| High-pressure treatment has been recognized for over a century as a potential food preservation technique because of its demonstrated ability to inactivate microorganisms without adverse effects on food quality. Recent developments in high pressure processing equipment technology have already brought into practice a number of successful commercial applications. Process development efforts are currently based mainly on observation of end effects from trial-and-error experimentation, and are further confounded by the inability to distinguish temperature from pressure effects because of the thermodynamic temperature rise that accompanies pressurization. The ability to predict such effects is further hampered by the complex composition of foods and by the fact that thermodynamic and transport properties, which govern the reactions and transformations taking place, are highly sensitive to pressure, temperature and food composition. The purpose of this research was to develop methodology for measurement of sound velocity in liquid foods under high pressure treatments from which a number of important thermodynamic properties could be derived and determined as a function of pressure, temperature and composition.; An ultrasonic high-pressure measurement cell was developed and instrumented for use within the sample chamber of a prototype high-pressure treatment unit equipped with independent temperature- and pressure-monitoring and control instrumentation. Measurements were taken over a range of pressures up to 600 MPa and temperatures between 10° and 30°C with four different simulated liquid food systems (binary aqueous solutions of sucrose, glucose and citric acid at different concentrations, and pure water). The resulting sound velocity data along with atmospheric pressure data on density, specific heat capacity and thermal-expansion coefficient were used to derive the important thermodynamic properties of specific volume/density, isentropic and isothermal compressibility and isentropic pressure thermal coefficient at elevated pressures. These results also led to an interpretation of the pressure-, temperature-, and concentration-dependence behavior of each property, allowing prediction of each property as a function of temperature, pressure and composition. The thermodynamic relationships of partial molar properties of solute and solvent in each solution have also led to a better understanding of the interactions between solute and solvent under the influence of pressure, temperature, concentration and solute type in model aqueous food systems. For predictive or numerical application purposes, regression coefficients were determined by fitting data to the appropriate model. |
