Comparing contemporary soaking practices with historical methods20 August 2007
Since the early 1900s, soaking was regarded as an equilibrium process. Practitioners and scientists1,2,3,4 of the time recognized the importance of a thorough soak for the proper rehydration of hides and skins and soaking for two to three days was the norm. During the eighties, new insight provided improvements that promised faster and more effective soaking. In particular, Bienkiewicz5 and Alexander6 et al began to unravel the biochemistry of the non-collagen impurities that inhibit isolation of collagen for tanning. Identification of the glycosaminoglycans that retard the opening up process has led to a more direct approach to removal of these impurities.
According to Bienkiewicz, glycosaminoglycans are acidic and neutral polysaccharides that complex with proteins to yield mucoids. They consist of hyaluronic acid, chondroitin sulfate, chondroitin, dermatan and keratin sulfates and heparin. He reports that there is a variety of glycosaminoglycans (formerly called mucopolysaccharides) present in hide. They are polyelectrolytes whose purpose is to control the viscosity of organic fluids in the extra cellular spaces of the hide and they help manage the flexibility of animal tissue by controlling plumpnes Glycosaminoglycans also affect metabolic processes by regulating the passage of all substances from cell to cell through the extra cellular spaces. Bienkiewicz is quite graphic in his description of the impact of hyaluronic acid, in particular, on the effective soaking of hides and skins. Hyaluronic acid is a long, non-branching polysaccharide chain that is highly hydrophilic. Fully hydrated, the hyaluronic acid molecule occupies nearly twice the volume of the non-hydrated counterpart. The net effect is to impede the migration of water and chemicals through the interfibrillar spaces within the hide. Until this gel-like substance is removed, it inhibits the removal of other non-collagen proteins and slows the opening up process, one of the main objectives of the beamhouse. Alexander et al reported that for adequate opening-up of hide fibre, two major cementing substances (mucopolysaccharides) need to be removed. Strongly acidic dermatan sulfate is the smaller of the two. The other is the very high molecular weight, hyaluronic acid, which does not appear to be bound to the collagen. He showed that hyaluronic acid is completely removed after soaking for 48-hours. Leafe7 recognised the significant place that fresh hides have as a raw material in modern tanning practice. He stated that there used to be a widely held opinion that fresh hides did not require significant soaking, since they were not dehydrated on arrival at the tannery. He further explained that this is inaccurate since the non-structured protein in fresh hides had not been subjected to the degradation that occurs during the storage of salted or brine cured hides. He proposed, in fact, that more intensive soaking is required for fresh hides. To date, nobody has presented a comprehensive review of soaking bovine hides as it relates to the theory of hydration of the hide and preparation of the hide matrix for subsequent leathermaking operations. Several questions beg attention. Most significant is: how does the soaking of fresh cattlehides differ from that of brine cured cattlehides? Similarly, there has been no significant reporting of the effect of soaking on the progress of hyaluronic acid extraction from the hide into the float and its effect on hydration. This paper addresses these issues. Tannery trials Full-scale tannery trials were conducted at Prime Tanning in St Joseph, MO, during March 2007. These trials were conducted according to a generic soaking protocol, except the chemical additions were varied. The generic procedure is to soak fresh hides in drums with 0.25% of an ethoxylated alcohol surfactant, 0.67% soda ash and 0.36% of a proteolytic enzyme for five hours in a 110% float at a temperature of 29°C. Chemicals were offered in some of the trials at normal dosages, while some trials were run with elevated dosages of soda ash, enzyme. For example, soda ash was increased to 1.25% and the enzyme was increased to 0.54% in several of the trials. In addition, some drums were run with specific chemicals omitted. The same soaking protocol for both fresh and cured hides was used, except that for cured hides the addition of surfactant to 0.40% and soda ash to 1.60% was increased. In addition to these levels, several trials were run with chemical combinations that included increased soda ash at 2.40% and increased enzyme at the 0.54% level. Again, select trials were run with specific chemicals omitted from the soak. For most of the fresh hide and cured hide trials the floats were sampled at T = 5-minutes and T = 5-hours, the normal soak time. For each of the fresh hide and cured hide trials, four float samples were taken, one each at intervals: 5-minutes, 1-hour, 2-hours and 5-hours. All of the float samples were analysed on site for temperature, pH and Baume and aliquots were chilled and sent to Buckman Laboratories Inc in Memphis for further testing. Results Various measures were used to track the extent of soaking. Among these, percent weight gain is one of the most frequently used indicators. Laboratory trials corroborate the most commonly utilised soak time in the US of 4-6 hours as the optimum when weight increase is the objective. The percent weight gain for