Some physical conditions affecting the setting of gelatin and the bearing of the results on the theory of gel formation

Some solutions of proteins, under certain conditions of temperature, protein concentration and hydrogen ion concentration, slowly set to a gel. The most familiar example of this phenomenon is the gelation of the heat reversible gelatin gel. Many theories of the mechanism of gel formation have been brought forward. The theory of a framework of hydrated gelatin was originally suggested by Hardy and further developed by Jordan Lloyd.This theory states that "gels are two-phase structures, which can only be formed in the first place from a fluid system ( i. e., by cooling). They consist (1) of a solid framework of precipitated neutral gelatin, with which is combined a certain amount of water; (2) of an interstitial fluid, which is a solution of gelatin in the salt form (either acid or basic). The volume of any system in the gel condition is determined by the equilibrium between two opposing sets of forces. These are (1) the elastic forces of the solid framework which tend to make the volume contract; (2) the osmotic forces of the ionised gelatin salts dissolved in the interstitial fluids which tend to make the volume expand. Neutral gelatin at the iso-electric point cannot make a stable gel, since the constituent of only one phase is present. The gel formed on cooling will be free to contract till it is solid throughout,i. e., it will no longer be a gel. Gelatin in the salt form cannot make a gel, since the constituent of only one phase is present. Gelatin in this form will form only sols with water. The proportion of neutral gelatin to gelatin salts in any system is determined by the reaction of the system."