Agricultural Materials as Renewable Resources - ACS Publications

Chapter 5

Preservation and Tanning of Animal Hides

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 16, 2016 | http://pubs.acs.org Publication Date: September 1, 1996 | doi: 10.1021/bk-1996-0647.ch005

The Making of Leather and Its Investigation by the U.S. Department of Agriculture William N. Marmer Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Wyndmoor, PA 19038

Animal hides are the most valuable co-product of the packing industry. They are a renewable resource of great export value, raw or tanned. Much of the current research in hides and leather researach at the Eastern Regional Research Center (ERRC) addresses environmental issues of the packing and tanning industries. Hides are traditionally preserved in NaCl brine. Research has focused on damage from halophiles and on alternatives to NaCl brine, e.g.,KClbrine or electron-beam irradiation. Pretanning steps, chemical and enzymatic, essentially leave a matrix of collagen-I. New methodology allows assay of bating enzyme activity during one such step. About 90% of all hides are tanned with Cr-III salts, and though the resulting product is not regarded as toxic, the toxicity of Cr-VI species raises public concern over all Cr-containing material. This has spurred a quest for alternatives both to chrome tanning and to disposal of huge amounts of solid, chrome-containing tannery waste. We have developed a molecular model of collagen-I to assist our understanding of the mechanism of chrome tanning and organic crosslinking, and we now can process the solid waste into chrome-free proteinfractionsand recyclable chromium oxide. Thefinalsteps of leather production involve physical and chemicalfinishing.Chemical and physical softening has been studied by acoustic emission. UV-cured finishes were demonstrated as alternatives to solvent-based spray finishes.

It is no surprise that the United States produces substantial numbers of cattle for beef and dairy needs, so it should not be surprising that we also produce substantial numbers of cattle hides. These hides are, in fact, the most valuable co-product of the

This chapter not subject to U.S. copyright Published 1996 American Chemical Society

In Agricultural Materials as Renewable Resources; Fuller, Glenn, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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meat packing industry. The domestic tanning industry only consumes somewhat less than half of our hide production, so hides represent a tremendous export product, worth about two billion dollars annually. The demarcation between packer and tanner is sometimes difficult to distinguish, and there is growing vertical integration; at least one of the three giants in the packing industry now processes some of its hides to what is called bluestock, i.e., hides processed through the first stages of tanning. The USDA's Agricultural Research Service maintains a substantial research interest in the processing of animal hides, and all of its research program on hide utilization is consolidated at its facility outside Philadelphia, the Eastern Regional Research Center (ERRC). Among the assortment of laboratories and testing rooms at ERRC is a very unique facility, housed in its own building, the research tannery. This pilot plant is the only such public facility in the United States; besides supporting the research program, the tannery is used by the industry as a training facility during short courses held every six months. To understand the significance of the ERRC program in leather research, a systematic review is provided of the traditional steps necessary to convert an animal hide intofinishedleather. The steps are outlined in Table I and discussed below: Overall, the hide must be preserved while it is awaiting tanning, and this is done traditionally by soaking the hide in sodium chloride brine (hide curing). The hides are then shipped to the tannery and stored. Leathermaking continues at the tannery, where the hide is trimmed, the salt and blood are removed (soaking), adhering adipose tissue is separated (fleshing), and hair, epidermis and soluble proteins are stripped away (liming and unhairing, using lime and sodium sulfide at pH 12.5, which also hydrolyzes the amide groups of the amino acids glutamine and asparagine). Thick hides, sometimes 8 mm thick or more, are then split into two layers, with the bottom layer sometimes diverted for collagen and gelatin by-products. For leather, lime must be neutralized and removed (deliming), and this is usually done with ammonium sulfate. Next comes an enzymatic treatment called bating, in which proteolytic enzymes remove hair follicles and other noncollagenous materials. The next step on the way to leather is crosslinking; this is accomplished for about 90% of all hides by salts of chromium ΠΙ. The hides are acidified to pH 2.5 (pickling) and treated with the chrome salt (tanning). The chrome isfixedby raising the pH. The hide is now preserved against putrefaction. Its thermal stability has been raised considerably, a necessary change for withstanding subsequent manufacturing steps tofinishedleather and leather goods, and for durability under user applications. The resultant bluestock material now can be called leather, and as noted before, the packing industry is moving toward selling its hides in bluestock form. The bluestock is wrung of excess moisture, trimmed of uneven perimeter areas, and split, with the bottom layer destined for suede and the top layer for top grain leather. Uneven bottom surfaces are shaven, generating tens of thousands of tons of landfill waste annually in the USA alone. Now, this bluestock isfinishedinto the leather the consumer knows. Retanning stabilizes chrome from leaching, imparts softness and body, and in some cases bleaches the blue color imparted by chrome. Coloring is ~ of course ~ dyeing, and fatliquoring conveys softness by lubrication of internal fibers. The grain layer is smoothed and excess liquids are expressed during setting out. Conventional drying


