The Project Gutenberg eBook ofAnimal ProteinsThis ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.Title: Animal ProteinsAuthor: Hugh Garner BennettRelease date: October 26, 2012 [eBook #41192]Most recently updated: October 23, 2024Language: EnglishCredits: Produced by Juliet Sutherland, Joanna Johnston and theOnline Distributed Proofreading Team at http://www.pgdp.net(This file was produced from images generously madeavailable by The Internet Archive/Million Book Project)*** START OF THE PROJECT GUTENBERG EBOOK ANIMAL PROTEINS ***
This ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.
Title: Animal ProteinsAuthor: Hugh Garner BennettRelease date: October 26, 2012 [eBook #41192]Most recently updated: October 23, 2024Language: EnglishCredits: Produced by Juliet Sutherland, Joanna Johnston and theOnline Distributed Proofreading Team at http://www.pgdp.net(This file was produced from images generously madeavailable by The Internet Archive/Million Book Project)
Title: Animal Proteins
Author: Hugh Garner Bennett
Author: Hugh Garner Bennett
Release date: October 26, 2012 [eBook #41192]Most recently updated: October 23, 2024
Language: English
Credits: Produced by Juliet Sutherland, Joanna Johnston and theOnline Distributed Proofreading Team at http://www.pgdp.net(This file was produced from images generously madeavailable by The Internet Archive/Million Book Project)
*** START OF THE PROJECT GUTENBERG EBOOK ANIMAL PROTEINS ***
Transcriber's note:Minor typographical errors and inconsistencies have been corrected. Some words had inconsistent hyphenation throughout the book; these have been made consistent.Ions are shown as Fe+++, instead of using superscripts. There is some inconsistency in the notation used in the original text for chemical formulæ such as Na2Cr2O7(2H2O). These have been regularized to use the modern mid-dot, for example, Na2Cr2O7· 2H2O.The index entry for Hemlock bark had no page number in the original text, so the correct page number, 34, has been supplied.On page 152 NaCO23has been corrected to Na2CO3. On page 212, the variable n has been replaced with the correctly subscripted forms n1and n2.
MEMBER OF THE SOCIETY OF LEATHER TRADES' CHEMISTS; FORMERLYASSISTANT LECTURER AND DEMONSTRATOR AT THE LEATHERINDUSTRIES DEPARTMENT OF THE UNIVERSITY OF LEEDSAUTHOR OF "THE MANUFACTURE OF LEATHER"
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LONDONBAILLIÈRE, TINDALL AND COX8 HENRIETTA STREET, COVENT GARDEN1921
The rapid development of Applied Chemistry in recent years has brought about a revolution in all branches of technology. This growth has been accelerated during the war, and the British Empire has now an opportunity of increasing its industrial output by the application of this knowledge to the raw materials available in the different parts of the world. The subject in this series of handbooks will be treated from the chemical rather than the engineering standpoint. The industrial aspect will also be more prominent than that of the laboratory. Each volume will be complete in itself, and will give a general survey of the industry, showing how chemical principles have been applied and have affected manufacture. The influence of new inventions on the development of the industry will be shown, as also the effect of industrial requirements in stimulating invention. Historical notes will be a feature in dealing with the different branches of the subject, but they will be kept within moderate limits. Present tendencies and possible future developments will have attention, and some space will be devoted to a comparison of industrial methods and progress in the chief producing countries. There will be a general bibliography, and also a select bibliography to follow each section. Statistical information will only be introduced in so far as it serves to illustrate the line of argument.
Each book will be divided into sections instead of chapters, and the sections will deal with separate branches of the subject in the manner of a special article or monograph. An attempt will, in fact, be made toget away from the orthodox textbook manner, not only to make the treatment original, but also to appeal to the very large class of readers already possessing good textbooks, of which there are quite sufficient. The books should also be found useful by men of affairs having no special technical knowledge, but who may require from time to time to refer to technical matters in a book of moderate compass, with references to the large standard works for fuller details on special points if required.
To the advanced student the books should be especially valuable. His mind is often crammed with the hard facts and details of his subject which crowd out the power of realizing the industry as a whole. These books are intended to remedy such a state of affairs. While recapitulating the essential basic facts, they will aim at presenting the reality of the living industry. It has long been a drawback of our technical education that the college graduate, on commencing his industrial career, is positively handicapped by his academic knowledge because of his lack of information on current industrial conditions. A book giving a comprehensive survey of the industry can be of very material assistance to the student as an adjunct to his ordinary textbooks, and this is one of the chief objects of the present series. Those actually engaged in the industry who have specialized in rather narrow limits will probably find these books more readable than the larger textbooks when they wish to refresh their memories in regard to branches of the subject with which they are not immediately concerned.
The volume will also serve as a guide to the standard literature of the subject, and prove of value to the consultant, so that, having obtained a comprehensive view of the whole industry, he can go at once to the proper authorities for more elaborate information on special points, and thus save a couple of days spent in hunting through the libraries of scientific societies.
As far as this country is concerned, it is believed that the general scheme of this series of handbooks is unique, and it is confidentlyhoped that it will supply mental munitions for the coming industrial war. I have been fortunate in securing writers for the different volumes who are specially connected with the several departments of Industrial Chemistry, and trust that the whole series will contribute to the further development of applied chemistry throughout the Empire.
SAMUEL RIDEAL.
It has been the author's chief concern that this volume should fulfil its own part in the programme set forth in Dr. Rideal's General Preface.
The leather, glue, and kindred trades have been for many years recognized as chemical industries, but the great development of colloid chemistry in the last few years has given these trades a more definite status as such, and they can now be placed in the category of applied physical chemistry. The time is probably not far distant when some knowledge of pure physical chemistry will be a first essential to students, chemists, chemical engineers, and to all engaged in these industries in supervision, administration, or control. It is hoped that this volume will stimulate the study of these industries from that standpoint.
