Improved hide quality and rapid unhairing

The quality of hides suffers from their treatment by the packing industry, who at best see it as a byproduct of meat production. This could be averted if the meat packing company were to develop a vertically integrated cattle operation in which the hide is a product in its own right. A key component of that innovative operation is the rapid unhairing of stunned cattle carcases prior to flaying in the slaughterhouse. Since it removes hair-associated manure, dirt and microorganisms, rapid unhairing greatly reduces the cross-contamination of microorganisms on the hide to the meat.

Consequential benefits are the allowance for rapid hide grading, splitting in the raw and reduced chemical usage, hence lowered costs in downstream tanning operations. Under a cooperative agreement, we worked with this company to develop an effective rapid unhairing process. Optimal process conditions were determined to be 6.2% sodium sulfide at 35ºC applied as a pressurised spray (in two applications) to warmed hide piece samples; an additional spray application of the sodium sulfide was used to remove the partially dissolved hair from the samples and subsequent neutralisation of residual sulfide (still on the sample pieces) with 3% hydrogen peroxide was brought about in less than 5 min of total reaction time. The process sufficiently removed hair and hide-associated manure balls, allowing for splitting of green hides.

The process was further developed to incorporate the recycling of the unhairing agent (sodium sulfide) and recovery of removed hair. Analysis of the recycled unhairing solution over 16 cycles showed a linear consumption of ca. 20% sulfide with a linear gain of ca. 1% nitrogen (proportional to dissolved hair protein content). More recently we have targeted the replacement of sulfide by systems that are safer to handle and yet preserve the integrity of the grain layer of the hide. Most successful, in terms of rapidity and extent of unhairing, were several formulations consisting of dilute alkaline peroxides and other substances (including potassium cyanate and lime). Results, including physical tests, are presented for these alternative, oxidative unhairing treatments. These unhairing systems may be adapted for use in the tannery.

Introduction

The unhairing of cattle hides, still associated with the carcase, in the packing plant not only promotes food safety, but also improves hide quality. This presentation reviews the operation of a novel meat packing facility, Future Beef Operations, LLC, (FBO), and its collaboration with the Agricultural Research Service (ARS) in the refinement of a rapid, sodium sulfide-based operation for unhairing of carcases prior to exsanguination and flaying.^1,2 This report additionally introduces research on oxidative unhairing systems as alternatives to sulfide-based operations. Primarily, FBO had planned to produce high quality fresh and pre-cooked beef for its customers. However, an additional goal was to consistently produce split-to-order, high quality, wet-blue stock, and to a lesser extent, collagen splits.

Uniformity in hide quality addresses traits highly desired by tanners and includes maximised area yield and shape, consistent hide thickness between belly, butt, and shoulder, reduction in hide defects (marks, brands, wrinkles), consistent grain thickness and pattern, and elimination of vertical fibre defect.

FBO planned to achieve this consistency via a regimen of integrated technologies including: 1) acceleration of desirable characteristics via selective and cross breeding of cattle utilizing multiple ovulation embryo transfer (MOET) and in-vitro fertilisation; 2) improved husbandry by suppliers/ranchers with an emphasis on details such as the prevention of hide damage due to insects and provision of vitamin supplements; 3) custom-developed computerised tracking with remote sensing capability would be based upon an ear tag system to identify each animal. Progress (health, weight gain, medications etc) of each animal’s development was to be registered from the ranch to the packing plant, where the identification would be duplicated and transferred to the hide.

Essentially, information derived from either the meat or hide quality would be traced to the origin of individual animals. This system of individual identification allows for the feedback of information regarding both positive and negative traits to breeders, geneticists, ranchers, and feedlot operators. This system would also strive to eliminate the practice of branding hides in prime locations (butt, side).

4) animals would be allowed to calm for 6 to 8 hours prior to harvesting, and 5) cattle received for slaughter would have a set fat content and weight range.

FBO planned to maintain this comprehensive programme by incorporating ranchers as shareholders. A cash incentive was to serve as a reward for ranchers that conformed to FBO guidelines in the treatment of their cattle. FBO hoped to increase the generation of A or B quality grade hides (those suitable for the production of full-grain or aniline leather) from the current less than 10% to as high as 70% within the first five years of operation.

The basis for ARS collaborative involvement with FBO was to refine the rapid unhairing of cattle carcases, a process step that follows those listed above. Rapid unhairing allows for grading of hides at a packing plant prior to delivery to the tannery as well as obvious food-safety benefits.

A radical alternative to conventional animal skin/hide unhairing with dilute sulfide salts in drums, rapid unhairing involves the spray application of concentrated sulfide salts to animal hides that achieves timely (less than 5 minutes) removal of hair. Dorstewitz and Heidemann (1977) first demonstrated rapid (46 min total) hair removal in a ‘Rapid Run-Through’ process.

