GLASS TRANSITION OF GELATIN - WATER SYSTEMS STUDIED WITH MODULATED TEMPERATURE DIFFERENTIAL SCANNING CALORIMETRY

Although gelatins have been studied for many years, their gelation process and the origin of their physical properties are still not fully understood. Investigations of the parameters that influence the crystallisation behaviour of gelatin are of great interest, not only to understand but also to control its structure development. This is important as in gelatin-based products the crystallisation of gelatin has an influence on the texture of the product. The melting behaviour, but also the glass transition, is important for the production and usage of polymers. Many studies have been focussed on the melting and on the glass transition of synthetic polymers. For biopolymers like gelatin and starch, this is much less the case as a result of experimental difficulties. For these products the melting point and the glass transition temperature of the dry sample are higher than the degradation temperature. Their melting behaviour and glass transition can, consequently, only be studied in the presence of water or another plasticizer. The role of water in modifying the stability and functional properties of biopolymers is, however, also of considerable scientific and commercial importance. To investigate gelatin gelation, it is necessary to know its state diagram. As the glassy solidification curves for the gelatin-water systems given in literature differ in a quite large extent [1], we had to study its glass transition first.
Modulated temperature differential scanning calorimetry (MTDSC) is chosen to study the glass transition of a commercial type B gelatin sample. MTDSC offers benefits for the study of (polymeric) materials, especially when overlapping phenomena occur. This is due to the determination of the two individual heat flow components. Material properties and transitions, depending on the heating rate, are usually found in the reversing signal and kinetic processes, depending on time and absolute temperature, in the non-reversing signal. MTDSC has also been shown to be a sensitive technique to study broad and weak transitions [2].
MTDSC measurements were performed on aqueous gelatin samples in the concentration range 60-88% (w/w). In industrial gelatin grains different fractions with different glass transitions are present. However, when these grains are dissolved in water, by keeping them at room temperature for some time and at at least 50°C for 45 min, only one glass transition is detected. Over the whole concentration range studied, the glass transition temperature decreases with decreasing gelatin concentration. For the 60.0, 63.6 and 65.0% (w/w) gelatin samples, cold crystallisation of water is observed just above the glass transition temperature in the total heat flow signal. The melting of this phase, which is also seen in the reversing heat capacity signal, follows immediately afterwards. This means that below a certain concentration, called maximal freeze concentration, gelatin samples are in a non-equilibrium state during cooling and phase separate during the subsequent heating. The determination of the glass transition would not be possible for these samples using only the total heat flow signal. Separation of the overlapping thermal events, as performed by MTDSC, is necessary. From our experiments we conclude that the maximal freeze concentration is about 67% (w/w). For gelatin samples with a concentration below 70% (w/w), the glass transition region is well below the gelation and gel melting region, so that the onset of gelation and gel melting is not masked by an overlap of these phenomena with the glass transition. At concentrations below the maximal freeze concentration, the cold crystallisation of the water-rich phase and the melting of the ice crystals formed hamper the determination of the onset of the gel melting of gelatin samples. The crystallisation and annealing of the gelatin sample also influences the glass transition.