As the author has previously written upon one of the industries involved herein ("The Manufacture of Leather": Constable & Co.), he has, rather inevitably, found it difficult to avoid altogether his own phraseology. The changes of a decade, however, together with the wider field and newer view-point, have made possible a radical difference of treatment.
The author desires to acknowledge the help he has received from the many books, essays, and researches which are mentioned in the references at the end of each section, especially to Procter's "Principles of Leather Manufacture," and also to thank Dr. Rideal for many useful suggestions. The author would like also to acknowledge here his indebtedness (as well as that of the trade generally) to the work of Dr. J. Gordon Parker, who,through his researches, lectures, and teaching work, has done more than any other man to disseminate a knowledge of practical methods of tanning.
The author's thanks are also due to his brother, Mr. W. Gordon Bennett, M.Sc., A.I.C., M.C., for assistance in proof revision, and to his father, Rev. John Bennett, for some literary criticism.
H. GARNER BENNETT.
Beverly,June, 1921.
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:—
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 acidradical 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.
The constituent groups have been investigated chiefly by hydrolytic methods. The chains of amido acids are split up during hydrolysis, and individual amido acids may thus be separated. The hydrolysis may be assisted either by acids, alkalies or ferments, but follows a different course according to the nature of the assistant. Under approximately constant conditions of hydrolysis, the products obtained are in approximately constant proportions, and this fact has been utilized by Van Slyke in devising a method of proximate analysis. It is not possible in this volume to enter deeply into the constitution of the different proteids. Reference must be made to works on pure chemistry, especially to those on advanced organic chemistry. It will be interesting, however, to mention some of the amido acids and groups commonly occurring in proteids. These comprise ornithine (1:4 diamido valeric acid), lysine (1:5 diamido-caproic acid), arginine (1 amido, 4 guanidine valeric acid), histidine, glycine (amidoacetic acid), alanine (amido propionic acid), amido-valeric acid (amido-iso-caproic acid), liacine, pyrollidine carboxylic acid, aspartic acid, glutamic acid (amido-glutaric acid), phenyl-alanine, serine (hydroxy-amido propionic acid), purine derivatives (e.g.guanine), indol derivatives (e.g.tryptophane and skatolacetic acid), cystine (a thioserine anhydride), glucosamine, and urea.
There are a few general reactions which are typical of all proteins, and which can usually be traced to definite groupings in the molecule. Amongst these is the biuret reaction: a pink colour obtained by adding a trace of copper sulphate and an excess of caustic soda. This is caused by the biuret, NH(CONH2)2radical or by similar diacidamide groups,e.g.malonamide, oxamide, glycine amide. Another general reaction is with "Millon's reagent," a solution of mercuric nitrate containing nitrous fumes. On warming the proteid with this reagent, a curdy pink precipitate or a red colour is obtained. This reaction is caused by the tyrosine group (p. oxy α amido phenyl-propionic acid). Another general reaction is to boil the protein with 1:2 nitric acid for some days. A yellow flocculent precipitate of "xanthoproteic acid" is obtained, and this dissolves in ammonia and caustic alkalies with a brown or orange-red colour. Another characteristic of proteins is that on dry distillation they yield mixtures of pyridine C5H5N, pyrrol C4H5N, and their derivatives.
On the subdivision, classification and nomenclature of the proteins much ink has been spilled, and it is impossible in this volume to go into the various systems which have been suggested. It should be noted, however, that some writers habitually use the terms "proteid" or "albuminoid" as synonyms for protein. The classification of proteins adopted in this work is used because it is the most suitable for a volume on industrial chemistry and has the additional merits that it is simple and is already used in several standard works on industrial chemistry. It is based upon the behaviour of the proteins towards water, a matter of obvious moment in manufacturing processes. On this basis proteins may be divided into albumins, keratins and gelatins.
Cold water dissolves the albumins, does not affect the keratins, and only swells the gelatins. The behaviour in hot water confirms and elaborates the classification. When heated in water, the albumins coagulate at temperatures of 70°-75° C., the gelatins (if swollen)dissolve readily, whilst the keratins only dissolve at temperatures above 100° C. Albumins and keratins may be distinguished also from gelatins by adding acetic acid and potassium ferrocyanide to their aqueous solutions. Albumins and keratins give a precipitate, gelatins do not. Another distinguishing reaction is to boil with alcohol, wash with ether, and heat with hydrochloric acid (S.G. 1.2). Albumins give a violet colour, keratins and gelatins do not.
Albuminsmay be first discussed. They are typified by the casein of milk and by white of egg. Their solutions in water are faintly alkaline, optically active, and lævorotatory. They are coagulated by heat and also by mineral acids, alcohol, and by many poisons. The temperature of coagulation (usually about 72° C.) is affected by mineral salts, the effect being in lyotrope order (see Part V., Section I.). The coagulated albumin behaves in most respects like a keratin. Some of the albumins (globulins) are, strictly speaking, not soluble in cold water, but readily dissolve in weak solutions of salt. The albumins are coagulated from these solutions, as usual, when heated. Into this special class fall myosin (of the muscles), fibrinogen (of the blood) and vitellin (of egg yolk). By a gentle or limited hydrolysis of the albumins with dilute acids in the cold, a group of compounds called albuminates are obtained. They dissolve in either acids or alkalies, and are precipitated by exact neutralization. They may also be "salted" out by adding sodium chloride or magnesium sulphate. They are not coagulated by heat. After further hydrolysis with either acids, alkalies or ferments, very soluble compounds are obtained called albumin peptones or albumoses. These are soluble in alkalies, acids and water, and are readily hydrolyzed further into amido acids and acid amides. They are very similar to the peptones obtained from keratins and gelatins. They are not coagulated by heat.
