Food products are multi-component heterogeneous systems which can be viewed as the result of changes related to the phase transitions that take place in the preparation process. These transitions are substantially irreversible and therefore cannot be described with a thermodynamic approach. The reason for their irreversibility is mainly related to the metastable character of the starting and/or the final state involved in the transition. Either of them can be amorphous or vitreous and the whole system can be finely, although not definitely, dispersed. Because of the large extension of the inter-phases, the average size of the solid particles (usually crystals) or the droplets (aqueous or fatty) or the bubbles (usually air) can directly affect the rate and/or the relevant onset temperature of these transitions. Although the driving force of these irreversible transitions is the relevant drop of the Gibbs function, the rate of their progress depends on other physical properties of the medium, like viscosity and/or permeability, which in turn can be modified by to the products formed. The overall process can therefore be referred to as an auto-catalysis (either positive or negative) of the overall progress toward the final state. One way to overcome the complexity of these processes can be found looking at the role of some key factors. When the food systems considered host large amounts of water shared between different phases (hydrophilic systems), the key factor that controls the overall progress of the phase transitions (and phase separations) is the local availability of water and its displacement from one to another region of the system. In systems where water is practically absent or strictly confined within micelles (hydrophobic systems) the phase transitions are governed by the rates of nucleation and growth of crystal phases in specific regions, e.g. around the liquid droplets and air bubbles. Thermal analysis is of great help in the follow up of these events. DSC and TGA are the most used techniques in these studies, but require skilful operators and adequate mathematical treatments of the data. Some examples will be given of phase transitions in either kind of systems together with the general lines of the relevant kinetic parameterization. The latter includes definition of TTT (Temperature, Time, Transformation) diagrams and kinetic models for the polymorph transitions. All these examples concern investigations performed with DSC and TGA equipments.
|Titolo:||Phase Transitions in Hydrophilic and Hydrophobic Food Systems|
|Autori interni:||SCHIRALDI, ALBERTO (Primo)|
|Parole Chiave:||Food ; Transitions|
|Settore Scientifico Disciplinare:||Settore CHIM/02 - Chimica Fisica|
|Data di pubblicazione:||lug-2005|
|Enti collegati al convegno:||AICAT (Ass. Ital. Calorim. Anal. Termica)|
|Tipologia:||Book Part (author)|
|Appare nelle tipologie:||03 - Contributo in volume|
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