This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
The extent to which gelatin swells in water is modified by the surface area, its initial pH, the presence of or addition of acids, alkalies, and in-organic salts, and its previous history. Bogue states that gelatin tends to absorb water equal to the amount of water it held at the last warming, but not very much over this. Thus the degree of swelling is modified by the previous history of the sample. This would include treatment during manufacture.
Ease of hydration of gelatins. The pulverized gelatin swells most rapidly, for it is finely divided, hence has a greater surface area. Granulated gelatin absorbs water quickly, not so rapidly as pulverized, but rapidly enough so that it is easily used in the home. Sheet gelatin may be thin or rather thick. The thin form is easily hydrated, but the thick does not hydrate so readily, and therefore requires longer soaking in cold water or a longer period of heating to dissolve. In preparing gelatin dishes the gelatin is usually allowed to soak in a small amount of cold water. During this soaking it swells. Bogue suggests that this swelling, by the formation of capillary spaces in the interior of the gelatin particles, increases the surface of the gelatin.
Solution. According to Bogue, soaked gelatin goes into solution more rapidly than unsoaked gelatin owing to solution occurring from the outer and inner surfaces, whereas if gelatin is put in hot water before soaking, swelling is inhibited to a certain degree and solution takes place from the outer surface only. The process of solution may be analogous to hydrolysis of collagen, i.e., just as gelatin is formed more readily from the swollen collagen, so solution may take place more readily from the swollen gelatin. Very little of the gelatin goes into solution while it is soaking in cold water. When the temperature is increased to about 35°C. the gelatin goes into solution rapidly. For food preparation this is often accomplished by adding hot water to the hydrated gelatin. It can also be accomplished by melting the hydrated gelatin over hot water and then adding cold water or fruit juices.
Initial pH. Bogue states that ordinary gelatin usually has a pH about 7. The pH of different commercial brands used in the laboratory for food preparation has been found to vary from pH 5 to 7. The isoelectric point of gelatin is given as pH 4.7. At this point it is least soluble, least ionized and swells the least. Bogue has found turbidity and foaming to be at a maximum at the isoelectric point. Loeb states that 1 gram of dry gelatin at the isoelectric point absorbs 7 grams of water. As the pH is decreased below 4.7 or increased above 4.7, greater absorption of water occurs until a maximum is reached. Bogue states that it is desirable from the standpoint of solubility for the gelatin to have a pH of 3.0 to 3.5, or 8 to 9. The variation of the pH of commercial gelatins is due to differences in salt content, which may increase the acidity or alkalinity. This acidity due to electrolytes, or to the treatment in acids and alkalies during manufacture, is often referred to as the residual acidity or alkalinity of the gelatin.
Acids. At a pH below 4.7 the amount of water absorbed increases with increasing acidity until a maximum is reached at a pH. of about 3.5. As the pH is lowered more than 3.5, the amount of water absorbed decreases. Loeb states that at a pH of 3.2 to 3.5 a gram of dry gelatin absorbs about 35 grams of water. In food preparation the acid fruit juices are added after the gelatin is in solution, so that the extent of swelling during hydration is affected by the initial acidity of the gelatin. In gelatin mixtures that contain acid salts, as acid phosphates and citric acid added for flavoring, the amount of swelling would be affected by the acidity.
Alkalies. Alkalies have the same effect as acids upon the swelling of gelatin, except that it is not so pronounced. Bogue states that the maximum swelling on the alkaline side is at pH 9.
Salts. Salts also affect the water absorption of gelatin. Some salts inhibit the swelling and others favor it. Ostwald states that when salts and acids are combined the salt usually decreases swelling below the amount which would result in the presence of acid alone, even though the salt alone may increase swelling. Sugar decreases swelling. Loeb states that, if the pH of the solution is kept constant, all ions of the same valency and charge have nearly the same depressing effect upon swelling. The depressing effect increases with increasing valency.