5. Conclusions
The origin of this study was motivated by the lack of thermodynamic parameters of olive oil under high pressure and at different temperatures. Knowledge of these parameters is indispensable for numerical modeling and optimization of highpressure food processing and preservation.
In this paper, the results of measurements of the phase velocity of the longitudinal ultrasonic wave and the density of the olive oil with known composition are presented. Measurements were conducted in a wide range of hydrostatic pressure, up to 600 MPa, for various values of temperature. The key factors that determine the speed of the ultrasonic wave in a liquid are adiabatic compressibility and intermolecular free path length . Using the measured sound speed and density data, the authors calculated adiabatic compressibility, isothermal compressibility, and intermolecular free length, as a function of pressure at various temperatures (isotherms).
Moreover, the kinetics of liquid-to-solid phase transition (crystallization) in olive oil was investigated. The results obtained show a strong influence of pressure and temperature on the kinetics of phase transitions in olive oil. As the temperature increases, higher pressures must be used to initiate phase transition. Moreover, the induction time and the transition time are longer. As the temperature is raised, the intermolecular free path length increases. Since the intermolecular free path length is inversely proportional to the sound speed, therefore with increasing temperature the sound speed decreases.
Under conditions of high pressure, after the phase transition, the molecular structure of the olive oil is changed. High-pressure phase of olive oil is more like a solid medium and forms a solid-like phase. The molecules are more densely packed.
Measurements of sound speed and density in edible oils as a function of pressure and temperature enable to evaluate some useful physicochemical properties and to investigate high-pressure liquid-to-solid phase transition in the oils involved. Changes in the time-of-flight, and consequently in the sound speed, can be observed with much higher resolution than corresponding changes in the oil density. Therefore, observation of changes in the speed of sound seems to be more reliable indication of the occurrence of the phase transition.
The results of this study are a novelty. Presented in the paper, the ultrasonic method can also be used to investigate (at highpressure conditions) the composition, purity, and quality of other liquid foodstuffs (e.g., fruit juices), not only the fats and oils. The results obtained can be useful for designing and optimizing new methods of food control, processing, and conservation, especially in high-pressure food processing.