cured hides steadily increased until it peaked at four hours. After four hours, weights of four of the five cured hide pieces declined significantly. Only the piece treated with enzyme alone failed to decline in weight. The results indicate that adding soda ash or enzyme by itself increased the rate of water absorption. The greatest weight gain was 45%, recorded when all three of the chemicals used in this study were offered at the same time (Trial 10). Equally interesting, the addition of a nonionic surfactant by itself, inhibited water uptake, compared to the control with no chemicals added. In addition to the results for the five cured hide trials, there was a weight increase for the single fresh hide piece that was soaked for a full 24-hours. Trial 5 which utilised all three chemicals had the highest degree of water absorption of any of the five fresh hide pieces. For this trial, the weight was nearly stable after only about one hour, with a weight gain of 14%. The percent weight gain for Trial 5 was significantly less than for any of the cured hide pieces. There was a vast difference in the float density for cured versus fresh hides. For cured hides, the optimal soak time, as measured by maximum float density, is eight hours or more, depending on the chemistry used. In contrast the float density increase for fresh hides comes at only four hours and thereafter significantly declines. Tannery results for soak liquor density resemble those seen in the laboratory and soaking of cured hides resulted in significantly higher float densities than for fresh hides. There does not appear to be a correlation with the chemicals added. Looking at the influence of soak time in the tannery trials, it was found that for the fresh hide trial incorporating all three of the chemicals under consideration at the normal tannery levels, the density quickly stabilised after one hour at 3° Baume. This density was maintained to the end of the 5- hour soak. On the other hand, even after five-hours soaking, the soak density for the cured hide trial with the same chemical additions was still increasing. The mass of hyaluronic acid extracted from 200g fresh or cured hide pieces during the laboratory soaking trials were calibrated. The maximum theoretical extraction, based on the report of Alexander et al is calculated at 192mg. For a cured hide piece soaked with all three of the subject chemicals added, the total hyaluronic acid extracted measured 93mg and 144mg after 8-hours and 24-hours of soaking, respectively. This represents just less than 50% and approximately 75%, respectively, of the total theoretical HA present in untreated hide substrate. Plotting HA-Extraction versus Soak Time reveals a logarithmic relationship that is characteristic of extractions in general. In contrast, the soaking of fresh hides yields very little hyaluronic acid in the float. Even with the full array of chemicals offered in these trials, the hyaluronic acid extraction after the full 24-hours was only 20.5mg, or less than 11% of the theoretical amount. The effect of chemical treatment for the laboratory soaking shows that all trials on cured hides yielded significantly more hyaluronic acid in the float after 4-hours soaking, than when fresh hide pieces were soaked. For fresh hide pieces, it appears that soda ash and proteolytic enzyme positively influence HA removal. The addition of surfactant, on the other hand, actually appears to be counterproductive. For cured hide pieces, each of the chemicals offered had a positive effect on HA removal. Addition of all three chemicals together yielded the best results for cured hides, with a total of 58mg/l HA detected in the float after 24-hours. The effect of chemical additions on HA extraction during the tannery trials showed that at equilibrium, complete extraction of the hyaluronic acid would yield approximately 500 mg/l in the float. In the tannery trials, maximum HA extraction of 147 mg/l occurred for cured hides when a proteolytic enzyme was used by itself at the normal dosage of 0.36%. This is less than 30% of the theoretical HA value. An increased dosage of enzyme to 0.54% by itself, also resulted in very substantial HA removal. When a full array of chemicals was added to the soak, the resulting HA extraction was 134mg/l. In contrast, the highest HA extraction measured for fresh hides was only 66mg/l, about 13% of theoretical. In addition, when no chemicals were used or when surfactant or soda ash were used by themselves, the HA levels in the float were markedly lower for both fresh and cured hides. Discussion Opinions regarding the optimum conditions for soaking cattlehides have changed and modern soaking practices differ widely from those reported in textbooks still in use in the industry. With better understanding of the chemistry of hide protein and the ancillary compounds surrounding collagen, new theories challenge the very objectives of soaking. In particular, removal of hyaluronic acid as an essential component of the soaking sub-process is now regarded as requisite to the ultimate success of the opening up of the hide structure. There has been a significant shift in the art of soaking towards the use of chemical adjuncts such as surfactants and proteolytic enzymes; both are widespread practices. Soda ash remains an essential component. Until now though, there has been little reported on the effect of these changes in soaking practice on even the most basic measures of soaking