## 3 Results and Discussion
### Oxidation  
The AVs, PVs and total oxidation products (TOTOX = 2 \(\times\) PV + AV) of enzymatically extracted oils are presented in Table 4. There were differences in AVs and PVs between the treatments. In the extractions from whole fish, AV was the lowest for the 35-min extraction with Alcalase, whereas the highest result was for the 70-min extraction with Protamex. With Baltic herring by-products, the lowest AV was achieved with the 35-min extraction using Neutrase, whereas the 70-min extraction with Alcalase resulted in the highest value, the difference being statistically significant (\(p<0.05\)). A longer extraction time resulted in a higher PV in all except one treatment. In addition to enzymes and extraction times, there was evidently an influence by the raw materials on the PVs of the crude extracts. The difference can be explained by interference of endogenous enzymes in the proteolytic process. With whole fish, the lowest PV was seen in the 35-min extraction with Alcalase, whereas the highest result was from the 70-min extraction with Protamex. For by-products, the lowest PV was from the 35-min extraction with Neutrase, which also had the lowest AV. The highest PV, on the other hand, was in the 70-min Alcalase-extracted oil, which also had the highest AV. According to the TOTOX values, the 35-min extraction with Alcalase was the best for whole fish in terms of oxidative stability. On the other hand, the 35-min extraction with Neutrase was the best for by-products. The Codex standard for refined fish oil states that the acceptable limits are PV \(\leq\) 5 meq/kg, AV \(\leq\) 20 and TOTOX \(\leq\) 26 [34]. In relation to these limits, all oil samples had a compromised oxidative status. A typical oil extraction process uses antioxidants, such as \(\alpha\)-tocopherol or synthetic antioxidants, as oxidation protectants which were not used in this study. Further, the oils were not refined. During the typically used chemical refining process, oxidation products are removed, which improves both AV and PV. Additionally, fish oil rich in EPA and DHA can lead to over-estimation of AV because the absorbance intensity is dependent of the unsaturation level of the aldehydes [35]. Recently, the suitability of PV and AV as methods to determine the oxidative status of fish oils is being reconsidered and alternatives have been discussed [30; 34]. Many oxidative compounds are derived from the degradation of long-chain PUFAs [36]. The degradation can also be visible as decreased DHA and EPA contents. In this study, Neutrase- and Protamex-extracted oils from whole fish had lower (\(p<0.05\)) DHA and EPA contents after the 70-min extraction compared to the 35-min extraction. On the contrary, Alcalase had the opposite effect for both n-3 PUFAs. When looking at the total PUFAs, Neutrase and Protamex show a decreasing trend with a longer extraction time, the difference being statistically significant for Neutrase but not Protamex. The PUFA content of the Alcalase-extracted oil increased between the time points (\(p<0.05\)). These findings do not correlate with the changes in AVs and PVs. Alcalase shows a significant increase in