## 1 Introduction  
Extra virgin olive oil (EVOO) is appreciated among consumers because of its specific flavor and nutritional properties. Due to its economic importance, EVOO is among the most common commodities subject to fraud and mislabeling. The European Union (EU) protects EVOO by the regulation mostly based on analytical and sensory controls [1] which generally succeed in detecting illegal manipulation with EVOO intrinsic properties (adulteration with cheaper refined and/or extraneous oils) and EVOO extrinsic properties (fraudulent misrepresentation of quality category). In the EU, most EVOOs of high economic value are additionally protected by Protected Denomination of Origin (PDO) [2]. Each PDO EVOO is produced according to a set of specific requirements prescribed by the holder of a designation in a specification document, governing aspects such as olive varieties used, cultivation, harvest and processing conditions, physico-chemical parameters, and sensory characteristics. Many PDO EVOOs are produced from olives of a single variety (monovarietal EVOOs), while the blends also owe a large part of their typicity to the unique olive assortment of a particular region. Nowadays, the information on the label about the varietal origin of EVOO is becoming more and more important and attracting, especially for the market segment of informed consumers interested in healthy, quality products with remarkable diversity and clear identity. Similar as in the case of wine, besides being linked to a given geographical origin and PDO, EVOOs from particular varieties are recognized, appreciated, and demanded on the market because of their specific nutritional and sensory properties. As a consequence, they often reach higher prices, and, given the obvious financial benefits associated with them, are very likely subject to fraud by mislabeling with respect to varietal origin. As regards EVOO varietal authentication within the process of protection by designation of origin (PDO), controls against counterfeiting include auditing of mandatory documentation and records that prove traceability in production and compliance with the requirements set up in PDO specification. The other part is the assessment of the conformity of EVOO with physico-chemical parameters and sensory characteristics laid down in the specification. The analytical parameters controlled more often (e.g., acidity, peroxide value, measurements in ultraviolet, etc.) do not specifically reflect varietal origin, and the limits that are established, although usually stricter than those prescribed by the official EU regulation [1], are regularly not designed to identify olive variety used. Similar applies for the sensory profiles commonly used to describe monovarietal/PDO EVOO [3] which are not highly discriminative. The mentioned measures are not sufficient to control varietal origin and avoid fraud. Fraud or false labelling might also be detected or confirmed chemically by analysis of other, minor EVOO compounds. The general strategy that is followed in various research laboratories is the detection of as many as possible EVOO constituents from a larger set of samples and application of multivariate statistical analysis to the analytical data in order to build up classification/prediction models based on varietal origin [4; 5]. Many EVOO compounds were found useful for this purpose, including sterols [6], tocoperbols [7], fatty acids [6; 8], etc. The chemical compounds whose amounts are regulated neither by the official EU regulation nor the PDO specifications, but are certainly the most involved in the typical sensory identity of PDO and monovarietal EVOOs and could serve as differentiators based on such criteria, are volatile aroma compounds and phenols [9; 10; 11; 12; 13; 14]. In fact, many successful reports were published which confirmed the utility of these constituents for EVOO varietal differentiation [12; 14; 15; 16; 17; 18; 19; 20].