The hydrophilic and hydrophobic molecules and their interfaces exert an influence on the interface behavior of gelatin. These characteristics determine its properties, such as the stability of its foams and emulsions, its adhesive properties and its solubility.
Owing to the fact that some amino acids have charged groups attached to their side chains, proteins have different electrical charges. Thus, depending on its amino acid composition, each protein has a different distribution of charge, which is also pH-dependent.
At the isoelectric point (IEP), there are an equal number of positive and negative charges within the molecule. In effect, the molecule is neutral. The isoelectric point of native collagen occurs at approximately pH 9.
Gelatin is produced by the partial hydrolysis of collagen. Type A gelatin, produced using the acid process, has an IEP between 8 and 9. Alkaline-produced gelatin (type B) has an IEP of between 4.8 and 5.4. These differences result from the partial deamination of glutamine and aspargine to glutamic acid and aspergic acid, respectively, during the alkaline pretreatment of the raw materials.
In the application of gelatin, the IEP plays an important role. The nearer the pH of the final product to the IEP of gelatin, the more chance of turbidity or precipitation there is.
Gelatin is a polydisperse system comprising different lengths of protein chains that, in turn, consist of long hydrophobic chain segments and short hydrophilic segments. This molecular structure is typical for substances showing surface activity. This is why gelatin is capable of decreasing the surface tension of aqueous systems; gelatin molecules attach themselves to the interface as a film. This ability can be utilized technologically in the formation and stabilization of multi-phase systems such as foams or emulsions.
Foams and emulsions
Depending on the application, a gelatin with particular foaming or whipping qualities may be required. Type A gelatin has better foaming qualities than type B. This at least applies in the pH range 3–6. This can be explained by the different surface charges as a function of pH (amphoteric behavior), which, in turn, leads to a greater or lesser deconvolution of the molecules. Despite these basic differences, both gelatin types are suitable for foaming and stabilization if their molecular design (amphiphilic behavior) is correspondingly adapted. This also applies to the formation and stabilization of emulsions.