## 5 Case Study: Panel Test and E-Nose for the Determination of VOOs' Shelf Life
### Virgin Olive Oil Shelf Life Assessment  
Nowadays, different methods are available to monitor the quality decay of VOOs as a function of oxidation process; for many of them, the current legislation defines the acceptability limits based on instrumental and/or sensory methods. Besides the conventional parameters (Table 2) in the literature, many examples of stability and SL studies are available, both in accelerated or real-time conditions, based on the VOO aromatic traits as well as on the loss of bioactive compounds [117] (Table 3). \begin{table}
\begin{tabular}{c c} \hline \hline
**Quality Indicator** & **Method** \\ \hline \hline Tocopherols & HPLC \\ \hline Polyphenols & COI/T.20/Doc No. 29 \\ \hline Oleeuropein aglycon content & HPLC \\ \hline Hydroxyprosol content & UNI 11702:2018 \\ \hline Tyrosol content & UNI 11702:2018 \\ \hline Carotenoids & Spectrophotometer \\ \hline Degradation products of chlorophyll a & ISO 29841:2009 \\ \hline Vocs analysis & HS-SPME-GC/MS; sensor arrays \\ \hline Hexanal & HS-SPME-GC/MS \\ \hline In vitro antioxidant activity & Spectrophotometry \\ \hline Color & Spectrophotometry \\ \hline \end{tabular}
\end{table} 
Table 3: Non-compulsory indices which can be assumed as markers to track the evolution of the oxidative degradation of virgin olive oils during storage and relevant analytical methodologies [6; 96]. Figure 1: The exposition of VOOs to oxygen or catalyst agents induces the oxidation of unsaturated fatty acids, with the formation of unstable hydroperoxides, which, during autoxidation, form off-flavors (secondary volatile products such as aldehydes, ketones, alkenes and alkanes) responsible for the VOO quality decay. According to the main focus of this review, only papers dealing with sensory analysis and sensor arrays for the VOO SL assessment will be taken into account. According to EU legislation, the panel test still represents the only official method (EEC 702/2007) to detect the increase in off-flavor in VOO for the quality classification of VOO samples, based on the organoleptic detection or not of the fruity attribute, together with the indication of the median value of the predominant defect [118]. Starting from this point, it seems extremely important that the accredited laboratories perform panel tests of VOO in compliance with a well-established quality system by applying the laboratory organization and technical conditions of analysis requirements (COI/T.20/Doc. No. 15/Rev. 8, 2015), together with the guidelines fixed in the Norm ISO 17,025 (COI/T.28/Doc. No 1, 2007--revised in 2017) [7]. As described in these guidelines, an accredited laboratory is asked to correctly monitor the panel proficiency and to periodically check the validity of its entire quality system, in terms of reproducibility and repeatability of the tests, to individuate and solve possible systematic errors early. With this aim, certified reference materials are available to support laboratories for both the correct training of panelists as well as to provide a useful tool to give a measure of the accuracy evaluations carried out during the panel test. Genovese and co-workers [105] recently published an interesting review about VOO flavor, with particular emphasis on the mechanisms affecting its production and release during tasting. Interestingly, the authors highlighted that the differences in VOO organoleptic characterization during storage, as reported by different panels, can be explained by the interaction among positive (i.e., bitter, pungent, fruity, green) and negative (i.e., rancid, fusty, moldy, winery) attributes during VOO tasting. For example, the high concentration of phenolic compounds that generally characterize fresh VOOs can completely mask the \"fusty\" character of the sample. In this case, the \"fusty\" defect cannot be detectable by panelists until the phenolic concentration decreases because of oil oxidation during storage.