Keratinsare typified by the hair of animals. They soften somewhat in cold water and even more in hot water, but are not dissolved until digested for some time at temperatures exceeding 100° C. With some keratins, however,the cystine group is to some extent easily split off by warm water, and on boiling with water hydrogen sulphide is evolved. The sulphur content of keratins is often greater than the average for proteids. All keratins are dissolved with great readiness by solutions containing sulphydrates and hydrates,e.g.a solution of sodium sulphide. In solutions of the hydrates of the alkali and alkaline earth metals, keratins behave differently. Some dissolve with great ease, some with difficulty, some only on heating and some not even if digested with hot caustic soda. They are dissolved (with hydrolysis) by heating with mineral acids, yielding peptones and eventually amido acids, acid amides, etc. Many keratins have a comparatively low content of nitrogen.
Gelatinsare very difficult to distinguish from one another, their behaviour being closely similar to reagents. They are also very readily hydrolyzed even with water, and the products of hydrolysis are even more similar. The gelatins are known together, commercially, under the general name of gelatine. Gelatins of different origin, however, have undoubtedly a different composition, the nitrogen content being variable. If the gelatins are not bleached whilst they are being manufactured into commercial gelatine, they are called "glue." Gelatine is colourless, transparent, devoid of taste and smell. It is usually brittle. Its S.G. is about 1.42, and it melts at 140° C. and decomposes. It is insoluble in organic solvents. When swelling in cold water it may absorb up to 12 times its own weight of water. The swollen product is called a "jelly." Jellies easily melt on heating and a colloidal solution of gelatine is obtained. This "sets" again to a jelly on cooling, even if only 1 per cent. gelatin (or less) be present. The solution is optically active and lævorotatory, but with very variable specific rotation. Some observers have thought that the different gelatins have different specific rotations and may so be distinguished. Gelatins are precipitated from solutions by many reagents, such as alcohol, formalin, quinone, metaphosphoric acid, tannins, and many salt solutions,e.g.those of aluminium, chromium and iron, and ofmercuric chloride, zinc sulphate, ammonium sulphate, potassium carbonate, acidified brine. Many of these precipitations have analogies in leather manufacture (see Parts I. to IV.). The gelatin peptones or gelatoses are formed by hydrolysis with acids, alkalies, ferment or even by digestion with hot water only. A more detailed description of the properties of gelatine is given in Part V., Section I. Gelatine is sometimes called "glutin" and "ossein."
Animals are much the most important source of proteins, especially of those which are of importance in industrial chemistry. Proteins occur in nearly every part of all animals, and the "protoplasm" of the living cell is itself a protein. The keratins include the horny tissues of animals: the epidermis proper, the hair, horns, hoofs, nails, claws, the sebaceous and sudoriferous glands and ducts, and also the elastic fibres. The gelatins are obtained from the collagen of the skin fibres, the bones, tendons, ligaments, cartilages, etc. Fish bladders yield a strong gelatin. The albumins are obtained from the ova, blood, lymph, muscles and other internal organs of animals.
The classification of proteins herein adopted fits in well with the scope and purpose of this volume. The keratins are of little importance in chemical industry, but are of immense importance in mechanical industry,e.g.the woollen trade, which is based upon the keratin comprised by sheep wool. The collagen of the hide and skin fibres is of vast importance to chemical industry, and is the basis of the extensive leather trades discussed in Parts I. to IV. The waste pieces of these trades, together with bones, form the raw material of the manufacture of gelatin and glue, as discussed in Part V. The proteids of animals' flesh and blood, milk and eggs form the source of the food proteins discussed in Part VI. The food proteins embrace chiefly albumins, but gelatins and even keratins are involved to some extent.
The term "hide" possesses several shades of meaning. In its widest sense it applies to the external covering of all animals, and is sometimes used derogatively for human skin. In this wide sense, it is almost synonymous with the term "skin." The term "hide," however, has a narrower meaning, in which it applies only to the outer covering of the larger animals, and in this sense is used rather in contrast with the term "skin." Thus we speak of horse hides, cow hides, camel hides, and buffalo hides. It is used in this sense in the title of Part I. of this volume. As such hides are from large animals, the leather which is manufactured therefrom is thick and in large pieces, and is therefore commercially designated as "heavy leather." From the standpoint of chemical industry hides are amongst the most important of animal proteins, and their transformation into leather for boots, shoes, belting, straps, harness, and bags comprises the "heavy leather trade," which is one of the largest and most vital industries of the country. The heavy leather trade predominates over other branches of leather manufacture, not only because of the comparatively large weight and value of the material handled, but also because the resulting products have a more essential utility. There is also a still narrower use of the term "hide," in which it applies only to the domesticated cattle—the ox, heifer, bull and cow—which use arises from the fact that the hides of these are both the largest and most valuable portion of the raw material of the heavy leather industries. In a very narrow sense theterm is also sometimes applied only to ox hides, which for most heavy leathers are the ideal raw material.
The Home Supplyof hides forms a large important proportion of the total raw material. Its importance, moreover, is rapidly increasing, for the excellence and abundance of the home supply determines the extent to which it is necessary for the industry to purchase its raw material abroad. The position of our national finances makes this an increasingly serious matter, for hides are comparatively a very expensive material.
