Innovations in Processing

Summary
The parts of the leather making process that cause tamers the most difficulty, with regard to perceived environmental impact, are unhairing and chrome tanning. The former contributes most to chemical oxygen demand and suspended solids in the effluent and the latter raises issues of the effects of heavy metals in effluent. This paper contains a number of innovations for processing, developed at University College Northampton, which address these problems. A new technology of enzymatic soaking, in which components of dung are targeted, produces clean hides prior to unhairing and liming. This allows a new approach to unhairing, in which the hair is shaved before chemical unhairing is initiated. Studies of the chemistry and biochemistry of hair reactivity after shaving have shown that effluent parameters can be improved, even using hair burning techniques. At the same time, higher grain quality is obtained, matching the advantage of hair save processes, in terms of empty follicles. In effect, shaving the hair provides a hair burning mechanism which gives the same result as hair saving. Complementary studies of the chemistry of liming reveal that little damage occurs to collagen in the early stages ofliming, providing guidelines for the use of enzymes in opening up of hide structure. A new test, based on the biuret reaction can reveal the depth in the hide at which proteolytic enzymes are operating. Studies in tanning science have resulted in a new theory of tanning being constructed. This allows predictions to be made regarding the effects of any tanning chemistry and points the way to new processes, based upon organic chemistry, that will yield high hydrothermal stability leather, to match the effects of chromium(III).

Introduction
Worldwide concern over the environmental impact of industry has resulted in tamers coming under pressure to minimise elements of their effluents. Hence, there has been acute interest in all aspects of cleaner technology, which might reduce the contributions of conventional processes to the mixed tannery effiuent'. In the list of priorities for improvement in the waste output of typical tanneries, highest are the byproducts from hair removal, the chemical or biochemical oxygen demand and the suspended solids, resulting from hair dissolving processes. Somewhere near the top of the list is the chromium(III) content of the mixed effluent.

Chemical oxygen demand is toxic to aquatic creatures by eutrophication. Since a substantial proportion of native hide or skin is non collagenic and hence must be removed during the beamhouse processing, leather production does yield considerable quantities of organic matter. The biggest contributor to this waste is degraded keratin, produced by dissolving the hair in the typical hair burning process using lime/sulphide2. The process is popular because it is ideally suited to closed drum operation; it is practically foolproof, to add the right chemical offers and let the drum run for about 18 hrs, to obtain acceptable grain quality. The alternative processes for hair saving, which would reduce the discharged keratin loading, require more monitoring and control, making them less simple to operate and therefore less attractive. The benefits of hair saving go beyond merely reducing the chemical oxygen demand in the effluent; the removal of the whole hair shaft by detaching the root fiom the follicle, produces a much cleaner grain, free of scud, giving clearer dyeings. The ideal for tamers would be a process which has the effects of a hair saving process, but which is based on hair burning, so that current chemistry does not have to change.

The trend in leather making science' is to move away from chemical processes, towards biochemical processes. A good example is the application of enzymes to the soaking process, to assist in the rehydration of the pelt, to initiate the opening up reactions and to solubilise the dung. The rehydration and opening up are helped by the presence ofproteases and, to a lesser extent, lipases, but neither they nor the often used amylases have any appreciable effect on dmg. In order to facilitate operations on the pelt prior to liming, it is necessary to have complete removal of dung and this is only biochemically achievable by using enzymes specifically designed for the purpose.

The use of enzymes in leather making has a down side; although these agents can be highly specific in their reaction, there is always the possibility of accompanying effects that are unwanted. Hence, it is important to know the extent to which these additional effects can occur and under what conditions they become a nuisance. In the case of soaking, the use of proteolytic enzymes can lead to excessive opening up during prolonged rehydration, so precise control over this aspect is needed, although often it is ignored. The use of proteolytic enzymes in unhairing is becoming more common and, in the same way, it is critical to understand how these enzymes can react with the collagen, since this too can result in damage to the pelt. Clearly, the enzyme reactions in these earlier beamhouse steps may have implications for the conditions under which bating is conducted. Therefore, it is necessary to be able to monitor the progress of the enzyme activity, at least to determine where, within the cross section they are operating.

