, as compared to 5-6 hours with quasi-isothermal method employed by currently available instruments. TPCell has advantages over traditional methods, as it avoids the decomposition of materials that result when achieving the quasi-isothermal state at higher temperatures. Temperature-dependent thermal properties were used to estimate the kinetic parameters of nutrient degradation during aseptic and conventional retort processing. Vitamin C and thiamin were selected as model nutrients for degradation study. Sweet potato puree was used as a food matrix. Aseptic processing had 50% higher retention of Vitamin C as compared to retort processing. Thiamin retention could not be quantified, as it survived well in aseptic as well as retort processing. The rate of reaction for ascorbic acid in aseptic processing and retort processing was 0.0073 min-1 and 0.0114 min-1 at a reference temperature of 127 °C, respectively. The activation energy for ascorbic acid in aseptic processing and retort processing was 26.62 KJ/g-mol and 3.43 KJ/g-mol, respectively. The kinetic parameter of thiamin could not be estimated due to insufficient degradation in aseptic as well as in retort processing.