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Rapid authentication of leather and leather products.

Introduction

Since ancient times, human beings have used animal skins and learned to make leather. Leather is animal skin that has been chemically modified to produce a strong, flexible material that resists decay. Driven by its wide applications in everyday life, the demand for leather products has become increasing from time to time (Shaw, 2000). The term hide is used to designate the skin of larger animals, for example cow, horse and buffalo, whereas skin refers to that of smaller animals such as goat, sheep, pig and dog.

Islam is a way of life, comprehensive religion guiding the lives of its followers through sets of rules governing the personal, social and public aspects. Driven by the fast-growing Muslim population, halal products are headed toward wider popularity. Thus, this depiction also shows that the needs for halal products in the future will also be increasing. In Islam, and may also be other religions dressing is also considered as a matter of worshiping God (ibadah). Thus, Muslims are supposed to make an effort to obtain good quality in ibadah.

It has been reported that some manufacturers choose to use pigskin as a main component in leather products. There are some premises retailing the non-halal leather products without proper labelling (Razlan, 2007). In Malaysia and other Islamic countries, this practice is considered as unethical since halal products are very sensitive issue among the Muslims. In fact, it is amazing that many Muslims do not know if they are actually using a leather product that is made from non-halal source such as pigskin leather.

Based on this concern, there is a need to develop rapid analytical techniques for the determination of leather from non-halal animals such as pig since insufficient of scientific studies and database regarding of this issue. There is the need for the development of a more rapid, accurate, efficient method for the detection of such animal species for leather product in order to protect the consumers from doubtful, fraud and adulteration.

Fourier Transform Infrared has a high energy throughput, excellent reproducibility and accuracy from the laser source. With the increasing use of computers, FTIR can easily manipulate spectral information, and its advance chemometric software is equipped to handle the calibration (Bell, 1974). It represents an important tool used for quality control and monitoring process in the food industry because it is less expensive; better in performance and easier to use than other method (Alexander and Bell, 1972). This technology also offers a fast and non-destructive alternative to chemical measurement techniques for qualitative characterization and quantitative measurements, (Bell, 1974).

This FTIR analysis was exclusively conducted to determine the types of functional groups present in leathers molecule. The functional group is represented by the peak obtained after the analysis. Since the absorption is based on the vibration mode of atoms and very specific, therefore each peak at different wavenumber represents only to specific functional group (Alexander and Bell, 1972). An average of 32 scans was carried out in order to determine the peaks which are significant for the statistical analysis, automatically done using the software provided.

The analysis was done in mid-infrared (mid-IR) region, where the wavenumber range between 4000 [cm.sup.-1] to 400 [cm.sup.-1]. Most IR application employs the mid-IR, but the near-IR (14285[cm.sup.-1] to 4000[cm.sup.-1]) and far- IR (400[cm.sup.-1] to 0 [cm.sup.-1]) regions can also provide information about certain materials, for example lattice vibrations (Beer, 1992). However, the majority of instruments are set up to scan only the mid-IR range (Stuart, 1996).

The objectives of this study is to differentiate pigskin leather and other leather products using FTIR spectroscopy complemented with Scanning Electron Microscope (SEM), identify the characteristics of pigskin leather as a source of non-halal leather products and authenticate natural leather and non-leather products (leather authentication).

Materials and Methods

Samples and Supplies:

Four types of leather were used in this study: cowhide, goatskin, sheepskin and pigskin. All leather samples were collected from Kulitkraft Sdn. Bhd. and Department of Chemistry Malaysia.

Physical Observation and Scanning Electron Microscopy:

The physical appearances of different types of leather were determined through physical observations and Scanning Electron Microscopy. The leather sample was cleaned with acetone. The sputter coater machine that was used is Polaron SC7640. SEM was carried out using a Scanning Electron Microscope Leica Stereoscan 440 (Leica Cambridge Ltd.) with X-Ray Analysis Version 3 and Link ISIS windows-based software. This SEM model was developed with Energy Dispersive X-Ray Spectrometer (SEM-EDX). For SEM technique, the grain surface and cross section of leather samples were analyzed.

