Gelatin is a nitrogenous food, but the nitrogen it contains is not available for tissue building. The ordinary commercial gelatin does, however, when made into jelly, give a good vehicle for the conveyance of other foods. Connective tissue of beef, veal, mutton and chicken will, by continued cooking in water, yield gelatin. For the sick, buy plain, unflavored gelatin.
Isinglass, fish gelatin, is obtained from the swimming bladders of sturgeons; and by many dietitians it is preferred to ordinary gelatin.
I doubt very much whether the ordinary calves' foot jelly made at home, is better than a quick jelly made from the commercial gelatin. The flavoring is different, and this may have something to do with the patient's liking for it.
Gelatin is insoluble in cold water, but it is wise to mix even granulated gelatin with cold water before pouring over the boiling water. Where gelatin is boiled, a larger quantity of gelatin or a less quantity of water must be used. After rapid boiling gelatin does not readily solidify.
Vegetable gelatin, clarified sea weeds, is quite nutritious.
Meat jellies made by condensing beef, veal or chicken stock are not more nutritious than gelatins made from commercial gelatin. But they are more stimulating and have a pleasant flavor, and for this reason are more agreeable to the invalid.