33-5 |
Understanding crystallization phenomena using calorimetry |
R. W. HARTEL, Dept. of Food Science, Univ. of Wisconsin, Madison, 1605 Linden Dr., A-13 Babcock Hall, Madison, WI 53706-1519 Differential scanning calorimetry (DSC) is often used for studying crystallization and melting in foods. The DSC allows us to characterize phase transitions through the enthalpic change associated with the change in state. Although the DSC is normally used in scanning mode, it can be used in isothermal mode to detect onset and rate of crystallization. The DSC used in scanning mode allows determination of onset of crystallization and enthalpy of crystallization. It also is used for measuring melting endotherms and final melting temperature. Under certain conditions, DSC can be used to isolate individual polymorphic forms of crystals (e.g., lipids). In isothermal mode, the DSC can be used to characterize an induction time for crystallization and a crystallization rate. The change in enthalpy with time can be analyzed according to kinetic models (e.g., the Avrami equation) to characterize both mechanisms and kinetics of crystallization in certain systems. However, there are numerous difficulties with using DSC as the only tool for studying phase transitions. The small sample size means that heterogeneous materials cannot be studied and slight differences in heat transfer to the sample can cause large shifts in the thermograms. Furthermore, the lack of agitation means that kinetic results found by DSC are not likely to apply to commercial applications since the effects of shear cannot be measured. In complex foods, other enthalpic changes may overlap with the desired transitions. Probably the most important limitation to using DSC to study crystallization is that the meaning of each peak observed may not be clear. Some other technique (as in x-ray diffraction) must be used to elucidate the physical phenomena associated with each peak. This is particularly true in lipid crystallization since peaks may be associated with different compound crystals or due to polymorphism.
Session 33, Applications of calorimetry in food and biological materials
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