Samples Preparation for FTIR Spectroscopy:

Three different methods were used for samples preparation. Firstly, leather samples were scratched to become thin by using scalpel and sterile blade. Samples were directly analysed using FTIR spectroscopy. For the second method, leather samples were scratched by scalpel and sterile blade resulting in the leather powder and then were used with KBr Die Model 129. The leather powder was mixed and compressed with potassium bromide (KBr) resulting in a transparent thin pellet for analysis by FTIR spectroscopy. For the third method, acetone was used to clean the surface of leather samples, and samples were analyzed directly with FTIR spectroscopy.

FTIR Analysis:

FTIR analysis was carried out using a Thermo Nicolet 6700 FTIR spectrometer, equipped with a room temperature deuterated triglycine sulfate (DTGS) KBr detector. The FTIR spectrometer was turned on, initialized, aligned and before use to scan an air background followed by scanning sample spectrum.

For the leather powder that was mixed and compressed with KBr, prepared sample pellet was placed on a 13 mm magnetic film holder and placed in the spectrophotometer used with Transmission E.S.P. The wavenumber was set from the region of 4000 [cm.sup.-1] to 400 [cm.sup.-1]. The spectra of both background and samples were obtained from 32 scans with the resolution of 2.0. Windows-based software program was used to obtain the frequency of each band using the label peaks command of the software or the vertical cursor was used by moving it to find the frequency at the maximum absorbance for the selected band. The data were collected for subsequent analysis.

For the second method, after leather surface was cleaned with acetone and dried, the prepared sample was placed directly under infrared pressure tower using Omni Sampler accessory. The wavenumber was set from the region of 4000[cm.sup.-1] to 675[cm.sup.-1]. The spectra of both background and samples were obtained from 32 scans with the resolution of 2.0. Windows-based software program was used to obtain the frequency of each band using the label peaks command of the software or the vertical cursor was used by moving it to find the frequency at the maximum absorbance for the selected band. The results were collected for subsequent analysis.

All data analysis were carried out using Omnic software version 7.3 and combined with TQ Analyst software for Discriminant Analysis.

Result and Discussion

Physical observation:

From physical observations, Figure 1 and 2 show the similarity characteristics between pigskin leather (natural leather) and synthetic polymer (polyurethane).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Pigskin is tough, rugged and durable leather but is somewhat stiff and intractable. Pigskin has the characteristic grain pattern produced by the hair follicles, which are arranged in roughly triangular groups of three. The holes remaining following removal of the bristles can be seen on the flesh side as well as the grain side. Figure 1 shows the triangle holes on pigskin leather at the grain surface. However, there is a polymer which is polyurethane that is looks like pigskin from physical observation. Polyurethane is a polymer which is printed in nicely triangular groups of three on the surface. From the report, some of the retail premises do not put labeling on this material and claimed that product is actually from leather products. This is because leather has its own characteristic which is leather's porous quality that can make air breathing from human's skin and for this reason also that polyurethane surface was made looks like the same holes from natural leather (pigskin). The analysis was done in order to protect the consumers' right, not only from Muslim customers but also other regions. (Hole and Whittaker, 1971). In addition, several characteristics from other types of leather: cowhide is tough, strong leather and very durable; sheepskin is thinner skin, flexible and soft; and goatskin is tougher and more tightly fibered than sheepskin, hard-wearing grain, has a distinctive texture by ridges and furrows in the grain and hair pits in groups all over the surface.

Scanning Electron Microscopy (SEM):

The Scanning Electron Microscopy is a method which is often used for the investigation of leather, collagen or plastics/polymers if light microscopy is insufficient. In particular that is the case if higher resolutions are needed. More preferences of scanning electron microscopy are large depth of field and high-contrast images of surface structures. Also the investigation of very dark surfaces or transparent materials is very favourable by using SEM technique.

Each species has a distinctive skin structure; the skins vary in total thickness, dimensions of the corium fibre bundles and in the proportion of the total thickness occupied by the grain layer. SEM was used for the verification of the leather types based on the examination of hair follicle patterns and the fibre structure. This was done for the identification characteristics of pigskin leather as a source of non-halal leather products (Figure 3). The characteristics and structures from the different types of leather were verified by scanning electron microscope using simple sample preparation technique.

