Mechanisms involved in the formation of fatty spues3 May 2001
Fatty spue is one of the defects that periodically appears on leathers and leathergoods. It is well known that the fatty spue is caused by free fatty acids, mainly palmitic and stearic, which have a high melting temperature. This can cause their precipitation onto the leather surface where they form a grey whitish patina. There are different opinions about the formation mechanism of the spues, although everybody agrees on the differences in the displacement of different fat components into the leather. Papers on the subject agree that the fatty acids and their esters initially form as a dissolved material in the liquid phase, with the liquidic oil acting as a solvent at room temperature. They come from natural fats and from fatliquors added in the process. This solution becomes oversaturated due to a high concentration of fatty acids and esters and/or to a fall in the room temperature. As a consequence the fats migrate to the outer surfaces of the leather and crystallisation of the fatty acids and/or glycerides occurs. The concentration of the acids and esters is due to a differential solubility of the components in the leather. The liquid oils are either partially or totally reabsorbed in the fibre network while the crystallised fatty acids and glycerides remain on the surface. In spite of the spue being an unsightly defect that is frequently found on leather and leathergoods, studies investigating the possible reasons of their occurrence were seemingly last carried out 40 years ago, without any instrumental techniques to help, and the authors were unable find any more recent papers. In the present study a contribution to a possible explanation of the phenomenon is proposed using High Resolution Gas Chromatography (HRGC). Different leathers, ovine and bovine, showing the defect were examined. Experimental Materials and methods The following leather samples have been used: * crust garment sheepskin * finished garment sheepskin * sheepskin gloving leather * finished bovine split The apparatus used was a Dani gas-chromatograph model 86.10 HT, with PTV (Programmed Temperature Vaporisator) injector and FID detector, or a PE gas chromatograph model autosystem XL with PSS (programmed Split-Splitless) injector and FID Detector Method The grey-whitish patina was removed by soft rubbing with a cotton plug, previously degreased by soaking with ethoxyethane. The plug was then washed with ether and the liquid injected into the gas-chromatograph and analysed for triglycerides. The same ether extract was also subjected to a derivitisation reaction with diazomethane to produce the methyl esters of the fatty acids. The resulting solution was analysed by a capillary column set up for fatty acid methyl esters (fame). The transformation of the free fatty acids to the respective esters makes for easier analysis of the fatty acids using a FAME dedicated column: moreover the sensitivity to the FID detector is increased in such a way it is possible to detect fatty acids in trace amounts. A control was carried out by rubbing an area where the spue was not present to verify if different components other than the fatty spue had been removed. Composition The total fatty matter was extracted with dichloromethane according to IUC4 - ISO 4048. The extract was treated with Trichloroacetic acid and hexane to separate the simple lipid fraction. The fraction of non-polar lipids contains mainly triglycerides and was analysed for triglyceride content after transesterification with KOH (2M) in methanol to produce the methyl esters of the fatty acids. Results and discussion figures 1 to 6 show the gas-chromatograms obtained. Figure 1 shows that, from the qualitative point of the view, the spue contains palmitic and stearic acids in more or less equal amounts. The same extract having undergone methylation by diazomethane and then analysed by fame, figure 2, shows that unsaturated fatty acids are also present though only in traces. Traces of palmitoleic acid (C16:1)), oleic (C18:1), linoleic (C18:2) and linolenic (C18:3) acids were found. The ratio between palmitic and stearic acid was about 1. This means that the displacement of the fat to the surface of the leather concerns unsaturated as well as saturated acids. In figures 3 and 4 the chromatograms are shown of the fatty acids found in the lipid extract of the ovine leathers. They show that the two compositions are the same and fall in the formal composition of the natural oils and fats. In figures 5 and 6 the chromatograms are reported for the glove leather and for the finished bovine split. In this case, as with the previous chromatograms, the distribution of the fats is representative of natural oils and fats. It has to be underlined that in general the usual distribution of the natural fat shows a palmitic acid content higher than the stearic, the palmitic/stearic ratio being normally 2-3. The lipid extract of these compositions is shown in figure 7. The triglycerides found include compounds with a total number of carbon atoms between 46 and 54, as is usual in natural fats and oils. In figure 7 the peaks with retention times of around 2-4 min agree with low boiling compounds. This fraction is due to hydrolysis of the triglycerides and includes, not well distinguished in these chromatographic conditions, the free fatty acids. In particular, for the gloving leather, the hydrolysis was so high that only traces of triglycerids were found. To analyse this light fraction of the lipid extract, the same sample has been analysed by a column for triglycerides using a programme starting from a lower temperature. It is clear that in all the examined samples the free acid content is due solely to the palmitic and stearic acids but with a ratio of C16 /C18 of less than 1, ie the stearic content is higher than the palmitic. In the glove leather, free acid from unsaturated acids was not found. Nevertheless, as said before, the fatty spue shows a palmitic/stearic ratio of around one. This is contrary to the ratio found in the triglyceridic fraction (Ž 2). It has to be underlined that the normal distribution of the fatty acids, as reported in figures 3,4,5 and 6, shows a high content of oleic acid, this can be considered as mainly responsible for the free acid in the natural fats and oils. It is, therefore, clear that a modification happened in the fatty composition concerning the double bond of the oleic acid that underwent an 'hydrogenation' process so producing its saturated homologous stearic acid. The same consideration can be considered for linoleic acid, which is quite common in the triglyceride and is not found in the free acid fraction of the lipid extract from the leathers. The transformation of unsaturated acids in the saturated homologue is supposedly due to many effects: * the high content of the stearic acid against the content of palmitic acid in the fat * the presence in the spue of the same amounts of palmitic and stearic acid * the total lack of unsaturated acids, found neither in the spue nor in the fatty part as free acids The only possible explanation of these experimental finds is a biological or enzymatic mechanism for the transformation from unsaturated to saturated fats. Conclusion Further indirect evidence of the biological nature of the phenomenon can be the seasonal occurrence of the defect which appears often when the enviornmental conditions promote mould or bacteria growth, sometimes after many days or months from the actual leather production date. Moreover, the scientific bibliography reports that fatty acids can undergo an hydrogenation process, thanks to some microorganisms, in the same way as hydrogenation in industrial processes in the presence of proper catalysts. It is possible say that fatty spue cannot be avoided by only controlling the acidity of the fatliquoring reagents used in the process. The main risk for this defect is the high degree of hydrolysis of the triglycerides. The hypothesis of a biological mechanism for the fatty spue is based on the fact that in the leathers we studied it was found that the palmitic and stearic acids were in the ratio of about 1, which is contrary to the ratio normally found in the lipid extract of the leathers where the stearic acid is very much higher than the palmitic. This higher amount can only be explained by an hydrogenation of the double bond in unsatured fatty acids like oleic through an enzymatic mechanism. Further studies are being carried out to analyse the change in fatty matter in the pelts throughout the leathermaking processes.