## 3 Results and Discussion
### Volatiles  
A total of 41 volatiles (Table 5) were tentatively identified from the oil samples based on the database NIST MS and comparison to external standards. Examples of total ion chromatograms for whole fish and by-products from the SPME-GC-MS are presented in Figures 15 and 16, respectively. Retention indices (RIs) of the volatiles with retention times of 12.93 min or higher in Baltic herring crude oils were calculated based on the retention times of these compounds and _n_-alkane standards (Table 5). Most of the identified volatiles were secondary volatile compounds formed from lipid oxidation including propanal, butanal, 1-penten-3-ol, hexanal, heptanal and nonanal. A larger volatile content indicates a higher degree of lipid oxidation in fish oil, because the majority of the volatile compounds are oxidation products. The oils produced from whole fish had a larger total volatile content compared to those from by-products (Figure 1). Based on the total ion chromatogram of HS-SPME-GC-MS analysis, the most abundant volatiles in oils extracted from whole fish were 2-propenal, 2-ethylfuran, 1-penten-3-ol, hexanal and two unidentified compounds (unknown 1 and 2), whereas in the oils extracted from by-products, the most abundant volatile compounds were propanal, 2,3-butanedione, 2-ethylfuran, 1-penten-3-ol, 2,3-pentanedione, unknown 2, and (_E,E_)-2,4-heptdiagnal. Hexanal and 1-penten-3-ol are typical oxidation products from n-3 PUFA degradation and they are both related to unpleasant odors and flavors [37]. Both of these compounds have shown increases in Baltic herring during storage, indicating oxidation of long-chain unsaturated FAs [38]. (_E,E_)-2,4-heptdiagnal is another oxidation compound related to rancid and fishy odors. It has also been detected in oxidized herring oil and could be a good indicator for the oxidative status of fish oil [39]. Furthermore, 2-ethylfuran has been detected in fish, for example triploid rainbow trout, and it is derived from the oxidation of n-3 PUFAs. The odor of the compound is described as \"rubber\", \"pungent\" and \"green bean\" [40; 41]. Out of all the identified volatile compounds, 17 were aldehydes including acetaldehyde, 2-propenal, propanal, potanal, 2-methylbutanal, 3-methylbutanal, (_Z_)-2-butenal, (_E_)-2-pentenal, hexanal, 2-methyl-4-pentenal, (_E_)-2-hexanal, heptanal, (_E_,_Z_)-2,4-hexadienal, (_Z_)-2-heptenal, octanal, (_E_,_Z_)-2,4-heptdiagnal and nonanal. Aldehydes are secondary oxidation products, which can also be measured with AV. The correlation between aldehydes and AV in the oils extracted from whole fish had a correlation factor of R\({}^{2}\) = 0.80, whereas the corresponding one for by-products was R\({}^{2}\) = 0.89 (Figure 2). The volatile data were analyzed with principal component analysis (PCA). The volatile data from whole fish (Figure 16) and by-products (Figure 16) were analyzed separately. The figures include scores and correlation loadings of the most important principal components (PCs). PCs 1 and 2 explain a total of 74% of the variance in the volatile data of oils extracted from whole fish. PC2 allows a quite good separation between faster extraction processes (35 min, lower scores on PC2) and slower ones (70 min, higher scores on PC2) independently to the enzyme used. The most influential compounds for PC2 are 2-pentylfuran, trans-2-(2-pentenyl)furan, 2-nonanone, 2,3-butanedione and heptane on the positive side of the X-axis, while 1-penten-3-ol was influential on the negative side. PC1 and PC3 (Figure 16,d) explain 64% of the variance in the volatile data. PC3 explains only 8% of the variance but it allows a rather good separation of the Alcalase-extracted samples from those extracted with the other two enzymes. The correlation loadings show that most of the volatiles are grouped close to each other on the negative side of the Y-axis. The only volatile on the positive side of the Y-axis with a significant impact (inside the further ellipse)is 2-methylbutanal.