## 4 Discussion  
Regarding the sensory analysis, the overall odor intensity of the degraded sample was evaluated with higher values in comparison to the undegraded sample, a score less than 4 in the undegraded sample (Figure 3A) and a score more than 5 in the degraded one (Figure 3B). Similar results were obtained from the analysis of the degree of smell of butter/milk and other attributes such as acidic, pungent, putrid, mold, propionic acid, and butyric acid. In other words, the scores given by the panelists to the undegraded cheese sample were in strong contrast to the scores given to the degraded sample. The degraded sample B was aged for 36 months, hence a high odor intensity is expected (Figure 3B). In the same sample, the smell of butter/milk was evaluated with low values, nevertheless, with a high variability among the judges. This confirmed the actual and real differences between the two classes of cheese samples. It was observed that the olfactory evaluation of cheese is a difficult task because it requires a lot of training and our perception of flavor and odor is extremely complex [36]. Despite some variation between the same class of samples, it was clearly noticed that the changes related to the quality and ripening period of Parmigiano Reggiano cheese took place in the odor and color of the product. These tests are generally used to make sure whether the samples are perceptively different. Figure 7: A histogram representing the VOCs from an undegraded high-quality cheese sample (blue) and a degraded Parmigiano Reggiano cheese sample (orange) identified by SPME-GC-MS analysis. As such, they can segregate the sensory properties of the cheese hence providing important and useful information to the instrumental analyses, especially electronic nose and SPME-GC-MS technique. On the subject of S3 analysis, the instrument, therefore, was able to distinguish the degraded cheese samples from the undegraded high-quality ones. The degraded samples are all grouped within the same cluster (black) on the left side of the graph, whereas the undegraded samples are grouped together within the same cluster (blue). This PCA explained 89.55% of the total explained variance within two PCs, which were represented by the cheese quality differences. A group of samples within the blue cluster were more scattered, probably accounting for variation between the aroma compounds, ripening time, and geographical origin of the cheese samples [23; 37]. Few samples were outside the cluster which could be minimized by improving the sensitivity of the sensors [38]. In general, the clusters related to the quality of cheese samples are well separated, so that it is possible to detect the headspace volatile components based on the classes of cheese with the application of S3. The result is also in positive correlation with that of the triangular test. From the graph in Figure 6, it can be easily concluded that the three clusters--blue, red, and yellow--diverge along particular directions according to different maturation period. In particular, the blue cluster groups the young cheese samples with 12 and 13 months of ripening; the red cluster includes the cheese samples with 16, 17, and 18 months of ripening; and the yellow cluster is represented by mature specimens cured for 36 months. The three clusters are very compact and well differentiated between them, therefore, the instrument was successful in sensing the differences between the aromatic profiles of the samples that constitute such groupings. This PCA model was more descriptive, the total explained variance was 95.25% within three PCs. The groups of samples at different ripening times were less scattered as well. The reason for this difference in the aromatic profile between the samples is due to the fact that, during the aging process, the product undergoes organoleptic and biochemical changes reflected in the cheese aroma and flavor [39]. The distinctive sensory characteristics of Parmigiano Reggiano cheese can be considered as evolving throughout the ripening of the cheese.