9. THE EVOLUTION OF THE GELATINE AND GLUE INDUSTRY * * * * * PART VI. MISCELLANEOUS PROTEINS AND BYE-PRODUCTS. 1. BYE-PRODUCTS OF THE LEATHER TRADES 2. BYE-PRODUCTS OF THE GELATINE AND GLUE TRADES 3. FOOD PROTEINS 4. MISCELLANEOUS ANIMAL PROTEINS INDEX ANIMAL PROTEINS INTRODUCTION Proteins are organic compounds of natural origin, being found in plants and in animals, though much more plentifully in the latter. They are compounds of great complexity of composition, and of very high molecular weight. The constitution of none of them is fully understood, but although there are a great number of different individual proteids, they present typical resemblances and divergences which serve to differentiate them from other groups of organic bodies, and also from one another. Proteins resemble one another in both proximate and ultimate analysis. They contain the usual elements in organic compounds, but in proportions which do not vary over very wide limits. This range of variation is given approximately below:-- Element. Per cent. Carbon 49 to 55 Hydrogen 6.4 to 7.3 Oxygen 17 to 26 Nitrogen 13 to 19 Sulphur 0.3 to 3.0 The most characteristic feature of the protein group is the amount of nitrogen usually present. This is generally nearer the higher limit, seldom falling below 15 per cent. This range for the nitrogen content is determined largely by the nature of constituent groups which go to form the proteid molecule. Roughly speaking, proteins consist of chains of amido-acids and acid amides with smaller proportions of aromatic groups, carbohydrate groups and thio compounds attached. In these chains an acid radical may combine with the amido group of another amido acid, the acid group of the latter combining with an amido group of another amido acid, and so on. Hydrogen may be substituted in these chains by alkyl or aromatic groups. There is obviously infinite possibility of variation in constitution for compounds of this character, the general nature of which varies very little. Practically all of the proteins are found in the colloid state, and this makes them very difficult to purify and renders the ultimate analysis in many cases doubtful. It is, for example, often difficult to ascertain their moisture content, for many are easily hydrolyzed with water only, and many part easily with the elements of water, whilst on the other hand many are lyophile colloids and practically cannot be dehydrated or dried. A few, such as gelatin and some albumins, have been crystallized.