The traditional chromium(III) tanning process is constantly under threat from the pressures%s of legislation, so it is prudent to consider alternative tanning methods. The immediate problem is: where might a practical alternative come from? The options are numerous: mineral tannages andorganic tannages, based on vegetable tannins or synthetic materials, are well known in the industry 5,6. However, the problems begin with the necessity of matching the hydrothermal stability conferred by chromium(III), which is a practical requirement for many applications of the leather. For many years, the only viable tamage that could reach the high shrinkage temperatures achievable by chromium(III) was semi metal tannage; this is retanning hydrolysable plant polyphenols with metal salts, preferably aluminium(III)', But these tamages do not solve the perceived problem of incorporating metals into the leather.

Recently it has been demonstrated that high stability can be achieved by both condensed vegetable tannins and synthetic resins, by adopting the right crosslinking chemistriess. Moreover, an analysis of tanning reactions, together with a thermodynamic approach to understanding hydrothermal stability, has led to a new theory of tanning being developedg. This is a practical theory, because applying the principles of tanning chemistry enables predictions of the effects of any tanning reaction on the shrinkage temperature of the leather produced by that tanning reaction. This paper is a not a review of the recent leather literature. It is a review of the innovations in leather science and technology that have been generated within the research group of the British School of Leather Technology and through collaboration with other researchers in UK; it incorporates the doctoral study programme, as well as the researches of the students on the Bachelor and Master degrees in Leather Technology.

Soaking
In typical processing, the function of soaking is to rehydrate the rawstock. This usually means the solubilisation and removal of salt from cured stock, to prepare for hair burning and liming. An accompanying effect is the cleansing of the rawstock of blood, dirt and dung. These reactions are often aided by the presence of nonionic detergent (preferred to the more substantive anionic detergents). It is increasingly common for tamers to include enzymes at this stage; enzymes available for this purpose are: proteases (active towards non structural proteins), lipases (active towards triglyceride grease) and amylases (active towards starch). These biological agents can be useful in assisting the rehydration of the skin structure and in initiating the opening up process. However, none of these soaking auxiliaries are effective at removing dung. Dried on dung can be solubilised by just warm water and surfactants may hasten the effect, but the reaction is often too slow to be completed within the usual soaking period. The available soaking enzymes are inactive in this regard.

Once dung has dried on the hide, the composite material is particularly strong and resistant to decomposition. Indeed, mixtures of dung and hair are traditional building materials, for example in wattle and daub. Therefore, it is common for dung to remain attached to the hide after soaking, to be removed along with the hair during hair burning. Such a situation is not always a problem in conventional processing, but there are two drawbacks: dung that is hard to remove may encourage prolonged soaking, with consequent risk of bacterial damage to the grain, and adhering dung precludes green fleshing, which yields a higher value byproduct than lime fleshing. Hence, there are sound economic reasons for introducing an effectilR dung removal procedure into the soaking step. An analysis of dung3·'0 summarised in Table 1, demonstrates why conventional soaking often does not solubilise it; the usual soaking enzymes do not target the major components.

The specificity of the reaction between enzymes and dung as a substrate is illustrated in Table 2, showing the degree of dung removal. It can be seen that those enzymes which do not attack the specific components of the dung are ineffectual, in comparison to the targeting enzymes. Note, there are two controls, to allow comparison of enzymes operating at their pH optima.

The enzyme systems are extended in Table 3, by using mixtures of the effective enzymes, cellulase and xylanase, together with ligninase, to obtain a synergistic effect. The mixtures are more effective because they are directed at the material which constitutes the cell wall of grass: this is cellulose fibres, to which hemicellulose is attached, with lignin fibres wrapped around.

It is frequently the case that enzymes are adversely affected by solution conditions, including dissolved electrolyte. Therefore, there might be a concern that these active enzymes would be deactivated by the high concentration of salt likely to be encountered in soak liquors. However, Fig.1 demonstrates that the activity is actually enhanced by salt.