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Table I. The Tanning Process Tanning Step

Explanation

Hide Curing Trimming Soaking Fleshing Liming/Unhairing

Temporary protective treatment for pelts; traditionally by NaCl Removes odd-shaped, unworkable areas of the hide Removes salt, blood; restoration of moisture Removes attached adipose tissue (also done prior to curing) Removes hair, epidermis, some soluble proteins; uses Ca(OH) and NajS Diverts bottom layer into food use (e.g., sausage casings) Removes alkali (using (NH ) S0 ) Removes noncollagenous materials enzymatically Lowers pH (using salt, acid) for reception of tanning chemicals Preserves from putrescence; imparts thermal stability; 90% of hides tanned with salts of Cr-III Removes excess moisture Removes unusable perimeters; generates Cr-containing waste Lower layer destined to suede; top layer to grain leather Adjusts thickness; generates Cr-containing waste (Minerals, vegtans, syntans) Adds body, softness; bleaches color Dyeing (with acid, metallized, direct and basic dyes) Lubricates forflexibility,softness (lipid derivatives; emulsifiers) Removes excess moisture; smoothing of grain surface. Removes all but equilibrium moisture Introduces controlled amounts of moisture for softness Mechanicalflexingfor softness Sanding of grain surface; generates Cr-containing waste Impregnates and coats with polymeric materials for abrasion and stain resistance, color effects, gloss and handle properties 2

Splitting Deliming Bating Pickling Tanning

4

Wringing Trimming Splitting Shaving Retanning Coloring Fatliquoring Setting Out Drying Conditioning Staking Buffing Finishing

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uses hot air, more recently supplemented by microwave or radiofrequency drying. Conditioning makesfinaladjustments to the moisture content. Staking mechanically massages the leather for additional softness. Buffing smooths out any rough surfaces and - like shaving - generates much solid waste destined to landfills. Finally, polymeric imprégnants, undercoats and topcoats are applied during finishing to add body, abrasion and stain resistance, color coats, and gloss (as with patent leather), and to alter tactile properties. Most domestic leather is targeted to the footwear, leathergoods and upholstery industries, and little to the garment industry. An excellent review of the leathermaking process was recently issued in revised form (1).


