## 2 Results and Discussion
### Chemical Changes during the Storage Experiment  
In order to assess the chemical changes that take place during the VOO shelf life, a storage experiment was carried out for 27 months under controlled conditions (see Section 3.1). Quality parameters, volatile composition and sensory characteristics (panel test) were monthly determined during this period of storage. The indicators of the VOO quality alteration showed an increase during the storage experiment at moderate conditions. Figure 1 shows the evolution of quality parameters and total phenol concentration per each sample during the long-term storage. In all cases, the final values were inside the \"extra virgin olive oil\" category according to the limits stated in EC regulation [13], except for K\({}_{270}\). Thus, this parameter exceeded the legislative limit stablished for the \"extra virgin olive oil\" category in the first months of storage (1-5 month) for the four VOOs. Furthermore, the increase of the K\({}_{270}\) is faster in VOO1 and VOO3 than in the other two VOOs in the first five months of storage. On the other hand, the total phenol concentration decreased during the storage experiment in all cases. Their concentrations showed a higher decrease during the first twenty months of storage compared to the seven last months in all VOO. These results agree with the results found by other authors [22; 23]. VOO3 and VOO4 underwent the highest concentration decrease, which were respectively 297.21 and 329.88 mg/kg. The other two samples showed a slighter decrease of their concentrations, with a reduction value of 133.24 mg/kg for VOO1 and 155.09 mg/kg for VOO2. In the course of the storage experiment, panelists identified some flavor changes in the samples, which resulted in a variation in their medians of the fruity attribute and the defect. Figure 2 shows the evolution of the medians of the defect and the fruity attributes for each sample during the storage. This figure also shows the variations of VOO category of the oils during the storage, according to the limits established in European regulation [13]. The sensory assessment results revealed that the pleasant odor attributes decreased during the storage. In all cases, a reduction of the median of the fruity attributes was observed. VOO2 showed the fastest decrease, so displaying a drop from 3.5 to 2.0 units in the median of the fruity attribute during the first five months of storage, while the rest of the samples kept their initial values at this time. On the other hand, the median of defect  
Figure 1: Time-course plots of peroxide value (PV), free acidity (FFA), ultraviolet absorbance at 270 nm (K\({}_{270}\)) and 232 nm (K\({}_{232}\)) and total phenols concentration per each virgin olive oil (VOO) (polynomial fitting). According to [13]: Limits for extra virgin olive oil (EVOO): PV \(\leq\) 20 meq O\({}_{2}\)/kg, FFA \(\leq\) 0.8%, K\({}_{270}\)\(\leq\) 0.22, K\({}_{232}\)\(\leq\) 2.50. Limits for virgin olive oil (VOO): PV \(\leq\) 20 meq O\({}_{2}\)/kg, FFA \(\leq\) 2.0%, K\({}_{270}\)\(\leq\) 0.25, K\({}_{232}\)\(\leq\) 2.60.  
showed that VOO2 was downgraded to \"virgin olive oil\" category rapidly, in the fifth month of storage (VOO2-5m) because assessors detected a winey-vinegary defect at this time. The time-trend changes of K\({}_{232}\) and FFA also pointed out that this sample was the least stable, since the final values of these parameters (2.45% and 0.32% respectively) were the highest compared with the rest of the VOOs (Figure 1). The next VOO undergoing a downgrade of category was VOO3. Thus, this oil changed to \"virgin olive oil\" category in the tenth month of storage (VOO3-10m) when assessors detected a winey-vinegary defect as well. Although the evolution of K\({}_{270}\) for this oil pointed out that this oil was unstable (the final value after 27 months of storage was 0.38), the time-trend changes of the other parameters pointed out its stability during the storage.