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Vol. 15, No. 7
T h e Use of Lime in t h e Glue and Gelatin Industry' By Robert H. Bogue LAFAYSTTE CO.LLEGE, EASTON, PA.
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0 UNDERSTAKD properly the function of lime, it is necessary to point out briefly the several steps in glue and gelatin manufacture. Several types of raw stock are used, which include hide pieces and trimmings from the tanner or the packer; fleshings, which consist of the under layer of the hides and are made up of loosely packed fibers of skin substance, fat cells, and thin muscles attached to the skin; sinews or tendons and connective tissue; ossein, which is the organic portion of bones left behind when the mineral matter is dissolved out with acids; and bones. All of these with the exception of untreated bones are conveniently grouped together as hide stock. Since bones do not receive a lime treatment, they will be omitted from the discussion. The hide stock is first washed to remove dirt and salt o r other preserving material that may have been added, and shredded to bring a greater surface exposure to the reagents employed later. This material is then allowed t o stand in vats with a suspension of lime (water-slaked) in water. After about two weeks the stock is forked out and a fresh suspension of lime in water is added. After two or three such treatments the stock will have attained a plump, uniformly swollen condition, and it is then removed and washed, first with water and subsequently with a dilute acid solution to neutralize the excess of lime. The neutralized stock is placed in a large, open tank, with water and steam admitted below a false bottom until the mixture has attained a temperature of 80" C. or higher. This operation, known as the boiling process, brings about an extraction of the gelatin. The liquor is run off after a few hours, and the boiling repeated a number of times with fresh lots of water. The liquors may then be filtered or clarified, and, being too thin to gel well, are concentrated i n vacuo and allowed t o form a jelly, after which they are dried and ground as desired.
BROUGHT ABOUT BY THE LIME The most obvious changes that have been induced b y the liming operation are the greatly increased volume and the loosening of the hair. The hide pieces have increased t o several times their original cross section, and have taken on a firm, rigid appearance. If the hair has not actually fallen off during the lime treatment, it will be found to be held so loosely that a gentle rubbing with the fingers will suffice t o remove it. Perhaps less obvious, but of more importance, are the chemical changes involved. If raw hide pieces were heated with water, solution would be effected very slowly unless a temperature above 100' C. (under pressure) were used. But gelatin, the constituent of glue which gives it the power t o form a jelly and upon which adhesiveness seems t o depend, is a heat-sensitive substance, and when exposed t o high temperatures rapidly undergoes a decomposition, breaking up into constituents which have very little or no value as jelly producers or adhesives. As a result of the lime treatment, however, the hide pieces are enabled to pass into solution by only a moderate heat treatment with water, and the valuable properties of the gelatin are preserved. Just how the lime functions t o bring about this result is not yet aItogether clear, but it seems that the alkaline solu* Received October 9, 1922. CHANGES
tion induces a loosening and separation of the fibers in tlie hide. When these are packed tightly, as in untreated skin, they present a limited surface exposure, but the distension produced by the lime offers a greater surface exposureand consequently permits a greater rapidity of solution, ALKALICONCENTRATION This plumping action appears t o be due to a particular concentration of hydroxyl ions or alkali. If portions of a neutral hide are immersed in ~ 5 t e rcontaining increasing amounts of alkali or of acid, the hide swells more as Iarger amounts of alkali or acid are added (there is a t first a slight decrease in volume on adding acid) until a certain maximum of swelling is attained, and if stiIl more alkali or acid are added beyond this point, the swelling becomes less and solution takes place. The alkalinity of a saturated solution of lime is very close to that alkalinity a t which maximum stvelling occurs. But if only a saturated solution of lime in water is added and the alkalinity of the solution is tested at regular intervals after introducing the hide pieces, it very rapidly becomes less alkaline, and is soon almost neutral. This would also be the case with any other alkaline or acid substance, and is due t o the ability of the hide t o absorb or react with hydroxyl or hydrogen ions, respectively. By using a suspension of the lime in water this difficulty is overcome, for a suspension is merely a saturated solution plus an excess of the undissolved sblid. As rapidly as the hydroxyl ions are removed by the hide, just so rapidly will the undissolved lime pass into solution. By this means the alkalinity of the mixture is automatically held constant. It is this property of a nearly foolproof and automatically controlled alkalinity that makes lime better than most other reagents for this service. SOLVENT ACTIONOF LIME Besides collagen, which is converted into gelatin by heating in water, glue stock contains several other proteins that have no value whatsoever in glue. Elastin is found in the connective tissue, mucin is found in cartilage, keratin is contained in hair, and albumins are always present. If these substances were allowed to be cooked with the stock, the product would be weakened and mould also give a glue that was turbid, muddy, or opaque. These proteins, especially the albumin and mucin, are soluble in alkaline solutions, and so are dissolved out of the stock by the lime. Acids would not effect such solution. Here again the lime is particularly well adapted for the purpose, as solutions of stronger alkalinity would dissolve also increasing amounts of the collagen and solutions of weaker alkalinity would not be effective in dissolving even the albumins or mucins. A s the albumins and elastin are attacked by the lime the roots of the hair become loosened, for they are embedded directly in the layer of elastin below the epidermis. The hair, having no longer any support, falls out or is easily rubbed off. Some of the fat of the stock is also acted upon by the lime, forming insoluble lime soaps that should be removed in the washing process if a clear product is desired. GERMICIDAL ACTION Lime itself is a very good germicide for most microbiological organisms. There are some forms of bacteria that
July, 1928
INDUSTRIAL A N D ENGINEERING CHEMISTRY
thrive in lime solutions and high counts have been obtained of these, but the putrefactive organisms are almost entirely killed or inhibited. In general, it is quite unnecessary to add other preservatives during the liming process. SELECTION OF THE LIME
It is important to use care in the selection of the lime for hide swelling, for it has been found that dolomitic limes which contain Large amounts of magnesia are decidedly inferior t o
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the high calcium limes. For some reason that is not entirely clear, the magnesia tends to offset the normal swelling induced by the lime. I n practice it is usually most satisfactory to procure a high-grade quicklime and slake it a t the plant just prior to use. The iron oxide content of the lime should likewise be low, as otherwise the color imparted by it to the finished glue or gelatin may be objectionable, and bleaching may then be necessary.
