Depth profiling of porcine adipose tissue by Raman spectroscopy Lotte Bøge Lyndgaard, a * Klavs Martin Sørensen, a,b Frans van den Berg a and Søren Balling Engelsen a Raman spectroscopy was applied on a depth profile of porcine adipose tissue (from skin to meat) with the purpose of (1) discriminating between fat layers and (2) estimating the variation in fatty acid composition as a function of fat depth and fat layer: total degree of unsaturation (iodine value), fractions of saturated, and monounsaturated and polyunsaturated fatty acids. The thickness and composition of the outer layer of porcine adipose tissue influences the final quality of backfat. A too-thick outer layer is associated with problems such as oily appearance, rancidity development, and difficulties in separating muscle and adipose tissue when cutting. From principal component analysis on standard normal variate preprocessed Raman spectra (1800–800 cm –1 ), it was possible to discriminate between the outer and the inner backfat layer. Principal component analysis loadings showed that the separation of layer was mainly explained by variation in the bands originating from vibration of double bond C = C stretching plus = C–H twisting and rocking. In the prediction of iodine value a three-component partial least squares regression model based on full range Raman spectra showed a root mean square error of cross validation of 2.00 and R 2 = 0.69. Applying Cauchy–Lorentz band fitting proved that information regarding fat unsaturation was found not only in band intensity, but also in band parameters such as location and width. The results suggest Raman spectroscopy as a potential measurement technique for rapid grading of pork carcasses. Copyright © 2011 John Wiley & Sons, Ltd. Keywords: Raman spectroscopy; fat layers; iodine value (IV); depth profiling; chemometrics Introduction Pork is one of the most important meat products worldwide. The backfat of porcine is very well studied because fat tissue is known to be an important aspect of pork carcass quality. Backfat consists primarily of triglycerides containing saturated, unsaturated, and polyunsaturated fatty acid (SFA, UFA, and PUFA), and the composition of these fatty acids is important in both meat processing and consumer acceptability. [1–3] From a health perspective it is preferred to have SFAs replaced with PUFAs as the evidence from epidemiologic, clinical, and mecha- nistic studies is consistent in finding that this will reduce the risk of coronary heart diseases. [4] However, from a production perspective fat containing mostly UFA causes problems with slicing yield and results in an unwanted oily product appearance when packaged. Furthermore, tendency to develop rancidity is increased with UFA content. [5–7] Consequently, Hugo and Roodt [3] concluded that saturated fat is necessary to manufacture high quality products, despite the fact that it is associated with inferior health properties. Porcine backfat becomes separated into two different layers (outer and inner) with increased age and fat content, and it has been shown that structure, chemical composition, and texture are different in the two layers. [8–12] The outer layer (next to the skin) has a high degree of UFA whereas the inner layer (muscle side) consists of more SFA. [13] The degree of unsaturation is very important for the firmness of fat, as the double bonds, only present in UFA, give bends in the methylene chain leading to a less structured and more fluid fat with lower melting point. [14] SFAs have straight chains and can pack closer together, which has a positive influence on fat firmness and consistency. The thickness of the backfat layers is an important quality parameter in porcine meat production. Measurements of backfat thickness can be a decision tool for optimizing breeding and conditions for growth, longevity in gilts, and for quality control and for carcass grading. [15] Currently, it is general practice to measure the total backfat thickness, and not the thickness of the individual layers. However, studies have shown that knowledge about the depth of the individual fat layers may help improve the quality. [16] Measurements of fat layers have been carried out with ultrasound and ultrasound imaging. [15,17] However, no chemical information about the fat layers can be withdrawn from ultrasound measure- ments. The chemical composition of each layer is potentially critical as it affects sliceability, visual appearance, and the oxida- tive stability. [7] Thus, although ultrasound can discriminate between fat layers, it does not reveal variations in composition of the individual layers. To the authors knowledge a depth profiling of the composition of fatty acids in backfat has not yet been proposed. The standard method for determining the fat quality (firmness) is iodine value (IV), which measures the degree of unsaturation through the total number of double bonds in the fatty acids. * Correspondence to: Lotte Bøge Lyndgaard, University of Copenhagen, Faculty of Life Sciences, Department of Food Science, Quality and Technology, Frederiksberg C, Denmark. E-mail: Lottebs@life.ku.dk a University of Copenhagen, Faculty of Life Sciences, Department of Food Science, Quality and Technology, Frederiksberg C, Denmark b Carometec A/S, Transformervej 9, Herlev, Denmark J. Raman Spectrosc. 2012, 43, 482–489 Copyright © 2011 John Wiley & Sons, Ltd. Research Article Received: 29 June 2011 Revised: 16 August 2011 Accepted: 26 August 2011 Published online in Wiley Online Library: 7 November 2011 (wileyonlinelibrary.com) DOI 10.1002/jrs.3067 482