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KC1 as a Substitute for NaCl in Hide Curing ERRC researchers have given considerable attention to hide preservation, especially because of the tremendous environmental impact of disposing of tons of sodium chloride both at the packing plant in the brine curing step and at the tannery during soaking. Kalium, Ltd., Regina, Saskatchewan, is a Canadian miner of potassium chloride. Representatives from Kalium and a meat packer, Lakeside Packers of Brooks, Alberta, posed the question, "Can we substitute KC1 for NaCl in hide curing?" The added expense would be offset by the utilization of the waste KC1 in fertilizer, thus saving a major cost of disposal of NaCl. But what about the quality of leather achievablefromKCl-cured hides? Under a cooperative agreement, ERRC scientists investigated the fate of these hides through all stages of tanning (2, 3). Under the agreement with Kalium, the effects of KC1 during curing were investigated. At Lakeside Packers, 100 hides were cut along the backbone and one set of matched sides was cured with KC1 and the other set with NaCl. These sets were then chrome tanned to bluestock at Dominion Tanners in Winnipeg. Lakeside also shipped 150 KCl-cured hides and 400 NaCl-cured hides to Teh Chang Tannery in Taiwan to test their fate during long-distance shipping. In the USA, IBP in Joslin, IL, prepared 900 hides in KC1 and 900 in NaCl and supped them to three American tanneries (Blueside in St. Joseph, MO, Garden State in Williamsport, MD, and Pfister & Vogel in Milwaukee, WI). Samplesfromeach set were brought back to ERRC in Philadelphia for evaluation. No discernable differences were found in tensile strength, extensibility, or area yield (Table II): Table Π. KC1 vs NaCl Brine Curing: Mechanical Properties of Resultant Leather

Tensile Strength (psi): Extension at Break (in): Area Yield (ft ) 2

KCl-Cured

Nad-Cured

3078 ±547 0.340 ± .030 21.88 ±1.66

2952 ±587 0.340 ± .040 21.74 ±1.55

Simplistically, one might expect no changes in treatment protocol or results when switchingfromNaCl to KC1, but attention is called to one major difference between the two salts, their solubilities as a function of temperature. The NaCl curve is flat, but KCl's solubility falls with falling temperature. It is important to maintain high salt concentration in the brine, and attention has to be given to the raceway during the winter season. It is also conceivable that under extremely cold conditions the concentration of KC1 brine within cured hides might fall below the minimum concentration for adequate bacterial protection. The approach of substituting sodium with potassium has broader potential than just for brine curing. Sodium-containing agents throughout the tanning process might be substituted due to the ever-increasing environmental concerns of industry.


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Halophilic Bacteria and Curing Brines Although one might think that traditional brine curing relieves concern over bacterial contamination, this is not quite true. Halophiles are a unique type of bacteria that only grow in concentrated salt solutions. The condition labelled "Red Heat" has been known by tanners and salt fish dealers for centuries. Red Heat is now known to be the manifestation of pigmented halophilic bacteria. Tanners associate Red Heat with hide damage even though up to now no one has positively demonstrated a causal relationship. ERRC researchers produced leatherfromexperimental brine-cured hides that were inoculated with halophilic bacteria isolatedfromcommercial brine-cured hides. The samples held at 106°F (41°C) for seven weeks showed serious erosion of the grain surface of the hide. No such damage was seen in samples that had been stored at room temperature (4). Scanning electron microscopy of damaged cross-sections showed an unraveling of thefiberstructure on the grain surface of the hide. Electron Beam Curing of Hides Another approach to hide preservation completely eliminates salt curing. Microbial contamination is destroyed by electron beam irradiation. Although the electron-beam apparatus is just a larger version of the electron gun found in an ordinary TV picture tube, the electrons in this case have much higher energies:from3 to 10 million electronVol.ts.Facilities to house e-beam irradiators are substantial in bulk, protective shielding, and - needless to say - capital cost. Nevertheless, electron beam irradiation is an established industrial process used in many plants all over the world. Most bandages and other soft medical supplies have been electron-beam sterilized inside the package after manufacture. The process is also used to crosslink rubber and various plastics. We have even experimented with electron-beams for curing polymeric finishes on leather. In cooperation with Ionizing Energy Corp. of Canada, ERRC staff investigated the use of electron beam irradiation as an alternative to brine curing for the preservation of cattle hides (3). In the earliest work, sterile packaging was required, using only a small amount of bactericide for safety. Preserved samples that were packaged and treated in 1986 are still on display at ERRC. Although preservation by electron-beam irradiation is good, there is some loss of tensile strength of the resultant leather as the irradiation dose is increased. Using lower doses coupled with refrigeration and bactericidal treatment seems to be the best protocol, and packaging and sealing each hide individually are no longer required. The hides need only to be covered in groups to prevent dehydration. Three- or 10MeV irradiators are employed with either double-passing each side at a dosage of 0.6 Mrad or single-passing at 1.2 Mrad; dosage is regulated by cart speed through the apparatus. The aim is to keep the hides in their "green state" while they are shipped overseas or to domestic tanneries. This method of preservation would be most affordable for a large packing company, and it would be a better investment for the packer were the process also usable for preservation of cuts of meat.