This new enzyme technology is achievable using not only pure enzymes, but also with mixtures produced in a growing medium by appropriate fungal species. Indeed, the precise nature of the mixture can be controlled by manipulating the growing conditions. Current studies of appropriate organisms and their effects on the biochemistry of dung decomposition will make this a practical optionfor tanners in the near future '

Unhairing
The arguments for introducing hair saving processes into tanneries have been well rehearsed, but a factor that has not been addressed is the effect on the economics of dung contamination in the hair. Since the dung adheres to the hair and not to the skin, dung which remains on the hide after soaking will be detached by the unhairing procedure. Dung will reduce the value of the hair for many added value applications; for that reason alone, removing dung during soaking is a good thing. In addition, it opens up the possibility of operating on the hide surfaces, before chemical unhairing. The obvious consequences are to make·green fleshing a practical option, thereby producing more valuable fleshings than are Currently obtained from lime fleshing, and to open up the possibility of green splitting, which would allow savings in all beamhouse chemicals. A less obvious consequence is the opportunity to shave the hair; this has not been attempted on bovine hide before, but it is not unknown in the field of sheepskin fellmongering.

Mechanical shaving
Equipment for shaving bovine hide has not been developed, but the principle is clear. So, in. these experiments , hide was shaved by hand,using conventional electric clippers. The immediate effects on the environmental impact of unhairing is illustrated in Fig. 2, showing how the chemical oxygen demand in the unhairing liquor changes during the hair burning period; Fig. 3 shows the p'oduction of total suspended solids during the same time.

At the end of hair burning, after 6 hrs reaction, COD is reduced to 40% and TSS is reduced to 25% of the normal values, respectively. Table 4 contains the COD and TSS results obtained at the end of 18 hrs, that is the total effects of both unhairing by hair burning and opening up by alkali. The final effect is to halve the waste produced by this process step.

Clearly, the precise quantitative effects of shaving depend on the closeness of the shave to the epidermis, but even this simplistic approach has a marked benefit. Examination of the grain surface after unhairing revealed that shaving also produces benefits in terms of grain cleanliness, shown in Table 4.

It can be seen from Table 5, that one effect of shaving is a reduction in the requirement for sulphide, with further savings if enzyme assistance is employed.

The mechanism of hair burning
One of the consequences of cutting the hair before beginning the chemical unhairing process is that the cut presents an additional surface at which chemical reactions can take place. This means that the mechanism of unhairing may be different, because degradation can occur inside the hair. Experiments were conducted on hairs which were either immunized or not and were either cut or intact. The effects of hair burning conditions, using lime/sulphide with or without trypsin (serine protease) or NUE, on different parts of the hair were noted: Table 6 refers to the hair root and Table 7 refers to the hair staple.

At the base of the hair, the most vulnerable site of attack by unhairing agents is the prekeratinised zone. The bulb is notably resistant to degradation by sulphide, but is easily degraded by proteolytic enzymes. Not surprisingly, this area is unaffected by cutting through the hair cross section and, since the stability is independent of the formation of disulphide cystine bonds, the immunization conditions do not have any effect on reactivity.

These results reflect the relative reactivities of the hair staple with respect to unhairing agents. When the hair is uncut, the burning reaction applies to the outer layers of the shaft, that is the cuticle and the cortex; the reaction is dependent on the presence of sulphide and is markedly affected by immunization. When the hair is cut, the reactivity of the medulla is revealed: it is unaffected by sulphide, largely unaffected by immunization, but is highly vulnerable to enzyme attack. This effect is illustrated in Fig. 4.

These studies have thrown some light on the mechanisms of hair degradation: more importantly, they have demonstrated an improvement to the hair burning process, in the presence of proteolytic enzymes. The effect of baring the medulla surface allows the enzymes to penetrate down the inside of the shortened hair shaft, to the prekeratinised zone and the bulb. In this way, the whole hair can be removed, thereby conferring to the conventional hair burning reaction the benefits of hair saving processes. Furthermore, enzyme attack inside the follicle, on the inner root sheath allows access to the outer root sheath, causing removal of this element of the epidermal layer. The consequence is a cleaner grain, with all that implies for aniline quality.  

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