1284 Research Article Received: 3 November 2008 Accepted: 26 February 2009 Published online in Wiley Interscience: 27 April 2009 (www.interscience.wiley.com) DOI 10.1002/jrs.2279 Visible Raman spectroscopy for the discrimination of olive oils from different vegetable oils and the detection of adulteration R. M. El-Abassy, P. Donfack, and A. Materny * We have investigated the potential of Raman spectroscopy with excitation in the visible spectral range (VIS Raman) as a tool for the classification of different vegetable oils and the quantification of adulteration of virgin olive oil as an example. For the classification, principal component analysis (PCA) was applied, where 96% of the spectral variation was characterized by the first two components. A significant similarity between sunflower oil and extra-virgin olive oil was found using this approach. Therefore, sunflower oil is a potential candidate for adulteration in most commercially available olive oils. Beside the classification of the different vegetable oils, we have successfully applied Raman spectroscopy in combination with partial least-squares (PLS) regression analysis for very fast monitoring of adulteration of extra-virgin olive oil with sunflower oil. Different mixtures of extra-virgin olive oil with three different sunflower oil types were prepared between 5 and 100% (v/v) in 5% increments of sunflower oil. While in the present context the adulteration usually refers to the addition of reasonable amounts of the adulterant (given the similarity with the basic product), we show that the technique proposed can also be used for trace analysis of the adulterant. Without using techniques like surface-enhanced Raman scattering (SERS), a quantitative detection limit down to 500 ppm (0.05%) could be achieved, a limit irrelevant for adulteration in commercial terms but significant for trace analysis. The qualitative detection limit even was at considerably lower concentration values. Based on PCA, a clear discrimination between pure extra-virgin olive oil and olive oil adulterated with sunflower oil was achieved. The adulterant content was successfully determined using PLS regression with a high correlation coefficient and small root mean-square error for both prediction and validation. Copyright c  2009 John Wiley & Sons, Ltd. Keywords: Raman spectroscopy; chemometrics; vegetable oils; adulteration detection; olive oil Introduction Olive oil has a high nutritional value and significant health benefits. [1–3] it is an essential component of the Mediterranean diet. According to the European Union (EU), olive oils are classified into various categories ranging from the high-quality extra-virgin olive oil to the low-quality olive pomace oil. [4] Extra- virgin olive oils are produced using only cold pressing techniques without any thermal or chemical treatments, while olive pomace oil, a fully refined olive oil, is obtained by solvent extraction processes from olive pomace. [5] Extra-virgin olive oils are the most desirable on account of their high quality and nutritional properties compared to low-quality pomace olive oil [6] and are therefore the most expensive grades. For the last reason, as a matter of fact and for commercial purposes, there is a tendency of mislabeling or adulterating extra-virgin olive oil with refined olive oil or cheaper and similar oils, such as sunflower oil and hazelnut oil. [7–9] Therefore, the identification of different oils and the detection of oil adulteration are of great importance from both market and health aspects. Classical methods based on wet chemical or chromatographic analysis for the determination of free acid radicals, peroxide value and fatty acid composition are typically used to detect adulterants and classify olive oil categories. Especially, a standard method based on quantification of stigmastadienes, which are derived from refining processes, is used in order to detect adulteration of virgin olive oil with refined oils. [10] Chromatographic methods such as high-performance liquid chromatography (HPLC) and gas–liquid chromatography (GLC) have been successfully applied to determine triglycerides [11] and the fatty acids, [12] respectively, in order to the check the quality and detect the adulteration in edible oils. In addition, triglycerides of olive oil and some seed oils (corn, cottonseed, palm, peanut, soybean, and sunflower) are used to detect olive oil adulteration by means of capillary gas chromatography. [13] In olive oil industry, these classical methods are not only time consuming and inconvenient for online process control, but they are expensive as well. Meanwhile, rapid and simple techniques are needed, which would offer online detection in order to boost up the production while reducing the costs and satisfying quality needs of the consumer. Spectroscopy techniques now have the potential to replace or at least complement these classical methods. Nuclear mag- netic resonance (NMR) analysis for detecting adulteration of olive oil has been reported. [14 – 16] With respect to vibrational spec- troscopy, near infrared (NIR) absorption has been applied for the ∗ Correspondence to: A. Materny, Jacobs University Bremen, School of Engineer- ing and Science,Campus Ring 1, 28759 Bremen, Germany. E-mail: a.materny@jacobs-university.de Jacobs University Bremen, School of Engineering and Science, Campus Ring 1, 28759 Bremen, Germany J. Raman Spectrosc. 2009, 40, 1284–1289 Copyright c  2009 John Wiley & Sons, Ltd.