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Colorimetric Assay of Bate Enzymes Bating, the proteolytic process to remove noncollagenous materials, used to be done in manure pits until Rohm and Haas Company cofounder, Otto Rohm, discovered that processed hog pancreases (mixed with wood flour) could effect the same results in a socially more acceptable manner. This material was introduced commercially prior to World War I as Rohm and Haas's veryfirstproduct, Oropon. During hide processing, bate activity should be controlled. The goal is effective removal of undesirable material while avoiding damage to the essential structural proteins of leather. A colorimetric bate assay recently developed at ERRC provides the information needed for such control (5). The substrate for the assay is hide powder azure, a denatured and insoluble collagen to which a blue dye is covalently bonded. Samples of well-dried hide powder azure are stored in tubes under silica gel. The test sample — either bate enzyme or the bath in which hides are undergoing bating — is incubated with the hide powder azure substrate. Proteolysis of the substrate produces solublefragmentsstill bearing bonded blue dye. Since the substrate is insoluble, vigorous mixing is required during incubation. Trichloroacetic acid is added to quench the reaction and filtration is accomplished rapidly through threefilters:a Vacutainer (a coarsefilter)and then medium andfineporosity syringe filters. Absorbance of the solubilized dye is measured at 594 nm. Concentration correlates linearly with the enzymatic activity of the bate. Serendipitously, it was found that ammonium sulfate, which is used in the prior deliming step, inhibits bate activity. The tanning industry is now taking care torinsethe hides thoroughly of ammonium sulfate prior to bating. Another approach to measuring enzymatic activity is under development, whereby the degree of collagen hydrolysis may be determined by viscometry instead of colorimetry. Molecular Modeling of Collagen-I One of the ERRC projects targets tanning and, as discussed below, "untanning." Chrome tanning accounts for 90% of all tanning, and for most purposes there is nothing known to effectively compete with chrome in its role as a hide preservative, as a hide stabilizer to temperature extremes, and as a well-fixed, non-leaching tanning agent. A molecular model of collagen-I, the major constituent of prepared hides, should be a good tool for developing an understanding of the mechanism of chrome crosslinking and the design of alternative crosslinkers. Building a molecular model of collagen-I has been a major undertaking. Collagen-I consists of a triple helix of 1014 amino acid residues per chain of repeating (Gly-X-Y) units, linked to the next helical segment by a telopeptide of 2030 residues. Two of the three chains are identical. Modeling work on the crystalline segment was accomplished using Sybil software from Tripos. Initially, a model of a single helix of glycine-proline-proline (Gly-Pro-Pro) was developed, and then hydroxyproline was substituted for the second proline. Three of these modified helices (Gly-Pro-Hyp) were combined to form a triple helix. Finally, the true amino acid sequence was introduced to produce the Smith model of a collagen microfibril, a bundle offivetriple helices (Figures 1 and 2) (6-8). In Agricultural Materials as Renewable Resources; Fuller, Glenn, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.



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Figure 1. Space-filling molecular model forfragmentsfromthefivetriple helices of Type I Collagen. The gap region is represented by the abbreviated length of the second triple helix. Four classes of sidechains are depictedfromlight to dark shading: structural (Gly, Pro, Hyp), nonpolar (Phe, e.g.) and neutral polar (Ser, e.g.), basic (Lys, Arg, His), and acidic (Glu, Asp). A color representation is depicted in Figure 3 of Reference 6.