The process exposed the hides to a series of concentrated sodium sulfide, sodium chlorite (for neutralisation of residual, hide-associated sulfide) and water (wash/rinse) sprays in addition to mechanical hair removal apparatus.

The majority of unreacted sulfide was filtered of particulate matter and recycled via acidification followed by reakalinisation.³ Applying this process to the rapid unhairing of hides that are still on stunned/immobile animal’s carcases not only removes hair, but also manure, dirt, and microorganisms that are associated with the hair/hide.

Therefore, potential cross-contamination by dirt or faecal matter-borne bacteria from the hide to the meat during flaying may be greatly reduced.^4,5

Thus, as a food safety initiative, Bowling and Clayton (1992) applied the principles of the Rapid Run-Through process to the rapid (approx 5 min) unhairing of whole cattle carcases. In one illustration of their process, concentrated sodium sulfide, hydrogen peroxide (neutralising agent), and water (rinse) sprays were utilised.⁶

Unfortunately, a drawback to the commercialisation of these rapid unhairing processes is that they may potentially generate large amounts of solid hair and unreacted sulfide diluted with copious amounts of water.

However, a more economical and environmentally benign alternative is to use the sodium sulfide solution as the carcase unhairing (depilatory) agent as performed in a closed system and filtration via vibrating screens for the removal of bulk solids.

Such a process is most conducive for the recovery of partially degraded hair and the further recovery and recycling of sulfide salts extracted from the bulk solids.⁷

An additional benefit of rapid unhairing is that hide quality can be assessed prior to tanning operations. Furthermore, subsequent tanning only needs to be performed on the thin grain layer of the hide, since separation into split layers is now possible following this early stage hair (and manure/mud ball) removal.

As a beneficial result, chemicals and time used for tanning are now minimised since only grain splits can be preferentially processed. It follows, therefore, that this process is ideally suited to a packing plant environment equipped with an integrated tannery.

Nevertheless, rapid unhairing may be developed for application to hides (already flayed) in a tannery; pure tanning operations would also benefit from reduced chemical usage and processing time. Unhaired hides can be readily split, since manure and mud balls are removed with the hair.

Subsequent collagen splits may be used for a variety of applications ranging from fertiliser, pet products and sausage casings to photographic films, cosmetic ingredients and pharmaceutical gel capsules. The grain splits see more rapid conversion into crust leather with at least a third less chemicals used.

The work addresses the collaborative efforts of FBO and ARS on the development, analysis and waste treatment of a sulfide-based, rapid unhairing process intended for animal carcases.⁸

Further, it presents more recent research on oxidative chemical alternatives to sodium sulfide that, for the most part, have demonstrated use for rapid unhairing.

Such alternatives hold promise for more environmentally sound unhairing operations while maintaining the same food safety, conservation of hide quality and lowered tanning chemical usage benefits. These latter oxidative reagents include alkaline calcium peroxide⁹ and alkaline hydrogen peroxide¹⁰ (with or without lime and/or potassium cyanate). Oxidative unhairing conditions and results (including extent of unhairing, grain assessment and physical testing) are presented.

Experimental

Procedures – Rapid Sulfide-Based Unhairing

Fresh hides (or hide pieces) were rapidly unhaired with sodium sulfide using the reaction conditions (concentration, time, temperature etc) reported herein. All unhaired hide pieces were converted to crust leather using the procedure described by Cabeza et al¹² (using Retanning ‘A’) with the exceptions noted by Gehring et al¹³; no sharpening agents were used during reliming unless otherwise indicated. Sulfide determination was performed using titration with ferricyanide.¹⁴ Total nitrogen was determined using the Micro-Kjeldahl method.¹⁵ Rapidly unhaired pigskins (4.5kg total skin weight) were converted to crust leather as previously described.⁸

Procedures—Rapid Oxidative Unhairing, Alkaline Calcium Peroxide

Alkaline calcium peroxide was prepared as follows: 1) sodium hydroxide solution (15% for pilot-scale; 30% for lab-scale) was made by dissolving sodium hydroxide pellets in tap water and allowing the solution to cool to about 50ºC; 2) accounting for purity, sufficient solid calcium peroxide was added to the sodium hydroxide to achieve the desired calcium peroxide concentration (5% for pilot scale; 15% for lab-scale); 3) the alkaline peroxide mixture was adjusted to 45ºC immediately prior to use; and 4) the alkaline peroxide was then either mixed before use for hairburn experiments, or the supernatant resulting from the settling of the solids was employed for hairsave experiments. (The percentages reported in this section are based on hide weight.)

