The swelling of fish skins in solutions of inorganic and organic acids

In experiments carried out partly in the laboratory and partly in breweries, it was noticed that when the method commonly used for the preparation of clarifying medium from isinglass was applied to fish skins, the latter were found to contain a high percentage of collagen, but they showed only slight swelling and very little gelatin diffused from them. When the organic acid used was varied, it was noticed that the behaviour of the skins changed considerably. In extreme cases shrinkage replaced swelling. The constitution of gelatin obtained from fish skins is different from that obtained from mammalian skins, and the great difference in the behaviour of gelatin according to whether it is obtained from fish skins or from mammalian skins may, with certainty, be attributed to this difference in chemical composition. No attempt is made in this paper to describe at length the chemical constitution of fish gelatin. We limit ourselves merely to a suggestion as to how its chemical constitution may influence the action of the reagents used to bring about swelling. Gelatin from fish skins is also likely to differ chemically from that extracted from the swim bladder. Though there is no exact knowledge of the chemistry of the two gelatins, the fact that the swimming bladder develops from the meso-blast, whilst the skin develops from the epi-blast, justifies one in looking for profound chemical differences. In this case also, the difference in chemical constitution is probably the cause of the difference in the behaviour of the respective materials to swelling agents. One specific difference between material of mammalian and material of fish origin may be noted here. Gelatin from calf skin, and the skin itself will absorb considerable quantities of water and yet retain the original structure. Fish skins, however, and the gelatin extracted from them, and isinglass absorb only a little water, up to a certain definite point. When this point is passed, there is disruption accompanied by considerable diffusion of nitrogenous products into the aqueous phase.