The quality of our home supply of hides is very valuable, being determined by the conditions of the animal's life, its precise breed, and by other factors such as age and sex. The best hides are usually obtained from animals which have been most exposed to extremes of wind and cold, as such conditions tend naturally to develop a thicker and more compact covering. Broadly speaking, these include the hides from cattle of the northern and hilly districts. The age of the animal when killed is also a dominating factor. Calf skins are very soft, fine grained and compact, the state of rapid growth favouring the existence of much interfibrillar substance. The youngest animals supply suitable raw material for various light leathers (see Part II., Section V., p.120), and are also very suitable for chrome work (see Part III., Section III., p.156). Bull and cow hides, on the other hand, are from animals whose growth is complete, and show in consequence a lack of interfibrillar substance, coarse fibres and a rough and often wrinkled grain. The resulting leather tends consequently to be spongy, thin, empty and non-waterproof. Intermediate between these extremes are the hides of the ox and heifer, large, yet of good texture, and well supplied with interfibrillar substance. These hides are much the best for sole leather, a firm, smooth-grained and well-filled leather being needed. The term "kip" is often applied to small hides and to hides from large calves. In the trade, however, "kip" is sometimes used also forlarger hides, as a verbal enhancement of value; just as a man with a few old fowls is said to keep "chickens." Cow hides tend to be "spready,"i.e.to have a large area per unit weight, and are therefore more suitable for dressing leather. Bull hides are thicker in the neck and belly, and thinner in the back, which characteristics reduce their commercial value.
Market hides are sold by weight, and are therefore classified chiefly by their weight, which is marked on near the tail by a system of knife-cuts. The animals are flayed after cutting the hide down the belly and on the inside of the legs.
Of the various breeds, "Shorthorns" yield a large supply of useful hides. The name, however, covers a variety of similar breeds, and the hides therefrom are rather variable in texture and quality. They tend to be greasy owing to high feeding. The "Herefords," obtained from Midland markets, are generally excellent hides for sole and harness leathers. They give a good yield of butt pelt, a stout and smooth shoulder, and are not often greasy. "Devons" yield a good-textured and well-grown hide, but are often badly warbled (see p.10). The "Sussex" cross-breeds yield somewhat larger hides. "Suffolk Red Polls," common in East Anglia, yield a good butt, and the cow hides make good dressing leather. "Channel Island" cattle yield very thin hides, but with a fine undamaged grain. Scotch hides possess deservedly the very highest reputation. The climatic conditions favour the production of a hardy race of cattle with thick well-grown hides, yielding a large proportion of butt. These hides are amongst the best obtainable for heavy leather, and particularly for sole leather. "Highlanders," "Aberdeen Angus," "Galloways" are typical breeds, with short neck, legs and straight backs. Cross-breeds are also excellent (e.g."Scotch Shorthorns"). The natural value of these hides is further enhanced by the usual care in flaying. "Ayrshires" yield good milch cows and consequently yield often a more spready hide. The Welshbreeds for rather similar reasons also yield valuable hides. The Irish "Kerrys" are small but stout, and yield hides suitable for light sole leather. Irish cross-breeds, Shorthorns, have a rather bad reputation, and are often ill flayed.
All the varieties of the home supply are subject to various defects, which influence seriously their commercial value. One of these defects is warble holes or marks, caused by the Ox Warble fly (Hypoderma bovis). This is a two-winged fly about half an inch long. The larva of this fly, the "Warble maggot," lives and thrives in the skin of cattle, and causes a sore and swelling. The life-history of this insect is still in dispute, but it is generally thought that the eggs are laid in the hair on the animal's back, and the young larva eats its way through the hide until just below the dermis, and there feeds until mature. It then creeps out of this "warble hole," falls to the ground, pupates for a month, after which the imago or perfect insect emerges from the chrysalis. Hides which have been thus infected have, in consequence, often quite a number of holes through the most valuable part of the hide, thereby rendering it unsuitable for many kinds of leather. Even old "warbles" which have more or less healed up are a weakness, and warbled hides and leather fetch a decidedly lower price than undamaged. Another of these defects is bad flaying. Clearly the hide should be as little cut as possible, but many of our market hides are abominably gashed and often cut right through. This, of course, often reduces seriously the commercial value of the hide. Careless treatment after flaying also results in another common defect, viz. taint. As the term implies, the hide is partly putrefied, sometimes only in patches, but sometimes also so extensively as to render the hide quite rotten and quite incapable of being made into leather at all. Hides are of course putrescible, and dirt, blood, dung and warm weather encourage rapid putrefaction. As market hides are usually uncured, this defect is constantly appearing, and is a cause of considerable loss. Other defects are due to injuries to the animal before it is killed,e.g.brands,scratches due to hedges and barbed wire, old scabs, goad and tar marks. All these reduce the value of the hide.
All the defects in hides involve a very serious loss to the community, and the time is rapidly approaching when their continuance is insufferable. The loss is not usually very considerable to any individual, though very large in the aggregate. The hide is a minor part of the beast's value, and a somewhat damaged hide does not involve a very serious loss to the farmer. Some with typical stupidity regard a few warbles as "the sign of a healthy beast." These defects involve practically no loss to the hide merchant, tanner or currier, as each pays less for damaged material. The loss falls upon the community, and the time is ripe for the community to insist upon the elimination of these defects. The national resources will be for some years strained to their uttermost, and preventable damage must be considered intolerable. The principal defects in hides are preventable, and ought to be prevented. The warble fly could, by a united effort, be rendered before long practically extinct, a task which is facilitated by the fact that it is not migrative. Bad flaying and careless treatment of hides resulting in putrefaction are still more easily remedied. The communal slaughter-house is long overdue from the standpoint of public health, and would, under conditions of cleanliness and skilled workmanship and oversight, also solve the problem of ill-flayed and tainted hides.
The question of the raw material is of first importance to the leather trades. There was, before the commencement of the European War, a steadily increasing scarcity of hides, causing a constant increase in their price. This was due partly to the fact that cattle were increasing at a less rate than the population, partly to the growth of civilization, and more extensive use of leather in proportion to the world's population, and partly to the constant discovery of new uses for leather,e.g.for motor cars, aeronautics, etc. The question of raw material was under these conditions serious enough. The terrificslaughter, necessary at the same time to provide the belligerents with food and the army with leather, is bound to result in a serious crisis for the leather industries; and in conjunction with the country's financial condition, will make it absolutely necessary that all care should be taken with the raw material of one of our most important industries. The farmer who pays no heed to the warble fly, the man who gashes the hide in flaying and who allows the hide to putrefy, are equally criminal with the man who throws bread crusts into the dustbin.