The alkaline calcium peroxide solution was employed in one of three methods: Rapid Oxidative Hairburn (ROHB); Recharged Rapid Oxidative Hairburn (RROHB) and Rapid Oxidative Hairsave (ROHS). All sulfide hairburn control hide pieces were converted to crust leather^12,13 but no sharpening agents were used during reliming unless otherwise indicated. All oxidatively unhaired pieces were converted to crust via the same procedure with the exception of the unhairing step and no sharpening agents were used during reliming unless otherwise indicated.

The rapid unhairing process was studied in the pilot plant tanning facility at ARS’s Eastern Regional Research Center. Of primary importance was the preservation of grain integrity while achieving rapid and thorough hair removal through the optimisation of unhairing agent (sodium sulfide) application parameters (temperature, concentration, spray pressure and exposure time).

Initial experiments served to gauge application conditions (temperature, time, sulfide concentration) required for satisfactory unhairing.

Limits on experimental conditions were also learned, revealing that sulfide solution concentrations of 6.2-12.4% applied at a temperature of >40ºC, caused grain damage/sueding (as observed in resultant crust leather) of hide pieces exposed for >10 min.

As investigations progressed, rapid unhairing solution was delivered to hide pieces using a hand-pump-pressurised spray can. After unhairing with sulfide solutions (30ºC) at different concentrations for varying reaction times, the hide pieces were rinsed with water, drummed with 3% hydrogen peroxide (to neutralise residual sulfide) for ca. 1 min and converted to crust leather in a tanning process that utilised no sharpening agents.

The crust was assessed for remaining hair and the extent of unhairing graded, ranging from ‘Poor’ (thick, visible hair though cleaved/removed to a length of no longer that 0.3-0.6cm) to ‘Marginal’ (patches of cleaved hair remain and/or stubble) to ‘Good’ (stubble only).

None of the pieces tested had any apparent grain damage and dyeing was level with even shading between all pieces. The best unhairing was observed with 9.3% sulfide solution that was reacted with hide pieces for 5 min or longer.

Although all of the tested conditions resulted in unhairing that was suitable to allow for early-stage splitting of any of the green hide pieces, further experimentation revealed that raising the temperature of the sulfide solution 5 degrees allowed for good unhairing with 6.2% sulfide solution.

Under these latter conditions, hair stubble is not generally perceived until the green hides are converted to crust. (However, the stubble could be readily eliminated if sulfide salts were employed during reliming, thus achieving complete unhairing.)

Conclusion

FBO built a meat-packing plant in Arkansas City, KS, with plans to process a full capacity of 1,650 head of cattle per day in one 8-hr shift. The plant housed a full-sized tannery that was in close relationship with the packing plant. Within one hour after the animal was stunned, unhaired hides would be pulled from carcases and rapidly transported via a ‘hide cannon’ from the killing floor to a tannery drum.

This would set a new precedence in the production of fresh hides and quality leather and eliminate the need for brine curing. In initial trials, much of the hair removed from the carcases was intact, separated (along with dirt and silage) from the waste stream and treated with acid (in a closed system) to remove and recover residual sulfide. The resultant hair, expected to be one ton per day at peak plant capacity, may have had additional value-added uses as a biomaterial, but it was destined for land filling.

In combination with the findings of previous work³ the results (unhairing conditions, sulfide depletion, nitrogen build-up, hair sludge separation etc) obtained in the ARS-FBO research collaboration were utilised as factors for scaling up to the commercial-sized unhairing system and sulfide reactor/regenerator that was housed in FBO’s facility.

It was hoped that the benefits of rapid unhairing might be realised by other industries. For example, rapid sulfide-based unhairing was successfully applied to whole porcine carcases. The rapidly unhaired pigskins were observed to yield high quality, full-grain leather. Rapid unhairing appears to be an economical alternative to costly scalding (pig carcase unhairing) operations and demand for full grain porcine leather has been on the rise.

ARS sought to replace the rapid unhairing process with an equally rapid yet sulfide-free unhairing process and demonstrate its utilisation (and similar benefits) in packing and/or tanning facilities. The pilot-scale RROHB study produced almost completely unhaired hides that exhibited no grain damage.

It follows that complete unhairing, achieved in the RROHB lab-scale study, might have occurred if the initial reagent solution temperature of 45ºC had been maintained throughout the course of the pilot-scale reaction. Alternatively, as exhibited by lab-scale trials, complete unhairing should have occurred if sharpening agents had been employed during reliming in the preparation of crust leather using the sides from these pilot-scale trials. (Select sharpening agents were observed to remove hair stubble that remained after lab-scale ROHB thus indicating that immunisation of the hair did not occur.)

It is important to note that this work is preliminary to rapid oxidative unhairing spray application studies. The applications (high temperatures, rapid reaction times of 5-10 min) presented herein probably would not be practical for application in a scaled-up industrial setting.

