THERMOANALYTICAL TECHNIQUES

This is a group of techniques in which the temperature dependence of a physical property of a material is measured by subjecting the material to a controlled temperature program. In this group are: Differential Scanning Colorimetry (DSC), Differential Thermal Analysis (DTA), Thermomechanical Analysis (TMA) and Thermogravimetric Analysis (TGA). Both DSC and DTA are being utilized in food research. Only DSC and DTA will be discussed here in relation to food applications and some of the thermodynamic concepts introduced in Chapter 2 will be employed.

Differential Scanning Colorimetry (DSC)

Heat processing is often employed to manufacture food products. Amongst major food applications that involve heat processing and heat effects are: cooking, extrusion cooking, sterilization/pasteurization and drying. Heat 'extraction' is also employed in freezing, freeze-drying or cold-storage of food products to prolong shelf-life of food products. The effects of heat or heat extraction on the functionality of food materials are, therefore, of great interest to the food industry and the food technologist. A basic understanding of the effects of heat exchange and related thermodynamic processes (phase transitions, etc.) in foods is necessary in order to reduce production costs and increase profits in the food industry. Thermoanalytical techniques such as SDC provide a wealth of information that needs be correctly interpreted in thermodynamic terms.

In both DSC and DTA measurements a sample and a reference material are simultaneously measured as a function of temperature while both the sample material and the reference are subjected to a controlled temperature program (Fig. 5-23A and B, respectively); in the case of DTA one measures the temperature difference in such a process, whereas in DSC one measures the difference between the energy inputs to be the sample and the reference. The modes of measurement are common in DSC: a power compensation method and a heat-flux method. In the former case, one measures the compensatory power flow, or the energy required to maintain a temperature (T) null-balance between the sample and the reference by compensating for heat exchange. Practically, DTA employs a single heat source, whereas DSC employs two separate heat sources for the sample and reference, as well as two Pt sensors for the temperature measurement (Fig. 5-23).

The output of a DSC (or DTA) instrument is a thermogram which plots the differential heat flow at Q/dT against temperature. A change in the dependent variable dQ/dT represents in fact an absorption or evolution of heat by the sample, that is an enthalpy change, in this case (for a definition of enthalpy see Chapter 2):

dQ/dt = dH/dT, where t is time.

At constant pressure one can determine the heat capacity, Cp as:

Cp = (dH/dT) = dQ/dT x (dt/dT),Eq. (5-1)

where (dt/dT) is the programmed heated rate. (For a more detailed discussion of heat capacity see also Chapter 2).

By weighing the sample before and after the measurement one can express the dependent variable in terms of the specific heat cp = Cp/m, if the sample mass is m and does not change during the measurement. Practically, one measures the specific heat of a food material relative to that of a known saphire standard (or 'reference'). This specific heat determination by DSC is represented schematically in Fig. 5-24).

Whenever there are phase transitions or structural changes occurring in a material by changing the temperature, the heat capacity of the material changes also because the material heat exchange properties depend upon structure and component distribution in the material. For example, the denaturation of proteins by heat results in a sharp change in the heat capacity of the protein, whereas prior to denaturation the heat capacity increases gradually. The latter process was interpreted to be the result of a gradual unfolding of the protein which would result in an increased number of side-chains being exposed to water (Privalov et al., 1971). In food research, the observed DSC transitions may relate to processes such as protein denaturation, gelling, starch gelatinization and fat crystal melting, or other phase transitions. Relatively sharp DSC peaks are observed for cooperative processes such as protein denaturation. A simplified example is drawn schematically in Fig. 5-25, where the integrated peak area, after T2

appropriate corrections, represents the enthalpy change [[Delta]]H = _ [[Delta]]Cp dT

T1

if the ordinate is calibrated in units of heat capacity. The temperature at the peak, Tmax, is also of interest as a "transition temperature" but it does depend on the rate at which the temperature is being changed (slope A-E in Fig. 5-25). For a two-state process one can estimate the Van't Hoff enthalpy, [[Delta]]HVH = (4RT)/[[Delta]]T1/2, by measuring the transition mid-point temperature and the peak width at half-height. [[Delta]]HVH can be then compared with [[Delta]]Hcal, the calorimetrically determined enthalpy in an adiabatic or isothermal calorimeter to indicate if the transition really corresponds to a two-state process or not.

Kinetic parameters and relatively slow reaction kinetics can be also estimated by DSC; however, these will not be discussed in this volume.

DSC was employed to distinguish between thermal stability of proteins and their tendency to renature after cooling. Proteins present in milk whey such as a-lactalbumin, xanthine oxidase, ribonuclease and lysozyme were shown by DSC to denature at relatively low temperatures but re-scanning indicated that these proteins slowly re-natured after cooling to room temperature. Hydration numbers of proteins of about 32 to 34% w/w or more were also proposed from DSC measurements.

Precise results and sharp transition peaks were observed by SDS for a variety of lipids in their hydrated state. The rigid endothermic peak for hydrated phospholipids was assigned to the gel-to-liquid-crystalline phase transition involving cooperative "melting" of the hydrocarbon chalius of phospholipid molecules (Ladbroke and Chapman, 1969). This transition is accompanied by a rather sharp DSC peak (Fig.5-26) indicating the high degree of cooperativity present in such systems as a result of hydrophobic interactions. The entalpy change of the transition increased together with the transition temperature for phosphatidycholines of increasing acyl chain lengths from 12 to 22 carbon atoms (C12 to C22) (Cabrey and Sturtevant, 1978). In foods, the polymorphism of commercial fats and oils is often studied by DSC. Because of the complexity of DSC analysis for polymorphic lipid or fat-containing foods, it has been useful to parallel the DSC measurements by X-ray diffraction in programmed temperature studies (Chapman et al., 1971). The identification of various oils in mixtures and the differentiation of lard from tallows could also be carried out by DSC. Solids content determination in fats and the determination of free fatty acid contents in several foods are also possible by DSC. These are only some of the food applications of the technique. Somewhat more controversial are those DSC studies concerning starch gelatinization where parallel X-ray diffraction studies of the DSC samples could greatly benefit the interpretation of the DSC/DTA thermograms. Nevertheless, it is rather interesting that certain higher-amylose starches tend to have higher gelatinization temperatures and that the gelatinization endotherm only appeared when water is present in potato starch in excess of ~32% (Duckworth, 1971). The threshold of 32% moisture content corresponds to 4 molecules of water per glucose unit or to the amounts of 'non-freezable' water present in the potato starch.

Various problems with the correction and interpretation of DSC thermograms of foods were recently reviewed by Wright (1984).

Ref: http://www.ag.uiuc.edu/~fs401/CH5-vol1.html#fn0