It is impossible to foresee, as yet, anything in the nature of a satisfactory solution to the problem of raw material, especially in respect to heavy leather production, for the food question will rank first in the popular mind, and the earlier slaughter enjoined for the more economical production of meat will scarcely tend to increase the proportion of heavy hides.
The Foreign Supplyof hides is also of great importance and value. In the case of imported hides precautions to prevent putrefaction are essential, and some method of "curing" is always used.
Saltingthe hides is one of the most satisfactory methods for temporary preservation. The action of salt is hygroscopic, and mildly antiseptic. Moisture is withdrawn from the hides, which are then under conditions no longer favouring the growth of bacteria. Well-salted hides will keep for years, especially if quite clean. A light salting is also useful for a short preservation, and is becoming common in hide markets and tanneries during the summer and autumn months. Salting is a method used extensively in the United States. The "packer hides" of the stockyards are carefully and systematically salted with about 25 per cent. of salt and stored in cool cellars. The hides are so piled up in heaps, that brine easily drains away. The great disadvantage of salting is the so-called "salt stains." These stains have been ascribed to the iron in the salt, to the iron in the blood, to calcium sulphate in the salt, and also to chromogenic bacteria, whose development is favoured bysalting. The relative importance of these factors is not yet satisfactorily determined, but cleanliness and pure salt tend to eliminate the trouble.
Dryingthe hides is a less satisfactory cure. The principle is similar, viz. removal of moisture. Dried hides are, however, much drier than salted, and are quite hard and horny, hence the name "flint hides." The hides also lose much weight, a considerable advantage in reducing freight. Tropical hides are often flint-dry, and where preservatives are expensive or unprocurable, it is often the only practicable method of cure. Nevertheless, the method has many serious disadvantages, and is difficult to execute. If dried too slowly the hides putrefy partially; if too quickly they dry on the outside, and the interior is left to putrefy. The fact that hides are of uneven thickness, and the climate often hot, increases the difficulty, and often results in partial destruction of the fibrous structure of the hide. When dried, moreover, the hides are still subject to the attacks of insect larvæ, for the prevention of which the usual sprinkling of naphthalene or arsenic is only an imperfect remedy. This method of cure is also a nuisance to the tanner, who has to employ labour, pits and time in attempting to restore the hides to their original condition, and often loses up to ten per cent. of the goods in so doing. Dried hides are also subject to the presence of anthrax.
Dry Saltingthe hides is an excellent method of curing. As the name implies, it combines methods of drying and salting which are used alternatively. The method is used extensively in South America. A modified form of it is also used for preserving the "E.I. kips," which are cured, however, not with common salt, but with earth containing up to 70 per cent. of sodium sulphate. Dry-salted hides are largely free from the defects of dried hides, but of course are more trouble to the tanner in the process of soaking (see Section II., p.16) than the wet-salted goods.
Freezingthe hides is now a commercial process. On the whole the process is satisfactory, but the expansion of water after freezing may tend to damage the hide fibres.
Sterilizing the hides has been frequently suggested, but no method has yet been advocated which does not interfere either with the tanning processes or with the quality of the finished leather.
Hides from the European Continent, usually wet salted and well flayed, exhibit much the same variable quality as the home supply, those from highland districts tending to be thick, yet even, well grown, tight textured and smooth grained, whilst those from lowland regions are less satisfactory. Thus hides from the Swiss Alps and Scandinavia have ranked high, whilst the spready Dutch cows are typical of a lowland hide. In the hides which once came from Germany the same features appear. Bavarian highland hides had an excellent reputation, whilst those from Berlin, Cologne, etc., tended to be long in shank and not well grown. French hides are often ill flayed, and Spanish and Portuguese are often subject to scratches. Italian hides have a very good name, being small but stout in butt.
The American supply is important. South America yields an excellent class of hide, salted or dry-salted. They are from an excellent breed of animals, slaughtered and flayed with every care, and efficiently cured. A most serious defect in this class of hide is the "brand," which is both deep and large and in the most valuable part of the hide. One side, however, is usually unbranded, so that each hide yields one good "bend." These hides,e.g."Frigorifics," have recently been much more extensively tanned in Britain because of the shortage in the home supply of market hides caused by the European War. South America also yields good horse hides. North American hides are usually wet-salted (e.g.packer hides). They are usually good. Central America yields mostly dried hides exhibiting usual defects.
The Asiatic supply comprises the frozen China hides, which are clean but small, with flaying of uncertain quality. There are the buffalo hides from Asia and East Europe, which are suitable for cheap and sole and strap leather, and also the dry-salted "E.I. kips," obtained from a smallbreed of Indian cattle, and extensively made into upper leather. The Asiatic humped cattle also provide a limited supply. The African supply is of increasing importance. The tropical parts yield dried hides of uncertain quality, but the more temperate parts of South Africa yield a growing supply of good quality.
REFERENCES."The Manufacture of Leather" (Bennett), pp. 27-37."Principles of Leather Manufacture" (Procter), pp. 33-56."The Ox Warble or Bot Fly" (E. Ormerod)."The Making of Leather" (Procter), pp. 2-22.
Before hides are tanned it is necessary for them to pass through a series of preparatory processes. The object of these processes is to obtain from the hide the true hide substance in a pure and suitable condition. Each class of leather has its own appropriate processes, the adjustment of which largely determines the quality of the finished article. So prominent is the influence of these preparatory methods that the paradox "good leather is made before tanning" is in trade circles almost a platitude. These processes, sometimes lumped together under the general name of "Wetwork," comprise soaking, liming, beam house work and deliming. These will be discussed in turn.
The term applied to the hide after these processes, but before tannage, is "pelt."
