This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Commercially there is little distinction between gelatins and glues, except that edible gelatin is a high-grade product and complies with the Pure Food Laws. It contains only traces of harmful ingredients, and Bogue states that it should be made from "such stock and by such sanitary methods, as not to be objectionable from the ethical point of view." Glue may be made from the same type of stock as the gelatin, but usually no effort is made to remove impurities.
Manufacture of gelatin. In making edible gelatin the stock is or should be selected with greater care than for inedible gelatin or glue. Some edible gelatins are made entirely from bones. Others are made from calfskin and others from pork skin. The bones of young animals contain more collagen than bones from older animals. For edible gelatin the bones are washed to free them from dirt, then degreased by extraction of fat by fat solvents. Next the soluble lime salts are removed by leaching the bones with acid. The principal salts in bone are the calcium phosphates, but some calcium carbonate and magnesium phosphate are also found. It is difficult to free the bones from all the traces of inorganic matter, and the electrolytes left influence the pH. of the resulting gelatin. The degreased bone with the inorganic salts removed is known as ossein and contains the collagen from which the gelatin is made. The ossein is soaked in lime water. In the lime water and in acid solution the collagen swells. This swelling makes it possible to hydrolyze the collagen to gelatin at lower temperatures than would otherwise be possible. This is important in the manufacture of gelatin, for with higher temperatures or long heating the jellying strength of the gelatin is lessened. The gelatin is then dried. The gelatin thus obtained has some ash and is not at the isoelectric point, but on either the acid or alkaline side of the isoelectric point. Putrefaction by bacteria must be prevented during the manufacturing process, as the bacterial action lowers the grade of gelatin obtained and odors develop that remain in the finished gelatin even if putrefaction is later stopped.
Market types of gelatin. Gelatin is put upon the market in three forms: granulated, pulverized, and sheet. The granulated is the form most commonly obtained.
Sheppard and Hudson have reported a new form of gelatin. It is called pressed foam gelatin. It is said to absorb water rapidly and to dissolve rapidly and it is not excessively bulky.