O. SCHLÜTER, V. HEINZ AND D. KNORR
___________________________________
Freezing of food provides a safe and convenient way of shelf life extension without negative effects on the nutritional quality. The subsequent thawing of the frozen food is a process assuming no meagre importance. 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 modifications of pure water with a higher density than the fluid exist under hydrostatic pressure above 200 MPa. However, 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. Nevertheless monitoring the extend of phase transitions during these processes and consequently control strategies are required to ensure the uniformity of the process.
From an engineering point of view, theoretically based heat transfer models that allow predicting the temperature history within products, undergoing pressure-shift freezing or pressure-induced thawing processes would be very useful: in analogy to processes at atmospheric conditions, they allow researchers, operators and equipment designers to calculate freezing and thawing times corresponding to processes and to gain insight into freezing and thawing kinetics at high pressure, to optimize the quality of end products, and to design and evaluate industrial equipment. Recently, research in this area has initiated and this will be discussed in more detail. When pressure is applied, the initial freezing point is shifted towards lower temperatures. Besides, pressure, as temperature, influences the properties to be used in the governing heat transfer equation and thus, introduces additional complexity. Considering the non-linearity introduced both by temperature and pressure effects, numerical methods seem advisable when modeling phase transitions during processes such as pressure-shift freezing or pressure-induced thawing. Work conducted at the Technical University of Berlin will be focussed in this presentation. Special attention is paid to adiabatic temperature increase/decrease, and thermal and physical properties required when modeling heat transfer during these processes. Experimental techniques will be reviewed which are appropriate for in situ detection of the transient temperature field inside the high pressure vessel. Temperature profiles and simultaneously recorded pressure variations will be presented to illustrate the pressure supported phase transitions of aqueous systems as well as the pressure supported phase transitions of edible fat (e.g. cocoa fat in chocolate).
|