Soakinghas for its object the cleansing and softening of the hides, chiefly by means of water. It aims at the removal of dirt, blood, dung, and curing materials by washing. The process is usually simple, and is much the same for all classes of leather. The ideal to be aimed at is to restore the hide to its condition when it left the animal's back. Cleanliness in leather manufacture is as essential at the commencement as anywhere, for the hide is in its most putrescible state. The soluble proteids (blood, lymph, part of dung, etc.) which always adhere to hides encourage the rapid growth of putrefactive bacteria, and cannot be washed away too soon. Dung is often difficult to remove, being caked on the butt end amongst the hair. Soaking only softens it, and mechanical removal is usually necessary. If such substances are not removed, they go forward with the goods into the lime liquors, causing stains,loss of hide substance, and counteracting plumping.
The detailed method and time of soaking are determined mainly by the nature of the cure. One of the purposes of the soak liquors is to dissolve the salt used in curing hides and to rehydrate the hide and make it again soft and pliable. As a 10-per-cent. salt solution exerts a solvent effect on hide substance, it is necessary soon to change the first soak liquor of salted goods.
Market hides, which are uncured, require the least soaking, the cleansing effect being most needed. The hides are inserted into pits ("water dykes") of water for a few hours, and the water changed once or twice. The soaking should not be prolonged as the hides are so putrescible, and where it is customary to leave the goods in a soak liquor overnight, it is advantageous to add a little slaked lime to the water before inserting the goods. This not only softens hard water, but is mildly antiseptic and plumping, and forms a suitable introduction to the liming proper. Each pit contains a "pack" of 30-50 hides, according to its capacity, which varies in different tanneries from 1000 to 2000 gallons. Tainted goods, which are indicated by a characteristic white colour on the flesh side and by loose hair, need a preliminary washing either in a "drum," "tumbler" or in a "paddle." This ensures a rapid change of liquor and the removal of most of the putrefactive agencies. Bad cases may need the application of antiseptics, such as immersion in 0.1 per cent. carbolic acid; but if possible these should be avoided, as they lengthen the time required for liming. After drumming or paddling, tainted goods should be placed directly into a lime liquor.
Salted hides need very similar treatment to uncured hides, but the soaking is longer, because of the dehydration caused by salting. Hence they receive also a greater number of changes of water, three or four usually, but often more. As much loose salt as possible should be shaken from the hides before insertion into any liquor. The employment of drum or paddle before pit soaking is extremely useful to effect the rapidremoval of superficial salt, and is also useful after pit soaking to remove the last traces.
Dried and dry-salted goods need a soaking still more prolonged, up to one week if water alone be used. With the assistance of caustic soda, however, the process can be shortened to about two days. The first soak liquor should consist of a 0.1 per cent. solution of caustic soda, and after the goods have been inserted twenty-four hours, they will be materially improved by a few hours' drumming or paddling. Another caustic soda soak will complete the process. Sodium sulphide crystals may replace caustic soda, but about three times the weight will be needed. Carbonate of soda and caustic lime also are a convenient commercial substitute for caustic soda. For 10 lbs. caustic soda, use 36 lbs. carbonate and 7 lbs. lime. Extra lime should be added in all cases when the water is hard. Acid liquors will also soften dried and dry-salted goods, but such processes do not fit in so well with the subsequent liming. The use of putrid soaks and stocks may be now considered out of date.
Limingfollows soaking, and consists essentially in immersing the hides for 7-10 days in milk of lime. The chief object in view is to loosen the hair and prepare for its mechanical removal. Liming takes place in pits, the tops of which are level with the limeyard floor. The lime is slaked completely and mixed well with water in the pit, being particularly well plunged just before the insertion of a pack of goods. Saturated limewater is only a 0.13-per-cent. solution. The goods are occasionally "handled"i.e.hauled out of the pit and reinserted after plunging ("hauling" and "setting"). This is necessary to keep the liquor saturated with lime. The hides are inserted one by one, each being "poked down" to ensure its contact with the liquor. The goods are invariably immersed first in a previously used lime liquor. Most tanneries now carry this out in a systematic way, so as to ensure regularity in the process. As the goods are large and heavy it is lesslaborious to carry out the whole process in one pit. In this "one-pit system" the goods are inserted for (say) four days in an old used lime liquor, with occasional handling; this liquor is then run to the drain and a new liquor made up in the same pit, into which the goods are inserted for (say) five days. They are then hauled and sent to the unhairers. Each pack thus gets two liquors, old and new.
A better method is the "three-pit system." In this case each pack receives three liquors and has (say) three days in each, first an "old lime," then a "medium lime," and finally a "new lime." This system ensures a greater regularity of treatment, and is deservedly the most popular method for liming hides for sole leather. After being used once as a "new lime," a liquor then becomes a "medium lime," and after being thus used becomes the "old lime" which receives the green hides from the soaks. The system involves the goods being shifted twice to another pit, which is more laborious than reinsertion into the old pit, but if the limeyard be arranged in "sets" or "rounds" of three pits, the shift is usually only to the adjacent pit. One special advantage of this system is that the top hides in one pit become the bottom hides in the next pit, andvice versâ. Rounds of more than three pits are sometimes used.
Many factories have now adopted systems in which there is no handling at all. The hides are suspended in lime liquors which are agitated by mechanical contrivances (e.g.Tilston-Melbourne process), or by jets of compressed air (e.g.Forsare process). The goods are soaked and limed "mellow to fresh" by changing the liquors by means of pumps, air ejectors, etc. Thus the hides need no labour from first being inserted until drawn for depilation.
