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Impact of the metastable state of water on the design of high pressure supported freezing and thawing processes

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Paper number  026197,  2002 ASAE Annual Meeting . (doi: 10.13031/2013.10852) @2002
Authors:   Oliver Schlüter, Dietrich Knorr
Keywords:   High hydrostatic pressure, pT-diagram, potato, ice I, ice III, ice V, metastable liquid, process optimization

Increased hydrostatic pressure influences the phase transition of water by way of depressing the freezing/melting point as well as reducing the latent heat of fusion. This results in shortened freezing/thawing times of food materials. Furthermore different solid polymorphs of pure water with a higher density than the fluid exist under hydrostatic pressure above 209 MPa. Taking advantage of the phase diagram of water various pathways of changing the physical state of food can be followed using external manipulations of temperature or pressure.

Recent studies on pressure supported phase transition processes focus mainly on the triple point of water/ice I/ice III at 209 MPa and -22 C where the lowest onset temperature for pressure shift freezing and the highest temperature difference (sample - pressure transmitting medium) by decreasing the melting point of a food sample due to pressurization is expected. Generally, an analogous slope of the melting curve of food compared to that of pure water was assumed, but extensive data sets which show the liquid-solid phase boundary of food systems (especially at a pressure above 209 MPa) have been missing.

Selected experimental techniques are appropriate for in situ detection of the transient temperature field of food samples inside the high pressure vessel. Temperature and pressure profiles detected clearly identify the pressure dependent melting curves of plant tissue (potato) in a pressure range up to 450 MPa. The curves derived from the experimental investigation can be defined by fitting the coefficients of the international equations for pressure along the melting curve of ordinary water (provided by IAPWS). Further the experimental data indicate a high degree of supercooling before the nucleation of water to a higher solid form than ice I takes place. When considering the experimental nucleation points a region of metastable water can be described in a range where ice III is thermodynamically stable. This behavior results in possible improvements for designing pressure supported freezing and thawing processes.

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