# Chemical Compositional Changes in Over-Oxidized Fish Oils
## Discussion  
### Hoki Liver Oil Over-Oxidation  
The present study reproduced the oxidative conditions employed by [PERSON] et al. (2018) as well as possible, even though a precise description of the experimental setup, such as the wavelength range and intensity of the illumination used, the rate of oxygen gas delivery, and the geometry and material of the vessel used for the incubation were not reported. By inspecting the temporal profile of PV in the present study, it was nevertheless possible to assess at which degree of oxidation the chemical changes would be approximately similar to those that may have occurred in the original study. [PERSON] et al. reported a PV of 48.8 meq O\({}_{2}\)/kg for the hoki liver oil after one month of accelerated oxidation ([PERSON] et al., 2018). The final PV attained in the current study was \(\approx\)125 meq O\({}_{2}\)/kg, indicating that the conditions applied in the present study were more intense. In the present study, a similar level of hoki liver oil oxidation was attained at approximately 15 days. Remarkably, only a relatively small increase in secondary oxidation was found in the study by [PERSON] and colleagues; after 30 days, p-AV had increased from 0.6 to 4.5, which is a value still well within the quality requirements for omega-3 EPA/DHA oils. For comparison, the present study showed a \(\approx\)3-fold higher p-AV (13.3) at 15 days and a \(\approx\)10-fold higher p-AV (43.8) at 30 days. A range of other indicators, such as oligomers and volatiles, confirmed the marked degree of secondary oxidation that was obtained. One of two plausible reasons may explain the apparently low level of secondary oxidation in [PERSON] et al. (2018). Either fatty acid hydroperoxyl radicals were not effectively further transformed into secondary oxidation products, which is highly improbable, or a substantial portion of the volatile _para_-anisidine-reactive aldehydes formed as secondary oxidation products were stripped out of the incubations more effectively than in the present study. The permanent oxygen gas stream is likely continuously removing a portion of the volatiles formed, suggesting a more intense contact (smaller bubbles) or longer contact time (a more vertically shaped container) of the gas bubbles with the oil in the study by [PERSON] et al. Both oxidation conditions employed in the present study represent good examples of \"open\" incubations, in which volatile compounds formed in the oil are easily lost to ambient air as the equilibrium is shifted toward net evaporation [13; 14; 61]. Since volatile compounds produced during the sustained oxidation incubations can evaporate from the incubations, the pattern of detected volatiles needs to be interpreted as those present in the oils at the moment of sampling. The accumulation of secondary oxidation products with boiling points below ambient temperature (oxidation condition A) or 50 \({}^{\circ}\)C (oxidation condition B) will not occur; i.e., they will already have evaporated to a large extent from the oils at the time of sampling. In Condition A, the formation of volatile secondary oxidation products is likely to be underestimated even more, since the continuous oxygen gas stream will strip volatiles out of the incubations faster than the spontaneous evaporation of volatiles from the incubations. This may explain the lower concentrations of volatiles observed in both oil types under Condition A compared to B. This situation does not correspond to the situation of encapsulated or bottled fish oils, which are closed systems without any, or very limited, gas exchange. It is probable though that the extent of secondary oxidation was markedly under-estimated in the study by [PERSON] et al., and that the levels of the secondary oxidation products with low volatility, such as the isoprostanoids and oxysterols, could be present at comparable levels as in the current study at day 15, or between day 15 and 30.