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Figure 2. Lateral and cross-sectional views of the space-filling model for a fragment of the Collagen I microfibril constructed from the triple-helical fragments of Figure 1. The gap region is visible at the top of the lateral view. Shading is as described for Figure 1, except that the three polypeptide chains of one triple helix are highlighted in three dark shades to emphasize the path of this fragment across the lateral view of the microfibril ending in the gap region. A color representation is depicted in Figure 2 of Reference 7.


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At the C-terminal end of each collagen chain is a 5 mer repeat of the Gly-ProHyp sequence, which serves to nucleate helix formation. In the microfibril, the Cterminal nucleating region of one triple helix is separatedfromthe N-terminus of the next triple helix by a gap region containing both N - and C- terminal telopeptides. These gaps are seen as dark areas in electron micrographs. The characteristic staining patterns observed by electron microscopy can be visualized on the model by assigning colors based on the ionic character of the amino acid side chain. New regions concentrated in -C0 " appear when amide-bearing residues (Asn, Gin) are hydrolyzed (to Asp and Glu, respectively). This change is thought to facilitate chromium uptake during subsequent chrome tanning. Regions with local concentrations of hydrophobic sites (e.g., Phe) are seen, as are neutral-polar regions (Ser, Thr). We are just beginning to experiment with the molecular model to study crosslinking during tanning of hides. In one example, we studied the spacial requirements of dicarboxylic acid crosslinks, using 6, 8, and 10-carbon straight-chain acids bonded to lysinyl amino groups in the model via salt bridges. Chain-length 6 is too short; C-8 is better; C-10 is best. These results corroborate with chemical experiments, which show a similar pattern in the ability of such binding to raise the thermal stability of the collagen samples.


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Enzymic Digestion of Chrome Shavings The shaving and trimming of bluestock leather generate a huge amount of chromiumcontaining solid waste, most of which ends up in landfills. Chromium in leather is Cr (III), not the toxic Cr (VI), and EPA exempts the industryfromhandling its chromium-containing waste as toxic material. Nevertheless, some local environmental authorities do not make a distinction between the two valence states. The tanning industry challenged us to develop a means of diverting this waste from landfills. Even if this waste is not regarded as toxic, it represents an economic loss of protein and chromium. The magnitude of this waste in this country alone is staggering; almost 60 thousand metric tons per year of bluestock waste are landfilled. Worldwide, this waste is an order of magnitude larger. Half the waste is chrome shavings and the rest is trimmings and buffing dust. (Further, huge amounts offinishedleather also end up in landfills, andfinishedleather waste is another significant challenge.) ERRC researchers discovered a cost-effective means of digesting the bluestock waste with alkali and alkaline protease enzyme; valuable chrome-free protein fractions are recovered, as is a cake of chromium (III) oxide, recyclable back into the tanning process (9, 10). The flow diagram shown in Figure 3 illustrates what now is a patented process (11, 12). The solid waste isfirsttreated under alkaline conditions (magnesium oxide, alone or combined with sodium hydroxide or carbonate, is most effective); this allows the isolation of a valuable high-molecular-weight product (13) - a gelable, çhromefree protein of molecular weight ca. 100 Κ - potentially usable in adhesives, as films, and for encapsulation. The remaining sludge is then treated with an alkaline protease enzyme to recover a protein hydrolysate and a precipitate of chromium (ΠΙ) oxide. The hydrolysate with the enzyme is recycled several times to digest more sludge.



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Gel Extraction Chrome Shavings Alkali Water Surfactant

100% Vary 500% 0.1%

Shake at 70-72°Cl6hr Centrifuge I Protein Solution

I Chrome Sludge

(Filter; store at 4°C)

(Store overnight at RT) Hydrolysis

Chrome Sludge Water* Alkali Surfactant* Shakeat70-72°C/1.5hr Alkali Enzyme* Shake at 70-72°C/3.5 Filter

r Protein Solution (Filter; store at 4 °C)

100% 150% Vary 0.1%

Vary

Agricultural Materials as Renewable Resources - ACS Publications

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