efficiency. Depending upon the measures one selects for monitoring the progress of the soak, different conclusions must be drawn regarding the relative effectiveness of the soak. If, for example, it is accepted that relative weight increase due to water absorption is the most important metric in soaking, then it must be concluded that the optimal soak time for cured hides is around four hours. On the other hand, the soaking data indicate that when soaking fresh hides, if percentage weight gain is the principal gauge of soaking progress, then there is little difference between soaking for one hour and soaking for 24-hours. Contrast the conclusions drawn when weight increase is the principle measure of soaking, versus using one of the measures of float density. When float density is the primary measure, it was concluded that optimal soaking occurs around eight hours for cured hides and four hours for fresh hides. If chloride is measured instead of density, the same conclusion results for cured hides. If, however, the extraction of hyaluronic acid from the hide matrix is considered a significant measure of soaking efficiency, then it must be concluded that current practices for soaking fresh hides are far from adequate. Within the physical constraints of float, time, temperature and mechanical action utilised in these trials and regardless of the chemistry currently employed in the industry, a small fraction of the hyaluronic acid present in cattlehide is in fact extracted from the hide into the soaking float. In the case of cured hides, the 'removal' of hyaluronic acid from the hide matrix varies widely. Clearly, curing itself plays a major role in HA extraction. Similarly, application of specific chemicals in soaking, namely soda ash and proteolytic enzymes, contribute to increased HA removal. On the other hand, some applications of surfactants may in fact inhibit the removal of this important glycosaminoglycan. Time is an important variable for HA removal. Under the conditions explored in our laboratory trials, soaking times in excess of 24 hours are needed to approach 100% removal of HA. While tannery conditions promoted improved HA removal, observed extraction values were well short of theoretical 100% removal values. The measurement of hyaluronic acid in the soaking float advances understanding of the mechanism of effective soaking. The data presented here support the theory that hyaluronic acid is impeding the penetration of water into the hide matrix during the soak. In particular, where hyaluronic acid removal was highest, water absorption peaked early, followed by a significant reduction in free moisture in the hide. Subsequent to this reversal in the weight gain versus time slope, the salt concentration in the float declines, further indicating the release of free moisture by the hide into the float. These results agree with the observations made by McLaughlin8 back in 1923. Conclusions The effect of several parameters on the overall efficiency of soaking as measured by the traditional measures of hide weight increase and float density were evaluated. According to those old metrics, some soaking practices currently regarded as state-of-the-art make sense. In particular, soaking either fresh or cured hides for 4-6 hours in 100% float at 20-30°C, with chemical augmentation (a combination of a nonionic surfactant, soda ash and sometimes a proteolytic enzyme), can be expected to maximise water absorption. These soaking conditions appear to maximise the float density for fresh hides. However, these same conditions appear inadequate for the complete removal of salt (to an equilibrium state) from cured hides; eight hours or more may be necessary. When one considers a more contemporary model of beaming as requiring the removal of hyaluronic acid to enable subsequent wetting and opening up of the hide matrix, entirely different conclusions are drawn. The soaking of fresh hides, in particular, under commonly encountered soaking conditions is woefully inadequate. With the removal of 13% or less of the available hyaluronic acid, it is unlikely that the fibre network in fresh hides undergoes any meaningful beneficial change. Similarly, it appears that for cured hides, while hyaluronic acid removal is markedly better than for fresh hides, it is far from optimal. Consequently, it is likely that a significant amount of time is spent during liming to remove hyaluronic acid as the requisite first step in opening up of the collagen. These results compare favourably with those found during an earlier soaking process evaluation conducted by the tannery. In that earlier evaluation, the glycosaminoglycan contents of the hides were measured before and after soaking. The percent removals of glycosaminoglycan (mainly hyaluronic acid) were quite similar to those calculated from soaking float extraction values reported here. The authors believe that their observations with respect to modern soaking practices will encourage leather practitioners and scientists alike to take a fresh look at the soaking sub-process. They are hopeful that the reevaluation and restatement of the basic objectives of soaking will lead to improvements in overall soaking effectiveness with concomitant improvements in overall leather processing. In particular, additional work is necessary to further identify those chemistries and practices that will maximise hyaluronic acid removal. It will also be important to relate HA removal to finished leather quality.