## 1 Introduction  
During the previous few decades, the olive oil scientific community and industry have become increasingly linked by the common goal of improving olive oil production and quality [1]. One of the most important permanent aims of this sector is to strengthen and improve the diversification of extra virgin olive oil (EVOO) on the market, because there are still a large number of uninformed consumers who consider olive oil as a standard commodity [2]. The consumers are often unaware of the great heterogeneity with respect to sensory and nutritional quality of oils within the category of the highest quality grade (EVOO), which is mostly present because the thresholds that are set by the European Commission regulation (EEC, 1991) for chemical parameters (acidity, peroxide value, \(K_{232}\), \(K_{270}\), \(\Delta\)K values, alkyl esters) are not rigorous and they can be met relatively easily, while the prescribed sensory analysis method practically only discriminates oils with certain fruitiness (median \(>\) 0) and without sensory defects from the defective ones [3]. As a consequence, rather different products comply with the requirements of the EVOO category, which often confuses the consumers, especially when it comes down to the large span of EVOO prices [4]. The mentioned heterogeneity of EVOO is strongly influenced by variations in pedoclimatic conditions, cultivar, agronomic practices, and technological factors, and especially by the level of care that is taken in order to avoid or minimize the negative effects from various sources [4; 5; 6], which is proportional to the costs of production and EVOO final price [4]. Consumers, mostly because of its pleasant aroma, bitterness, and pungency, as well as its health effects, appreciate EVOO. The chemical parameters that are not regulated by the legislation, but they are certainly among the most meaningful for evaluating and understanding EVOO sensory and nutritional quality and they could serve as differentiators that are based on such criteria, are volatile aroma compounds and phenols [7; 8; 9]. The typical fruits and green aroma of high quality EVOO mainly consists of C5 and C6 aldehydes and ketones, enzymatically formed by lipoxygenase (LOX) and hydroperoxide lyase (HPL) during olive processing, later partly reduced to C6 alcohols by alcohol dehydrogenase (ADH) and then transformed to C6 esters by alcohol acyl transferase (AAT) [10]. In addition, there are other important classes of molecules in EVOO, such as hydrocarbons, acids, and terpen, which contribute to generating either positive (wood, lemon, rose) or negative notes (rancid, butter, vinegar) [11; 12]. Phenols in EVOO are responsible for its taste characteristics, such as bitterness, pungency, and astringency, and they are characterized by a diversity of chemical families, including phenolic alcohols and acids, hydroxy-isochromans, flavonoids, lignans, and secoridoids. The most abundant among them, secoridoids, are specific compounds of _Oleaceae_ plants and they differentiate olive oil as unique among the other vegetable oils [13; 14]. Secoridoids, more specifically oleuropein glucoside, and its aglycon, are the key contributors to EVOO bitterness [15], although it has been observed that benzoic and cinnamic acid derivatives are also responsible for bitter mouthfeeel [5]. EVOO pungency also derives from secoiridoids, especially from the dialdehyde form of decarboxymethylelenolic acid linked to tyrosol, also known as \(p\)-HPEA-EDA or oleocanthal [16]. EVOO phenols exhibit antioxidant activity which is widely responsible for its oxidative stability and shelf life, as well as its nutritive value [17]. In fact, the European Food Safety Authority (EFSA) allowed for olive oil producers to declare a health claim on the bottle label regarding their positive effect on blood lipids (European Commission, 2012), confirming that the content and composition of phenols can be directly used as an indicator of EVOO quality.