## 3 Results
### Peroxide Values, Protein Carbonyls and Oxidation-Related Volatile Compounds
The ratio of 2-ethylfuran/hexanal was significantly higher in the pH-shift-processed isolates from B. herring compared with roach, which could be associated with the higher content of n-3 PUFAs in isolates from B. herring compared with the isolates from roach (Table 3), as 2-ethylfuran is produced from hydroperoxides of n-3 PUFAs through peroxyl radical cyclisation, while hexanal can be produced by scission of the lipid molecules on either side of the radical and is more associated with the lipid oxidation of n-6 PUFAs [59]. [PERSON] et al. [54] reported the volatile composition of alkali- and acid-extracted protein isolates, surimi, and mince of farm-raised hybrid catfish (Clarias macrocephalus \(\times\) Clarias gariepinus). Acid-extracted isolate showed the highest content of certain oxidation-related volatiles, such as octanal, nonanal and decanal, and also the highest TBARS values. In the present study, alkali-extracted protein isolate was more abundant in oxidation-related volatiles and was also the most oxidized based on PV and carbonyls. 2-Pentylfuran and 2,6(_E_,_E_)-nonadienal, and in the case of roach also 2(_E_)-hexenal, were only detected in the pH-shift-processed isolates (A and B) and not in the hydrolyses produced by Protamex, Neutrase and Corolase. This difference may be related to the higher lipid content (Table 1), the higher DHA content (Table 3) and the higher content of PLs (Figure 1) in the pH-shift-processed isolates compared with the enzymatically hydrolyzed samples. In general, enzymatically produced hydrolyses showed lower volatile formation than the isolates produced by pH shift, indicating less lipid oxidation. In the case of roach, hexanal, 1-penten-3-ol and propanal, and in the case of B. herring, hexanal, 1-penten-3-ol, and heptanal were the most abundant volatiles in the hydrolyses. 1-Penten-3-ol was the only volatile, out of the selected volatile lipid oxidation indicators, that was equally or more abundant in the hydrolyzed samples than in the pH-shift samples in both fishes. 1-Penten-3-ol was one of the most abundant volatile compounds in all hydrolysates of Atlantic salmon by-products produced using varied process parameters, such as enzyme/substrate ratio and hydrolysis time [62]. In the roach hydrolysates, the peak area of propanal was also higher than in the acidic and alkaline isolates from roach. The intensities of volatile compounds derived from lipid oxidation were in line with PV and protein carbonyl measurements. Based on PV and volatile data, protein isolates and hydrolysates produced from roach showed slightly less signs of lipid oxidation compared with the respective isolates and hydrolysates produced from B. herring (Table 1). The difference is most likely due to differences in lipid composition, an extra centrifugation step in the production of B. herring protein isolates and hydrolysates (Supplementary Figure S1), and possible differences in the levels of pro-oxidants, such as hemoglobin. In a recent study by [PERSON] et al. [45], pH-shift-extracted protein isolates from herring and cod backbones were seen to be more susceptible to lipid oxidation than the product from salmon, and oxidation was found to correlate significantly with hemoglobin and iron contents. On the contrary, total content of PUFAs showed a low and non-significant correlation. In the present study, the proportion  
Figure 4: Peak areas of seven oxidation related volatiles, analyzed using headspace solid-phase microextraction-gas chromatography-mass spectrometry, in protein isolates from roach (**A**) and B. herring (**B**). Different letters indicate a statistically significant difference (\(p<0.05\)) between samples. of PUFAs did not differ to a large extent between the isolates and hydrolyses of roach vs. B. herring (25.4-33.5 vs. 27.2-33.8%). B. herring raw material and protein isolates and hydrolyses extracted from it, however, contained 2-8 times as much PLs compared with roach raw material and protein isolates and hydrolyses.