Proof of concept, at a scaled-up level, will need to be conducted with alkaline calcium peroxide optimally adjusted to pH12.8-13.3 and applied to hides by high-pressure spray in a temperature-regulated environment.

Rapid oxidative unhairing is on par with rapid sulfide-based unhairing in terms of reaction time and extent of unhairing.^8,32 Crust leather, produced using the sulfide-free RROHB unhairing method (lab-scale), had comparable properties (lack of hair, grain appearance, physical testing and hydrothermal stability) in addition to a similar feel, dye shading and dye levelness (data not shown) to crust produced via unhairing using conventional hairburn with sodium sulfide.

Though some hair stubble remained associated with the grain, ROHB-treated hide pieces yielded crust leather very similar to that from RROHB. Rapid oxidative unhairing procedures may find utility in either the meat packing or tanning industries, but sharpening agents may have to be employed during the reliming process to eliminate any residual hair stubble.

If a hairsave process is desired, ROHS may be employed and any residual hair stubble may apparently be eliminated with a subsequent ROHB process.

The processes were all conducted at 45ºC (initial temperature) to promote rapid unhairing in a timeframe suitable for application in a meat packing plant.

No grain damage was observed for the 5 min ROHB or ~7-10 min RROHB processes. Sueding that was observed with the pilot-scale ROHS treated side might have resulted from allowing the highly alkaline unhairing agent to dwell on the side as hair samples were carefully collected.

The oxidative unhairing processes presented are expected to elicit desired benefits of rapid sulfide-based unhairing potentially including food safety, early-on grading of hide quality, splitting-in-the-raw (a neutralisation step, employed immediately following alkaline peroxide unhairing, would first be required) and subsequently lowered chemical usage during tanning of only the grain split, and perhaps the simplified separation of intact hair from the waste stream.

There are several questions that remain from the rapid unhairing study with alkaline hydrogen peroxide. The first is what is the role of lime in the rapid unhairing reaction? The physical test results for leather from the hide sample that had undergone unhairing with NaOH/lime/H₂O₂ were very similar to the results obtained for leather from hides unhaired with alkaline calcium peroxide. At high base concentrations and elevated temperatures it is possible to produce calcium peroxide in solution.^33-35

These conditions are probably met, since the base concentrations for the NaOH/lime/H₂O₂ unhairing mixtures are greater than 20% and the mixing of the reagents is exothermic (>70ºC).

It was observed that the lime in the mixture turned a yellowish brown colour, the same colour as solid calcium peroxide. It is, therefore, likely that calcium peroxide is being generated in the unhairing mixture. The role of KOCN in the unhairing reaction is not known though the disulfide bond may be cleaved by the cyanate ion in a nucleophilic substitution.³⁶

Oxidation of the reaction products to the sulfonic acid would take place immediately after cleavage of the bond. It is also possible that a peracid derived from KOCN/H₂O₂ plays a role in the unhairing reaction. ARS is currently investigating the mechanism of hair removal when KOCN is added to the unhairing reaction.

It appears possible to replace conventional hairburn or rapid unhairing with sodium sulfide with an oxidative unhairing protocol. The use of alkaline hydrogen peroxide yields good leather but amending the unhairing mixture with either lime or KOCN yields better results.

The KOCN unhairing mixture is very attractive as all reagents are water soluble and it is possible to split the green hides after unhairing. This study was carried out in drums and the reaction conditions need to be modified for spray application. The results from these experiments will provide a starting point for the determination of those reaction conditions.

The results presented for the oxidative unhairing alternatives (magnesium peroxide, sodium perborate, sodium percarbonate, and potassium monopersulfate) were poor in terms of damage to grain and diminished tensile properties. However, this research area still holds promise if better control is placed on the regulation of temperature, which is inversely proportional to unhairing solution pH, and timing of such reactions.

Acknowledgments

We thank the following from the Eastern Regional Research Center, Agricultural Research Service, USDA, for their technical support and assistance in this project: Chad Mazenko, Dale Allen, Rocco Caveng, Nicholas Latona, Joseph Lee, Cheng-Kung Liu, Derek Pace, John Phillips and Jiming Yao. In addition, we gratefully acknowledge the participation of and key discussions with Rainer Dorstewitz (Ingenieurburo Dr Dorstewitz, Germany) originator of the ‘Rapid Through-feed Unhairing System’, as well as with Fred Abraham (PPG Industries Inc), Jerry Anderson (CHAD), Douglas Gehring (National Refrigerants Inc), Scott Timmons and Mark Van Dyke (Southwest Research Institute, San Antonio), and Leguin Williams (Vibra-Screen, Phoenix E & P Technology). We are grateful to the following for their critical review of these proceedings: John Ezzell (Dionex Corp), Peter Irwin (ERRC, ARS, USDA), and Dennis Shelly (Leather Research Institute, University of Texas).