[FIGURE 3 OMITTED]

For cowhide, goatskin and sheepskin from the grain side and the cross-section that looks different than the cross section of pigskin leather. From the grain surface of pigskin leather under SEM, the holes remaining following removal of the bristle can be seen on the flesh side as well as grain side. Grain of pigskin is quite nice compared to other animal because it has less hair and grain less crowded. The cross section of pigskin leather is shown in Figure 4 that shows hair follicles penetrate until flesh side although grain of pigskin has less hair compared to other animals. This is the uniqueness characteristics that can be seen from pigskin leather compared to other leathers, even after the pigskin leather was processed and treated the holes which are arranged in roughly triangular groups of three still remained and existed, hair follicles penetrate until flesh side. From grain surface of goat skin, there was small holes pattern arranged below the big holes of hair follicles, thus, this characteristic very essential to differentiate between goatskin and sheepskin leather based on the grain surface under SEM.

[FIGURE 4 OMITTED]

Leather Authentication:

For leather authentication, polyurethane is a polymer which is printed in nicely triangular groups of three on the surface. The nature of polyurethane is that the holes cannot be seen on the back side and the holes are straight. The comparison between pigskin leather and polyurethane can be seen in Figure 5, holes from pigskin leather are slanting. Moreover, previously from the sample preparation, finishing on the surface of the samples had been removed by swabbing with acetone, different from other leathers; polyurethane has become shrinking when introduced with acetone. That was initially differentiated between natural leather and non-leather, because polyurethane is derived from polymer.

FTIR Spectra of Halal and Non-halal Leathers (Region of Interest):

Recent years have seen a dramatic improvement in the ease with which analytical instrument can be used, and the quality and quantity of the analytical data that they can produce. Publications during the last 25 years show the acceptability of FTIR spectroscopy (Cronin and McKenzie, 1990; Guillen and Cabo, 1997), as a modern analytical technique.

[FIGURE 5 OMITTED]

Although information has been published concerning the characteristics of leather using a few analytical and molecular biology methods are available and most of them are either difficult to perform or time consuming. For example, in leather analysis using molecular biology technique, the extraction of DNA from leather sample is very difficult due to DNA degradation and which may be destroyed at elevated temperatures or the chemical treatment, a process known as denaturation. The sample may be limited in quantity, thus, accurate sample analysis is critical. This sensitivity to low level of DNA also brings the challenge of avoiding contamination. Besides, it can also be denatured by placing it in a salt solution of low ionic strength or by exposing it to chemicals denaturants (Butler, 2005). All of these factors need to be considered because leather itself undergoes the series of processes. Thus, the choice of Fourier Transform Infrared (FTIR) complemented with Scanning Electron Microscope (SEM) is considered the most suitable technique for the detection of pigskin leather.

FTIR spectroscopy offer rapid, consistent and reproducible analytical technique that could be used as a quality measurement of all states of the sample. Further benefits using FTIR spectroscopy is that the tedious time and chemical consuming standard chemical methods can be avoided. FTIR allows the analysis to be carried out directly on neat samples, without prior extraction, the analysis time is reduced, more reliable and the solvent waste disposal problems are minimized.

The most important part in this study is to determine non-halal pigskin leather using a new rapid analytical technique and more reliable which is FTIR analysis. Four types of leather and leather products were used in this study which are consist of cowhide, sheepskin, goatskin and pigskin. Besides, there were also polyurethane and PVC that were scanned to obtain their RTIR spectra and compared to the natural leather spectra used for leather authentication. The status of the samples are already known and verified by Department of Chemistry Malaysia and the Malaysian company named Kulitkraf Sdn. Bhd. Each of the samples was analysed in triplicate and the results were studied and compared for differences and/or similarities of each samples using the same software.

In this qualitative analysis, it was found out that all the spectra of the leathers were almost the same. This is because leather is made up of protein which is collagen, which is the major protein from which skin is formed (Mirghani, 2010). However there are several regions that make different and fundamentally useful to differentiate between pigskin leather and the other types of leather. This is called as 'regions of interest' in this study.