T h e Determination of Solids (or LA40isture) by Means of a Gauze Dish' By Armin Seidenberg
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CHEMICAL LABORATORY, DEPARTMEKT OF HEALTH, N E W YORK,N. Y.
The determination of solids or dry substance (moisture by difference) of viscous organic liquids is one of the most frequent operations in chemical investigations. I n spite of its seeming simplicity, it has generally been found impossible to secure strictly quantitative results bg a n y of the methods so f a r proposed. Pumice stone, sand, and other finely divided materials usually used as distributing media, have a tendency, after being heated, slowly to adsorb gases and liquids. The slight traces of adsorbed moisture are held very tenaciously and tend greatly to increase decomposition of solids, so that it is dificult to differentiate between the loss due to evaporation of moisture and that due to decomposition of solids. For this reason it is not possible to secure a definite end-point followed by a correct constant weight that indicates the actual solids present. In order to secure more satisfactory results a dish of corrugated wire gauze has been proposed. T h i s permits the liquids to be dis-
tributed over a wide surface; it also permits uniform dehydration, is unaffected by chemical action, and does not change in weight during cooling or weighing. It has no tendency to retain moisture, and largely for this reason the decomposition of the solids distributed over it has been found to be much less than is the case with pumice or sand. I t is always possible with the gauze dish to secure a sharp end-point followed by a true constant weight that remains constant on prolonged heating and that indicates the correct amount of solids present. A large number of comparative determinations were made on trarious sugar and other solutions between the gauze dish and pumice stone methods, and very considerable differences were noted in the results by the two methods--equal in m a n y instances to 3 per cent and more of the solids present. B y using higher temperatures rapid results (in about h a y a n hour) can be secured on many materials with the gauze dish.
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SUCROSESOLUTIONS RESULTSWITH PUMICE SToivE-Several hundred parallel determinations a t temperatures ranging from 55' to 175' C. were made under exactly the same conditions, by the gauze dish and pumice stone method^,^ on sucrose solutions of varying concentrations. The pumice stone used was subjected to careful preliminary purification; it was used only after standing under atmospheric conditions for several weeks and then dehydrated in each instance in the manner recorded in the table. The pumice, before being weighed, was always placed in a desiccator for exactly 10 min. It was found that pumice, as well as any other material consisting of finely divided particles or containing openings of capillary size, had a tendency to adsorb both gases and liquids, particularly after having been heated. Material in a colloidal state such as this is affected by the conditions of temperature and pressure so that its weight changes, not only during evaporation, but also during cooling and weighing. Considerable evidence will be presented in another paper4 indicating that, although decomposition of organic residues distributed over pumice does not take place in the presence of a large excess of water, it is greatly accelerated by the slight traces of adsorbed moisture held throughout the dehydration by the pumice. Radically different results were secured by the pumice stone method, dependent upon the manner in which the pumice had been dehydrated before being weighed. In those instances (KO. 3, Table I) where it was first heated over the Bunsen flame, results were secured which after a
HE most favorable condition for the thorough removal of the volatile portion of a liquid is a wide and uniform distribution of the liquid over a large surface which will itself remain entirely unaffected throughout the drying operation. In a previous paper2 the writer has described a device, a modification of which was found to meet these conditions t o a marked degree. This device, or gauze dish, is made from a fine mesh wire gauze with an area of 200 sq. cm., corrugated into 31 to 33 lateral ridges and compressed in this way into an area of 8.5 X 5.5 cm. The gauze dish referred to in this paper was made of platinum and rested upon a platinum-gold stand. I n order to protect hygroscopic material it was placed during weighing in a closed dish made of thin, light-weight lead sheets. The drops of any liquid when delivered from a slight height (about 2 cm.) upon a gauze in this form are broken up by the impact of the fall and held entirely as a fine film within the meshes. I n this way the grooves formed by the corrugations exert a capillary action and 5 cc. of any liquid can readily be distributed over the gauze dish without any going through the meshes. Owing to the l u g e surface and to the effect of the meshes and corrugations in maintaining a wide and even distribution of the liquid, dehydration is greatly facilitated. Evaporation of the liquid takes place from both surfaces. After all the water has been removed the solids will be found to be held uniformly within the meshes, forming an integral part of the gauze dish. All dehydrations were carried on in an electric oven, those in parallel being conducted in dishes placed side by side upon a small cardboard tray. 1
Received October 25, 1922.
*THISJOURNAL, 7 (19151, 769.
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Assoc. Official Agr. Chem., Methods, 1920, p. 101. T o be published in J . Assoc. Oficial A g r . Chem., August 15, 1923.
