Gelatin is an example of a cold-setting thermo-reversible gel: when a solution of gelatin molecules is cooled below a certain temperature a gel is formed, but when it is reheated the gel melts. Egg-white is an example of a heat-setting thermo-irreverisble gel. When an egg is heated above a temperature where gelation occurs a characteristic white gel is formed, however, when the egg is cooled back to room temperature the gel remains white, i.e., it doesn't revert back into the liquid from which it was formed. For ingredients that gel it is important to know the temperature at which gelation occurs, the gelation rate, and the nature of the gel formed. Thus thermal analytical techniques are needed by food scientist to measure these properties.
3. Experimental Techniques
A variety of different analytical techniques have been developed to monitor changes in the physical properties of food components that occur in response to controlled changes in temperature. A number of the most important of these thermal analysis techniques are described below.
3.1. Thermogravimetry
Thermogravimetric techniques continuously measure the mass of a sample as it is heated or cooled at a controlled rate, or is held at a particular temperature for a period of time. Thermogravimetry is useful for monitoring processes that involve a change in the mass of a food or food component, e.g., drying, liberation of gasses, absorption of moisture. To mimic the various types of processing and storage conditions that a food might normally experience, thermogravimetric instruments have been specially designed to allow measurements to be carried out under specific environments, e.g., controlled pressures or atmospheres. Gravimetric instruments typically consist of a sensitive balance situated within a container whose pressure, temperature and gaseous environment can be carefully controlled.
The mass of a sample may either increase or decrease with temperature or time depending on the specific physicochemical processes occurring. Heating often leads to a reduction in mass because of evaporation of volatile components and various chemical reactions that liberate gasses. On the other hand, the mass of a food may increase due to absorption of moisture from the atmosphere. The ability to be able to carefully control the temperature, pressure and composition of the gasses surrounding a sample is extremely valuable for food scientists, because it allows them to model processes such as drying, cooking, and uptake of moisture during storage.
3.2. Dilatometry
A dilatometer is a device that is used to measure the change in density of a material as a function of time or temperature. Dilatometry measurements are routinely used for monitoring the crystallization and melting of fats in foods. A weighed amount of melted fat is poured into a graduated glass U-tube that is thermostatted in a temperature controlled water bath. The sample is then cooled at a controlled rate and the change in volume of the material is measured as a function of temperature. The density of a solid is usually greater than that of a liquid, thus the volume of a sample decreases when crystallization occurs, and increases when melting occurs. Dilatometry can therefore be used to provide information about the melting and crystallization of fatty foods. For food scientists, the most important information is the temperature at which melting or crystallization begins, the temperature range over which the phase transition occurs, and the value of the solid fat content at any particular temperature.