Instruments methods and procedures

The principles of thermal analysis and calorimeiry are described in a recent book (Haines, 2002).

All thermal analysis and calorimetry equipment available (e.g. high pressure DTA, heat flow or Calvet type calorimeters, power compensation DSC instruments) give valuable information. Even the recently introduced modulated (or alternating) differential scanning calorimetry (MDSC or ADSC) can provide useful data, especially related to Tg.

Either scanning (heating and cooling) or isothermal modes are applied. In the scanning mode, two runs are often performed to clarify whether the observed phenomena are reversible. A heating scan showing melting transitions is often followed by a cooling scan under the same conditions to evidence any hysteresis between crystallization and melting temperatures. In order to obtain measurements with high sensitivity, a Micro-DSC (with high sample amount) and/or fast scanning rates should be used. In contrast, a standard DSC (with small sample amount) and/or low heating rates is generally selected to obtain measurements with high resolution.

For our studies on food materials we have used high pressure DTA analysis (Netzsch-Geratebau 404H, Selb, Germany), heat flow or Calvet type calorimeters, a heat flow Micro-DSC (Micro-DSC III, Setaram, Caluire, France) and standard power compensation DSCs (DSC7 and Pyris 1, Perkin Elmer, Norwalk, USA). Oxidation of lipids was studied isothermally by DSC under oxygen flow in order to have an excess of oxygen. The measurements in relation with process safety were performed on a DTA under pressure (25 bar of oxygen for example)

or with heat flow calorimeters using sealed cells. These sealed cells may be fitted with a pressure sensor in order to perform thermomanometry. For process safety research, adiabatic calorimetry (accelerating rate calorimetry or ARC, Columbia Scientific Industries, Milton Keynes, UK) was also used as it represents the worst situation if one considers thermal conditions.

Since calorimetric and thermal analysis techniques alone do not give complete information about the physicochemical properties of foods and food constituents, they are often used jointly with other analytical techniques such as X-ray diffraction (XRD), optical methods, rheological techniques such as dynamical mechanical analysis (DMA) or dynamical mechanical thermal analysis (DMTA), as well as high- and low-field nuclear magnetic resonance (NMR) spectroscopy.

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