## 5 Case Study: Panel Test and E-Nose for the Determination of VOOs' Shelf Life  
### EVOO Quality and Main Stability Issues  
The term virgin olive oil (VOO) shall be taken to mean \"the oil extracted from the fruit of the olive tree (_Olea europaea_), solely by means of physical or mechanical process\". Therefore, in order to produce a VOO, olives must be subjected only to washing, de-canting, centrifugation, and filtration; oils produced by chemical extraction, re-esterification or oil blending processes do not fall within this definition [7; 95]. VOO quality, in terms of chemical features and sensory profiles, is affected by several elements, such as genetic features, pedoclimatic conditions, training system, harvest period, extraction procedure and storage conditions [96; 97; 98; 99; 100; 101]. It is therefore possible to classify VOOs into three main categories according to their chemical and sensory parameters (Table 2): (i) extra virgin olive oil (EVOO), recognized as the highest grade of olive oil with beneficial health effects, (ii) VOO and (iii) lampante olive oil (LOO), which is destined for technical uses or refining processes only. A VOO is composed essentially of triglycerides (98-99%, distributed as follows: mo-nounsaturated fatty acids (MUFAs = 65-83%), polyunsaturated fatty acids (PUFAs = n - 6: 6-15%; n - 3: 0.2-1.5%) and saturated fatty acids (SFAs = 8-14%)), with minor constituents such as phenols (i.e., phenolic alcohols, such as hydroxytyrosol and tyrosol; secoriidoids such as oleocanthal and oleuropein derivatives; lignans such as pinoresinol), tocopherols, intergenic acids, sterols, 4-methylsterols, carotenoids, chlorophylls, mono and diglycerides, free fatty acids, esters and volatiles, which give peculiar sensory properties and several bioactive functions [102, 103, 104]. Water is also present in very small, but essential amounts as micro-droplets, which are associated with polar or amphipathic substances belonging to the minor constituent's group [102]. The volatile molecules constituted by five or six carbon atoms, produced in the _lipoxygenase_ pathway, represent the biggest part of volatile compounds in VOOs and exert an essential role in the expression of the green attributes of VOO [105, 106]. Among these, worthy of particular attention for their desirable effects, are hexanal (cut grass), guaiaol (soapy, olive paste), octanal (citrus, lemon), (E)-2-decneal (soapy, fatty), 1-penten-3-ol (green plants, grassy), (Z)- 3-hexenyl acetate (fruity), (E)-2-hexenal (green) and 6-methyl-5-hepten-2-one (nutty) [95]. Nevertheless, there are also several compounds responsible for negative organoleptic perception and belonging to the class of \"off-flavor\" (i.e., rancid, winey, fusty, vinegary, frozen), generally caused by the oxidation process [95]. Moreover, due to the high content of MUFAs and PUFAs, along with the presence of enzymes such as _lipase_ and _peroxidase_, VOO lipids are susceptible to oxidation and enzymatic hydrolysis, which promote autoxidation [107]. This degradative process reduces VOO's nutritional and healthy effects, together with its economic value, since it is the main cause of VOOs downgrading to a lower quality classification (i.e., EVOO in VOO or LOO). Many factors influence the lipid oxidation process during production and storage: artificial light exposure, sunlight, storage temperature, humidity and air exposition, as well as packaging features, together with the oil chemical composition [97, 98, 108, 109, 110, 111, 112, 113]. As reported in Figure 1, VOO compounds (i.e., fatty acids or liposoluble vitamins) are oxidized and turned into unstable products which ultimately trigger further degradation reactions, forming off-flavors and toxic compounds [6, 114]. In this context, VOO SL could be defined as the storage time during which safety, sensory and quality parameters are within accepted limits for the specific commercial category (see Table 2) [107].