The most important part in this study is to differentiate between pigskin leather as main source of non-halal leather and cowhide, sheepskin, goatskin leathers. Figure 6, shows the comparison between these types of leathers. There are three regions of interest in this study which are regions are 1200 - 1000 [cm.sup.-1], 700 - 600 [cm.sup.-1] and 500 - 400 [cm.sup.-1]. From all of these regions, it can be very useful point to differentiate between halal leather and pigskin leather.

Firstly, peak at wavenumber 1033 [cm.sup.-1] has shown the peak that was obtained from pigskin leather was fainted and it is not sharp compared to the sharp peak in the spectra obtained from leather of other animal species (Fig 6, region a). That is mean; peak for pigskin in this region is the weakest compared to others. This might be pigskin has small amount of the molecular absorption for the functional group of carbonyl, amide group, or aromatic aldehyde.

[FIGURE 6 OMITTED]

The second one is 669 [cm.sup.-1], at this region (Fig 6, region b) of pigskin leather spectrum, the peak is decreased (like slopping around this region) when compared to other spectra at same region. Pigskin just has small shoulder peak and that one still cannot clearly be seen. Pigskin has low of molecules interaction at this peak base on the vibrational wagging from the functional group of aliphatic amide.

The last region peak of interest was 472 [cm.sup.-1], cowhide peak was clearly visible at this region (Fig 6, region c). It is useful that we can compare between leather speciation based on animal species. Thus, at this region we can know this peak is belonging to cowhide leather. This might be cowhide has highest amount of the molecular absorption for the functional group of carbonyl, amide group, or aromatic aldehyde based on in-plane deformation of vibration.

All in all for these three regions of interest, pigskin leather's peak was among the lowest peak and this feature useful to differentiate non-halal (pigskin) leather between halal leather and leather products. Table 1 shows interpretation of the peaks of interest used in qualitative analysis in Figure 6.

Leather Authentication (PVC and Polyurethane)

For leather authentication, FTIR spectroscopy is very useful and rapid technique to distinguish between leather and non-leather products. It can clearly be seen in the Figure 7 which is the qualitative analysis between natural leathe

r and PVC. Besides, from Figure 8 shows the reliability of the natural leather and polyurethane. Besides, FTIR analysis is very accurate qualitative analysis because from figures 7 and 8 it can be seen that the same result was obtained when using two different samples of PVC and polyurethanes. It could be concluded that, the spectra of non-leather products which are PVC and polyurethane were also different, showing different peaks and they are very different from natural goat or pigskin leathers.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Figure 8 shows the accuracy of the FTIR analysis for leather authentication. The spectra of non-leather product which is polyurethane very different from natural leather.

Discriminant Analysis:

Cooman plot showed that all leathers are clustered into distinct group which could be distinguished clearly. Discriminant analysis is a very useful method for -halal- leather screening and enhances the process of -Halal-leather products authentication. A TQ analyst software (TURBOQ.EXE) classify an unknown sample spectrum by finding the spectrum or group of spectra most closely match to the sample spectrum or by verifying that the sample spectrum is similar to the spectra in a specified group.

[FIGURE 9 OMITTED]

When comparing the spectra obtained from the same type (species) of leather, for example different products of the pigskin leather, they have also some peaks. In fact, their spectra are not totally hundred percents same. It is because every hide and skin is unique, and varies not only from species to species, but even between individual animals. Each type of leather carries its own unique characteristics of the individual animal from which it originated (Mirghani, 2010).

Conclusion:

Currently, the most widely method that has been used for leather analysis is Scanning Electron Microscopy (SEM). Many studies that have been carried with SEM focused more on the leather characteristics; however none of them is on the detection of non-halal leather. Therefore in this study, SEM was used to obtain pigskin leather characteristics, FTIR spectroscopy was used to determine the types of functional groups present in the leather products. The functional group was presented by the peak at different wavenumbers from the analysis. The study contributed to the fundamental understanding of animal leather speciation especially for non-halal (pigskin) leather determination. Three different types of sample preparations for FTIR spectroscopy were used to obtain spectra of each leather product. The use of FTIR spectroscopy technique for this study can be applied as one of the established techniques for pigskin detection of leather product. Besides, this is also useful to make a distinction between leather and non-leather products.

