## 3 Results
### Lipid Classes  
The composition of the lipid classes retained in the protein isolates and hydrolyses, and the lipid classes in the fish raw materials were assessed semi-quantitatively by analyzing the lipid classes using LC-MS. Figure 1 presents the sum peak areas of the detected lipid classes of lipids extracted from the protein isolates and hydrolyses, as well as relative changes in lipid composition compared with the lipid composition of the raw material. All peak areas are presented in an equal amount of lipids. [PERSON] et al. [47] reported B. herring lipids to comprise 53.5-91.2% TAGs, 7.6-41.3% PLs, 0-2.2% diacylglycerols (DAGs), 0.6-1.9% cholesterol, and 0.1-1.2% free fatty acids (FFAs), the proportions varying depending on the season. The highest relative content of TAGs was observed in fish caught in the late autumn when the PL content was at its lowest, whereas the lowest relative TAG content was observed in the fish caught in the summer and the PL content was at its highest. Based on semi-quantitative data, the lipids of B. herring used as raw material contained approximately 78.3 \(\pm\) 0.8% TAGs, 18.8 \(\pm\) 0.7% PLs, 2.1 \(\pm\) 0.2% DAGs, 0.2 \(\pm\) 0.0% monoacylglycerols (MAGs), and 0.6 \(\pm\) 0.1% FFAs. The B. herring was caught in September and hence the results are in line with the study of [PERSON] et al. (1985) [47]. The lipids in roach raw material, on the other hand, contained 77.9 \(\pm\) 1.0 TAGs, 7.7 \(\pm\) 0.5 PLs, 11.7 \(\pm\) 0.5 DAGs, 0.7 \(\pm\) 0.1 MAGs, and 2.0 \(\pm\) 0.1 FFAs, having less PLs and more DAGs, FFAs, and MAGs of total lipids, when compared with B. herring. Higher relative contents of DAGs, MAGs, and FFAs can be partly explained by the difference between the two species but possibly also by the hydrolysis of TAGs or PLs during the frozen storage of minced roach (3 months at \(-\)22 \({}^{\circ}\)C) prior to processing. [PERSON] et al. [29] reported findings that strongly indicated high endogenous protease activity in the roach raw material, as opposed to B. herring. It is possible that endogenous lipase activity may also be higher in roach compared with B. herring, and thus explain the relatively high proportions of DAGs, MAGs, and FFAs observed in roach raw material. Classes of lipids in fish protein isolates and hydrolyses have seldom been reported, apart from some studies that reported the PL content [15; 24]. According to the results of the present study, there were significant differences in the composition of lipids retained in the protein isolates or hydrolyses obtained with different processes (Figure 1). In the case of roach isolates/hydrolysates, the lipids in alkaline and acidic pH-shift-processed isolates contained statistically significantly (\(p<0.05\)) more PLs/total lipids compared with enzymatically produced hydrolyses. Acid- and alkali-extracted roach isolates also exhibited 167% and 85%, respectively, higher peak areas of PLs compared with the lipids in the roach raw material. Similarly, pH-shift-processed protein isolates from B. herring, contained a statistically significantly higher proportion of PLs compared with enzymatically produced hydrolyses of the same raw material (Figure 1B). However, when compared with the B. herring raw material, a statistically significantly higher peak area of PLs was only seen in the case of the acidic protein isolate (Figure 1D). The observed increase in DHA (22:6 n-3) in the acidic protein isolate of B. herring may have been due to the increased PL content, as membrane PLs are rich in PUFAs [23]. Membrane PLs are suggested to be the main substrates of muscle tissue lipid oxidation [15] and therefore their removal during  
Figure 1: Sum peak areas of triacylglycerlycerols (TAGs), diacylglycerols (DAGs), monoacylglycerols (MAGs), free fatty acids (FFAs), and phospholipids (PLs) in protein isolates and hydrolyses of roach (**A**) and B. herring (**B**), and relative changes in lipid class peak areas in lipids of protein isolates and hydrolyses compared with the fish raw material (**C,D**).