The gelling properties of gelatine are principally dependent on its gel strength; in the gelatine industry, this is given in Bloom grams according to the method defined by the American scientist Oscar T. Bloom. In addition, they are dependent on the gelling time, the gelling temperature, the melting temperature and the viscosity.
Gelling power. Determined by Bloom value and concentration.
The primary property of gelatine is its ability to form thermo-reversible gels. This characteristic property is not only of technological but also of economic importance; it is therefore an important quality aspect of gelatine. The gelling power is measured in Bloom-grams – this is the firmness of a 6 and 2/3% gel after 17 h storage at 10°C.
The Bloom values of commercial gelatine are between 60 and 300 Bloom. The firmness depends the Bloom value but also on the concentration of the gelatine being used. Examples are shown below for low-, medium- and high-Bloom gelatines.
The higher-Bloom gelatines are recommended for many applications. Their advantages include:
• Higher melting and setting points
• Quicker setting times
• Lower quantities required
• Lighter colors
• Neutral odor and taste
However, these advantages are influenced more or less by factors relating to the practical handling of the gelatine, e.g.:
• Concentration of the gelatine solution
• Gelling time and temperature
• Thermal pre-treatment and duration
• pH of the solution
• Salt content
Thus, for the uniform quality of the final product, not only is it necessary to select the right type of gelatine but also to know about the above-mentioned influencing factors. Examples are shown in the diagrams below:
Loss of gelling power as a function of temperature and time
Change in gelling power as a function of pH and type of gelatine
Setting and melting points.
A further characteristic is the temperature at which a gelatine solution transforms from a liquid to a solid and vice versa (setting and melting temperatures). High-Bloom gelatines have high setting temperatures.
To determine the visco-elastic behavior of gelatine gels, an oscillating rheometer is used. Here, in addition to the elasticity or storage modulus G', the viscosity or loss modulus G'' is determined. The cooling curve of a gelatine solution shows that the viscous properties dominate when the loss modulus G'' is greater than the storage modulus G'. In contrast, elastic properties dominate when the storage modulus is greater than the loss modulus; this corresponds to a gel state. The transition area, where the viscous part is the same as the elastic part, is known as the sol-gel transformation point. It can be seen that the cooling and heating curves of gelatine solutions are not identical; there is distinct hysteresis. This means that the melting process is at a higher energy level whereby the melting temperature is generally approx. 5 °C above the setting temperature of the corresponding gelatine.
The flow behavior of a particular type of gelatine is important as this considerably influences the process properties. The viscosity of a gelatine solution is dependent on the following factors: concentration, temperature and Bloom value. The raw materials and the manufacturing process have an indirect influence on viscosity.