References

Alexander, R.W. and R.J. Bell, 1972. The Cooly-Tukey Algorithm. In Bell R. J. (1974). Introductory Fourier Transform Spectroscopy. 2nd printing. Academic Press Inc.

Beer, R., 1992. The Basic Principle of Fourier Transform Spectroscopy in Chemical Analysis. Vol. 120. Wiley Interscience, 153 pages. Bell, R.J., 1974. Introductory Fourier transform Spectoscopy. 2nd Ed. Academic Press Inc., pp: 23-44.

Butler, J.M., 2005. Forensic DNA Typing: Biology, Technology and Genetic STC Markers. 2nd Ed. Oxford: Elsevier Academic Press.

Cronin, D.A. and K. Mckenzie, 1990. A Rapid Method for the Determination of Fat in Food Stuffs by Infrared Spectrometry. Journal of Food Chemistry, 35: 39-49.

Guillen, M.D. and N. Cabo, 1997. Infrared Spectroscopy in the Study of Edible Oils and Fats. Journal of the Science of Food and Agriculture, 75: 1-11.

Hole, L.G. and R.E. Whittaker, 1971. Structure and Properties of Natural and Artificial Leathers. Journal of Materials Science, 6: 1-15.

Mirghani, M.E.S., 2010. Techniques for the detection of non-Halal leather and leather products. Halal Pages, (2010 / 2011). pp. 40-46. ISSN 1675-2465. ((Available as e-Halal at: www.yellowpages.com.my/halal)).

Razlan, M.S., 2007. Kasut, Beg Kulit Babi: Peniaga Enggan Pamer Tanda Amaran. Harian Metro, 21 February: 15A.

Shaw, R.B., 2000. Modern natural: Creating Sophisticated Interiors with Wood, Leather and Stone. Page One Publishing Pte Ltd.

Stuart, B.H., 1996. Modern Infrared Spectroscopy. In Ando, D.J. (Ed.), New York: John Wiley & Sons, Ltd.

Windfordner, J.D. and I.M. Kolthoff, (Eds.), 1980. Remote Sensing by Fourier Transform Spectroscopy. Toronto: John Wiley & Sons, Ltd.

(1) Mohamed Elwathig Saeed Mirghani, (1) Hamzah Mohd Salleh, (2) Y.B. Che Man, (1) Irwandi Jaswir

(1) International Institute for Halal Research and Training, International Islamic University Malaysia, Gombak P.O. Box 10, 50728 Kuala Lumpur, Malaysia.

(2) Halal Products Research Institute, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia.

Corresponding Author: M.E.S. Mirghani, International Institute for Halal Research and Training, International Islamic University Malaysia, Gombak P.O. Box 10, 50728 Kuala Lumpur, Malaysia.

E-mail: elwathig@iium.edu.my Tel: 603-6196 5749
Table 1: Interpretation of spectral regions a, b & c (Fig 6) used to
differentiate between cow, goat & pigskin leathers.

Region   Wavenumber     Functional group    Predicted chemical
(Fig 6)  ([cm.sup.-1])     assigned             structure

a        1200-1000      Carbonyl, amide     [FORMULA NOT REPRODUCIBLE
                        group or aromatic   IN ASCII]
                        aldehyde

b        700-600        Aliphatic aldehyde  [FORMULA NOT REPRODUCIBLE
                        (- NH wagging)      IN ASCII]

c        500-400        Acetamide,          -C(=O)N[H.sub.2] and -C-N
                        propanamide  &      (in plane deformation)
                        butanamide
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Title Annotation:Original Article
Author:Mirghani, Mohamed Elwathig Saeed; Salleh, Hamzah Mohd; Man, Y.B. Che; Jaswir, Irwandi
Publication:Advances in Natural and Applied Sciences
Article Type:Report
Geographic Code:9MALA
Date:May 1, 2012
Words:3658
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