In liming, the whole of the epidermis as well as the hair is loosened, and is subsequently removed in depilation. The corium or true hide substance becomes much more swollen by imbibation of water, and when taken out of the new lime is "plumped" to very firm jelly. This plumping is a matter of prime importance to the tanner. The coarser fibres are thereby split up into the finer constituent fibrils, which fact assistsvery materially in obtaining a quick and complete tannage, good weight, and a firm leather. During the liming, the natural grease of the hide is saponified or emulsified, which prepares for its removal in scudding. Liming is thus a complex process: the hair is loosened, the hide is plumped, and the grease is "killed." All these results may be hastened by the use of other alkalies in addition, and most heavy leather yards assist the liming by adding also sodium sulphide or caustic soda or both. Sodium sulphide is a powerful depilatant, and will alone unhair hides easily in strong solutions even in a few hours. As in solution it forms caustic soda by hydrolysis, it possesses also the powerful plumping and saponifying powers characteristic of the latter. The addition of arsenic sulphide (As2S2) (realgar) to the lime when slaking causes the presence of calcium sulphydrate in the lime liquors thus made. This is also a powerful depilatant, but not much used for heavy leather.
The function of the lime in depilating is complex and has occasioned much discussion. Its main purpose, however, is that of a partial antiseptic. When hides putrefy, one of the first results is that the hair is loosened. In America depilation by "sweating" is carried out commercially by such a mild putrefaction, the lime liquor permits a similar fermentation at a slower rate, and all tannery lime liquors are swarming with putrefactive bacteria. Liming is thus a safer method than sweating, which may be easily carried too far. Various workers have isolated specific organisms—Wood abacillus, Schmitz-Dumont astreptococcus—but it seems highly probable that the limeyard bacteria are just the common organisms of putrefaction sorted out or selected by the exact nature of the liquor and the method of working the limes. Many putrefactive bacteria are very adaptable and could easily accommodate themselves in this way. It is known that the exact nature of the culture medium has a great influence on the rate of development of such organisms, and which particular species thrive and obtain predominance in any limeyard will depend upon the amount and nature of the dissolvedorganic matter available as food, and upon the exact alkalinity and the concentration of other apparently inert substances, such as common salt and sodium, calcium and arsenic salts. Hence no two lime liquors operate alike, and approximate regularity is only assured by systematic method. In handling and shifting, the organisms are subjected to further selection, and the most adaptable survive. It is probable that different species may act symbiotically. The depilating organisms of lime liquors are probably mostly anærobes, but some may be anærobic by adaptation. It is probable that ærobic ferments commence the depilation, but this will be done before the goods are put into work, or at any rate before they reach the limes. More strictly, it is the enzymes secreted by bacteria which are directly responsible for the hydrolytic work; these enzymes are chiefly proteolytic (proteid splitting), but the lipolytic (fat splitting) enzymes have also a place.
The lime, however, not only limits and selects the course of the putrefaction, but also affords more positive assistance. Lime plays its own hydrolytic part and assists the depilation by purely chemical action. Lime will unhair without the assistance of bacteria, but its action is slow and forms a minor part of the operation in the average limeyard. This action is due chiefly to its progressive formation of calcium sulphydrate from the cystine group of the softer keratins. Lime also plays an essential part in assisting the putrefactive fermentation. It softens the keratins and thus assists the bacterial attack, it hydrolyzes other proteids and provides the bacteria with food in solution, the calcium ion increases the proteolytic action of certain enzymes, and finally the apparently inert excess of undissolved lime has an accelerating effect on the bacterial activity.
In the average limeyard these various functions are inextricably mixed up, and it is impossible to assign any definite proportion of the total depilatory effect to any of the factors at work. Lime alone will unhair, bacteria alone will unhair, and sulphides will also unhair without limeor bacteria, but in the limeyard all three agencies are at work. Putrefactive fermentation, however, obtains a good start. Ærobic fermentation commences with the slaughter of the animal, and the anærobic organisms soon commence their part, and are at work in the hide house and soaks. On entering the limes, the purely chemical hydrolytic action of lime is added to that of the bacterial enzymes as well as the action of lime as bacterial assistant, and the three continue to operate side by side. Each gives rise to the formation of calcium sulphydrate, whose own special solvent effect is superadded. If sulphydrates be deliberately added to the liquors there is yet another factor assisting. Speaking broadly, the bacterial enzymes have their maximum activity in the old limes, and the chemical action of sulphydrate formed from the keratin cystine is also at a maximum in these liquors. The chemical action of added sulphide, and the simple hydrolytic action of calcium hydrate have their maximum activity in the new limes. Most observers would agree that in practice the bacteria shoulder the greater part of the work.
From the limeyard is taken about the only waste bye-products of the tannery, viz. the residues from the soak and lime pits. These consist mainly of lime and chalk, with some hair and dung, and possibly a little sulphide. The sludge possesses some value as a manure, especially if from the soak pits on account of the greater nitrogen content. (Part VI., Section I.)
The Beam House Workconsists in the mechanical removal of those parts of the hide not wanted for leather manufacture.Unhairingremoves the hair and the epidermis made loose in liming. The hides are placed over a sloping "beam" with a convex surface, and the hair scraped off with a blunt concave and double-handled knife. The hides are then thrown into a pit of water. The hair is carefully collected, washed well with water, preferably centrifuged, and then dried out by a current of warm air. It forms a valuable bye-product. White hair is usually keptseparate and fetches a higher price.Fleshingis the next process. The hides are again placed over a beam, with the flesh side (i.e.the side nearest the flesh) uppermost. Skilled workmen then cut off, with a sharp convex knife, the fat, flesh and connective tissue left in flaying.Roundingis usually the next process. The unhaired and fleshed hide is spread out flat and cut up into butt, shoulder and a pair of bellies. These parts have different commercial values, and may afterwards be tanned by different methods for very different purposes—for dressing leather, and sometimes even for sole leather.Scuddingis the last piece of beam work. The fleshed hides (whether rounded or not) are washed, or at least rinsed, with water, and again placed on the beam grain side up. They are then scraped with a rather sharp concave knife, to remove "scud," which consists of hair roots and sheaths, lime soaps, fat, pigment and other dirt. Short hair is shaved off by a very sharp hand knife.
