## 3 Results and Discussion
### Principal Component Analysis (PCA)  
Principal component analysis (PCA) was performed on the entire volatile compounds profile of _P. erygii_ and _P. ostreatus_ strains, in order to investigate the existence of any groupings or associations (Figure 2). The first two principal components, explaining 98% of the data set variance (i.e., PC1: 75.4% and PC2: 22.6%), allowed a clear discrimination of two _Pleurotus_ species and eight strains studied (Figure 2a,b; ellipses drawn at a confidence level of 0.95).  
However, due to the high number of identified compounds, it was impossible to acquire some additional information regarding the volatile compounds that were responsible for the species/strain discrimination through the interpretation of their loadings. Therefore, a PCA was performed on the basis of the volatile compounds' classes, which revealed a clear separation at species and strain level (Figure 3a,b, respectively). Furthermore, the first two principal components explained an even higher percentage (98%) of the data set variance (i.e., PC1: 95.7% and PC2: 2.9%). Separation is taking place in both axes (PC1 and PC2), with _P. erygii_ strains mainly placed in the 4 th quartile and _P. ostreatus_ strains in the 2 nd quartile. The observation of the loadings (i.e., the compound classes) revealed a positive effect of ketones, alcohols and toluene at separating _P. ostreatus_ strains (Figure 3c). On the other hand, aldehydes and fatty acid methyl esters presented a stronger effect on the separation of _P. erygii_ strains. Both observations are in agreement with the previously determined content in volatile compounds for these particular species/strains.  
PCA was also performed on the basis of volatile compounds' classes for cooked (roasted) and raw (fresh) _P. erygii_ strain CS3 and _P. ostreatus_ strain LGAM 3002 cultivated on wheat straw, to detect any groupings and to acquire additional information. The PCA allowed a clear discrimination of samples both in terms of species/strains as well as of the roasted/raw treatment (Figure 4a,b). As regards the latter, in particular, separation took place across PC1 (X-axis) with the fresh samples being placed on the negative side of PC1 and the roasted on the positive side. By examining the PCA loadings, it appears that pyrazines, sulfur compounds and toluene exert a significant role at separating the roasted samples. However, PCA was not able to discriminate mushrooms according to the cultivation substrate on the basis of the volatile compounds or compound classes studied (data not shown). This possibly indicates a low effect of the cultivation substrates on the  
Figure 2: Score plot of principal component analysis and discrimination on the basis of the volatile compounds detected of _Pleurotus_ mushrooms at (**a**) species and (**b**) strain level (Pos: _P. ostreatus_; Per: _P. erygii_).  
mushrooms content in volatile compounds, contrary to the outcomes of our previous works regarding antioxidant properties and bioactive microconstituents like ergosterol, phenolic and terpenic compounds as well as free amino acids in the same mushroom species [5; 44].  
Figure 3: Score plot of principal component analysis and discrimination on the basis of classes of volatile compounds detected in _Pleurrotus_ mushrooms at (**a**) species and (**b**) strain level, as well as their (**c**) loadings. (Pos: _P. ostratus_; Per: _P. errggii_).