# Natural Variation of Volatile Compounds in Virgin Olive Oil Analyzed by HS-SPME/GC-MS-FID
## 1 Introduction  
Virgin olive oil (VOO) is an essential component of the traditional Mediterranean diet, and attracts the interest of the scientific community for its health-promoting properties [1; 2; 3; 4; 5; 6]. VOO is unique for its high levels of monounsaturated fatty acids and the presence of a wide range of minor components that are largely responsible for both their health-promoting properties and their organoleptic characteristics. Regarding the organoleptic characteristics, some of these minor components are volatile and consequently responsible for the aroma of VOO, which is characterized by a distinctive balance of green and fruity attributes. [PERSON] et al. [7] established that most of VOO volatiles are synthesized when enzymes and substrates join during the olive fruit milling in the oil extraction process, and that the enzymatic activities within the lipoxygenase (LOX) pathway are involved in this synthesis. Aldehydes and alcohols of six straight-chain carbons (C6), as well as their derived esters, are the major compounds in VOO aroma, either qualitatively or quantitatively [8; 9]. Linoleic (LA) and linolenic (LnA) acids are the main substrates for this synthesis. LOX enzymes catalyze the production of 13-hydroperoxide derivatives in the first step of this pathway, which are later cleaved heterolytically by the hydroperoxide lyase (HPL) enzyme, forming C6 aldehydes [7; 10; 11]. C6 aldehydes are then reduced by alcohol dehydrogenase (ADH) enzymes to C6 alcohols [7; 12], and finally converted into the corresponding esters by alcohol acyltransferase (AAT) enzymes [7; 13]. The relevance of the five straight-chain carbons (C5) compoundsin VOO aroma has also been suggested [9]. They seem to be produced via a parallel branch of the LOX pathway, as demonstrated in soybeans [14; 15]. The synthesis of VOO volatile compounds seems to depend primarily on the availability of fatty-acid substrates to be catabolized through the LOX pathway during the VOO extraction process. In addition, LOX activity also proved to be an important limiting factor for the production of these volatile compounds, and this limitation is seemingly characteristic of each olive cultivar [16; 17]. Along with its possible health benefits, its excellent organoleptic properties may well explain the continued demand for VOO. As mentioned above, volatile compounds are responsible for the aroma of this product, which is composed, when obtained from sound fruits, of a mixture of green and fruity odor notes, spiced with some other positive notes that make VOO a unique, edible oil. Odor perception and pleasantness are determined by the functional groups in the volatile compound [18; 19]. To understand the relationship between volatile compounds and odor attributes in VOO, [PERSON] and [PERSON] [20] developed a statistical sensory wheel through a compilation of the data obtained from trained VOO sensory panels across Europe. As a result, odor notes with similar description and volatile compounds with a given sensory perception were grouped together into sectors. Among the most relevant to VOO aroma are green or ripe fruit odor notes. Moreover, concentration and odor threshold determine the sensory contribution of each volatile compound in the oil. The ratio between the concentration of the volatile compound and its odor threshold is the odor activity value (OAV). Those volatile compounds with OAV below one do not theoretically contribute to the aroma of VOO. The volatile fraction of VOO is also responsible for the off-flavors present in some oils. This is of absolute importance because VOO classification into commercial categories requires a sensory analysis. This classification is officially ruled [21] and carried out by trained test panels, which assess the green and fruity odor notes typical in the aroma of virgin olive oil, when extracted from healthy, fresh olive fruits, as well as aroma defects in the oil.