The beam work demands a certain amount of skill from the workmen, especially from the flesher, whose sharp knife may prove very wasteful in incompetent hands. Hand labour was slowly but surely being replaced by machinery before the war, and war-time conditions have greatly accelerated the rate of transition. Beam house machinery is rapidly becoming universal. The machines are cumbrous and expensive in cost and in power, but machine work is quicker, less laborious, and needs much fewer workmen. Many types of machine have been suggested, but the most useful are those in which the hides pass over rollers and are simultaneously acted upon by a rapidly revolving cylindrical knife with spiral blades, one half being a left-handed and the other a right-handed spiral, so that the hide is scraped outwards as well as in the direction of motion. The part of the hide being acted upon rests on a pneumatic roller. By changing the type of spiral knife cylinder the machine will unhair, flesh or scud.
Delimingis a general name covering a number of similar operations whose primary object is the neutralization and removal of the caustic lime and soda in the plumped pelt, or at any rate on the surface of thehide. This is a preparation for the tan liquors. All the tannins and many associated substances darken rapidly with oxidation when in alkaline solution, so that to place the fully limed hide in a tan liquor would give a dark-coloured leather. A short insertion in a bath of weak acid would secure the elimination of surface lime and the disappearance of this difficulty, but there are other purposes in deliming. The more completely lime is removed the more the plumped pelt "falls" into a soft, pliable, unswollen and relaxed condition, and this change assists very materially in the production of a soft dressing leather, suitable for boot uppers, bags, etc. For such leathers, therefore, the deliming must be much more complete than for sole leather, in which the object is to obtain a firm and plump leather.
In the case of the softer dressing leathers, experience indicates the advisability of allowing some further bacterial action on the interfibrillar substance in order to produce the requisite pliability and softness. This is secured by "bating" the hides. This process consists in immersing the goods into a cold fermenting infusion of hen or pigeon dung. The infusion is made in a special tub or pit with warm water and allowed to stand for a day or two until the fermentation has commenced, and then run into the bating pit through a coarse filter such as sacking. The hides are immersed for some days, but are handled frequently to ensure an even effect. The bate is always slightly alkaline. The caustic alkalinity increases rapidly at first owing to the diffusion of caustic lime, then at a slower rate, afterwards slowly declining. This is explained by the production of organic acids, and their salts with weak bases from the dung infusion by the action of bacteria. The total alkalinity of the bate liquor increases rapidly at first owing to the diffusion of lime and its liberation of organic bases, then very slowly, but towards the end of the operation the total alkalinity increases very rapidly indeed, owing probably to the commencement of a violent anærobic fermentation which produces ammonia and other organic bases, and which heralds the approach of aputrefactive action, which if allowed to continue for even a short time will ruin the hides. Bating is consequently a risky process, and needs experienced oversight. For goods which need only a mild bating, there is the alternative of giving a longer liming in older limes. This of course involves more bacterial hydrolysis, and perhaps does it in a safer, more economical and certainly in a less offensive manner. Bating is often followed by a further deliming by acids. Boric, lactic, acetic, formic and butyric acids are all used, and with care even hydrochloric and sulphuric acids may be employed. Innumerable "artificial" bates have been put on the market, but most are merely weak acids, acid salts or salts of strong acids with weak bases. An American "bacterial bate" consists of a lactic fermentation of glucose in the presence of glue.
Closely similar to bating is "puering," investigated by Wood (see p.94).
Drenching is another fermentive deliming process. In this the goods are inserted into an infusion of bran. This is made by scalding the bran with hot water, and allowing it to stand until it is about 70°-90° F. The infusion is then "inoculated" with a few gallons of old drench liquor, and the goods are immersed. This fermentation has been examined carefully by J. T. Wood. First the enzyme cerealin converts bran starch into glucose, which is then fermented by the drench bacteria with the production of lactic acid, some acetic acid and small amounts of formic and butyric acids. The butyric fermentation is liable to become too violent. These acids, as they are formed, neutralize the lime in the hides and plump the pelt slightly (see pp.107-109).
Various gases (carbon dioxide, hydrogen, nitrogen, methane and sulphuretted hydrogen) are involved, and the proportion produced in the pelt itself has a peculiar opening effect on the hide fibres. The activity of the drench can be decreased by dilution and by using a less starchy bran, and can be increased by adding pea meal or rye meal.Drenching usually follows bating. Scudding sometimes follows deliming.
The theory of the volume and elasticity changes of pelt during preparation will be better understood after considering the behaviour of gelatine gels (pp.200-219). The determining factors are thenettcharge of hydroxyl ions on the disperse phase, resulting from ionic adsorptions, and the lyotrope influence of dissolved substances on the continuous phase.
In softening dried hides the swelling may be due to either influence, but the latter tends to loss of hide substance and the production of soft leather.
In liming, the nett adsorption of hydroxyl ions is the principal factor, but the lyotrope influence of the alkali cations and of the impurities is important. Plump pelts are those in which the contained water is in a relatively greater average state of compression. Few substances can assist plumping, but many can hinder it. In plumping all lyotrope influence is objectionable, and "sharp" (pure) alkali solutions are required. Mellow limes reduce elasticity and plumpness by lyotrope influence.
In bating and puering the essential change is that before the process the swelling is due chiefly to adsorption of hydroxyl ions, whereas afterwards it is due chiefly to a composite lyotrope influence.
REFERENCES."Principles of Leather Manufacture," Procter, pp. 108-184."The Manufacture of Leather," Bennett, pp. 49-113."Lyotrope Influence and Adsorption in the Theory of Wetwork,"Bennett,J.S.L.T.C., 1920, pp. 75-86."Analytical Examination of Bating," Bennett,Leather Trades Review, 1911, p. 972, and 1912, p. 28."The Bating, Puering and Drenching of Skins," by J. T. Wood.