Mechanism action of thickeners and their application in food processing(2)

The gelling process is essentially the formation of these joining zones.

The physical arrangement of these joining zones in the network may be influenced by various parameters such as temperature, the presence of ions and the inherent structure of the thickener. There are three mechanisms for the gelation of thickeners, namely ionic gelling, cold set gelling and thermoforming gelling. Ionic gelling occurs through the cross-linking of thickener chains with ions, usually negatively charged polysaccharide cation-mediated gelling processes such as alginate, carrageenan and pectin, and ionic gelling through diffusion setting or internal gelation.

In cold set gels, colloidal powders are dissolved in warm boiling water to form a dispersion which, on cooling, leads to enthalpy-stabilised interchain helices forming individual chain segments to form three-dimensional networks, such as agar and gelatine. Heat setting gels require heating of the gel, usually only where the food needs to be heat set. The heat setting mechanism occurs through the unfolding of natural starch proteins and their subsequent rearrangement into networks.

Role of the linkage region in the gel

The linkage region plays an important role in the gelation process of thickeners, influencing the properties and function of the gel. In gelatine, the association zone is formed by three molecules through hydrogen bonding. In carrageenan, six to ten molecules form the association zone, while in type I carrageenan only two molecules are involved. The higher the number of molecules in the linkage region, the higher the rigidity of the gel. Thus, the multi-molecular linkage region of K-type carrageenan is rigid and less likely to be reconstructed when disturbed by shear, whereas I-type carrageenan gels have a more flexible structure and are less sensitive to shear. The association zone of carrageenan and alginate consists of two molecules, but carrageenan gels can withstand more deformation before rupture than alginate, which has almost the same strength.

The thermal behaviour of the gels also varies depending on the association zone. Gelatine melts at lower temperatures because the linkage region is bound only by weak hydrogen bonds. The quality of the solvent is also another important factor. Hydrogen bonds in high methoxylated pectin gels can only be formed with the addition of sugar, which reduces the water activity sufficiently.

Other factors influencing gel formation

The various factors affecting gel formation from thickeners include the concentration of the gelling agent, the pH of the medium, molar mass, degree of polymerisation, temperature, ionic composition and solute. In addition to specifying the factors affecting gel formation from thickeners, the gels formed from them should be characterised, usually by microstructural and rheological characterisation, which can assist in the addition of thickeners as gelling agents. For example, the effect of the addition of sucrose and aspartame on the compressive properties of thickener gels, i.e. K-type carrageenan, kinked cold gels and K-type carrageenan acacia bean gum, has been investigated; the addition of sucrose resulted in an increase in the true fracture stress of all these gels. However, the addition of aspartame at low concentrations did not affect the textural compression parameters.

Furthermore, the main factors determining the sweetness of the gels are related to the mechanical properties of the gels (gel strength, fracture stress, fracture strain, etc.) and, in particular, to the amount of deformation required to break the network and its resistance to deformation. In addition, co-solubles such as sucrose, concentration of hydrolysed colloids, shear rate and temperature are also important variables affecting the rheological state of the gel.