## 3 Results and Discussion
### Evolution of Polyphenols during 12 Months of Light Exposure  
The evolution of all the phenolic compounds expressed as the sum of 3,4-DHPEA, \(p\)-HPEA, oleacein (3,4-DHPEA-EDA), 3,4-DHPEA-EA, oleocanthal (\(p\)-HPEA-EDA), and lignans ((+)-1-pinoresinol and (+)-acetoxypinoresinol) was also evaluated. The data (Figure 6; Table S9) showed a general decrease in the initial concentration, with a loss ranging between 57.1 and 86.9% of the total amount. However, while VOOsImp (Figure 6A) exhibited a depletion from 76.7 to 86.9%, in VOOsmhp, (Figure 6B), the loss was attenuated by approximately 57.1 up to 85.9%; in particular, lower decreases (57.1-65.8%) were registered in those VOOsmhp with an initial concentration of polyphenols equal to or higher than 900 mg/kg (samples S1, S4, S5, and S6) (Figure 6B). Figure 6: Evolution (reported every two months) of the polyphenol concentrations (mg/kg) over 12-month storage with light exposure in VOOImp (**A**) and VOOmhp (**B**). Legend: VOOImp: Virgin olive oil with low-medium polyphenol content; VOOmhp: Virgin olive oil with medium-high polyphenol content. The values are expressed as the sum of hydroxytyrosol (3,4-DHPEA), tyrosol (p-HPEA), oleacein (3,4-DHPEA-EDA), oleuconnal (3,4-DHPEA-EA), oleocanthal (p-HPEA-EDA), ligstroside aglycon (p-HPEA-EA) and lignans ((+)-1-acetoxypinoresinol and (+)-pinoresinol). Accordingly, [PERSON] et al. recently observed a significant decrease in phenolic content loss (46%) in Greek VOOs during usual storage with light exposure over 12 months [44]. Additionally, we showed that VOOsmhp with higher polyphenol contents were the same demonstrating higher resistance to oxidation dur to their lower values of K\({}_{270}\), \(\Delta\)K, (\(E\), \(E\))-2,4-decadienal and (\(E\))-2-decneal. Thus, the antioxidant effect of VOO polyphenols, in contrast the photo-oxidation was highlighted. These findings confirmed the results of [PERSON] et al. [7, 8], who observed, in previous VOOs' real-time shelf-life studies, higher resistance to oxidation in accordance with the initial polyphenol concentration [7, 8]. Even [PERSON] et al. [15] observed a huge decrease in oleocanthal and oleacein during the storage of 3 VOOs stored at 25 \({}^{\circ}\)C under daylight exposure, but at different rates: in fact, in VOOs with low polyphenol contents, oleocanthal and oleacein decreased quickly, and after only 8 months of storage, they were no longer detectable. Conversely, in the other two VOOs, containing a concentration of polyphenols, the two secoiridoids gradually decreased and were undetectable after 11 and 12 months, respectively [15]. Evolution of the Oxidized and Non-Oxidized Form of the Oleuropein Aglycon during 12 Months of Light Exposure  
This is the first study where the 3,4-DHPEA-EA degradation product (oxidized form of oleuropein aglycon) was monitored over time to evaluate its potential capability in monitoring the oxidative status of the VOO as an earlier and reliable indicator. In this regard, [PERSON] et al. [25, 26] and [PERSON] et al. [7] showed the accumulation of the oxidized form of 3,4-DHPEA-EDA in VOO, already demonstrating its potentiality as a marker of VOO freshness. Similarly, [PERSON] et al. [28], following the evolution of the VOOs' quality real-time shelf life, concluded that it is also possible to predict the residual shelf-life of a VOO by measuring the relative quantity of oxidized phenols [28]. In our study, we chose to monitor only the oxidized form of the oleuropein aglycon instead of the sum of the oxidized forms or the single forms of \(p\)-HPEA-EDA and/or 3,4-DHPEA-EDA because the chemical structures of the last two ones usually absorb at a wavelength of 278 nm, which is the same wavelength absorbance of the relative non-acidic forms [15]. This means that spectral discrimination between the non-acidic and acidic forms, could be a long and difficult procedure, limiting the chemical evaluation as a reliable and easy method for monitoring VOO freshness over storage.