## 3 Results and Discussion
### Sensory Evaluation, Odor-Active Compounds, and Other Explanatory Factors
Trimethylamine has been determined to be one of the most important fishy odor sources in fish products [73]. All the samples, apart from COLL1, contained trimethylamine. The prevention of TMA formation is crucial for producing fish proteins with mild odor and taste. Recently, [PERSON] et al. [74] demonstrated enzyme-mediated conversion to TMAO as a working approach to reduce the TMA content in fish proteins. 3-Methylbutanal is formed by the degradation of isoleucine and the concentration increases as the degradation of fish progresses [75]. 3-Methylbutanal has previously been found, for example, in surimi samples [76] and Baltic herring [28]. This sour-bread-smelling compound could be affecting the overall odor and taste intensity in processed fish products. The concentrates were found to be fishier and more sea or seaweed-like than the hydrolysates, which have been reported to be an issue with all fish proteins regardless of the processing method [8; 22; 32; 58]. In addition to TMA, fishiness of the sample has largely been associated with secondary lipid oxidation products [32; 68; 77]. However, fishiness did not seem to be associated with TBARS, since the values were the same between, e.g., samples HYDR2 and CONC3, but the fishy odor and taste intensity differed largely between the samples. Association between fishiness and lipid oxidation was found from the odor-active volatile compounds. Fish protein concentrates had the most abundant amounts of lipid oxidation derived volatile compounds, such as 2,3-butanedione, (\(Z\))-4-heptenal, octanal, heptanal, and hexanal. These compounds have been associated with fishy odor and taste development [73]. 2,3-Butanedione, (\(Z\))-4-heptenal, and hexanal were previously reported as significant odorants in pH-shifted protein isolate from Baltic herring [28]. COLL1 was the whitest, and mildest in taste and odor, which are desired properties of fish protein. The COLL1 was positioned on the opposite side of the ellipse from odor-active compounds such as TMA, (\(Z\))-4-heptenal, methional, and 3-methylbutanal (Figure 7). This indicates that this study successfully identified the odor-active compounds of fish commercial fish proteins. However, rotting-cabbage-smelling methanethiol, which was detected also with GC-MS-O, was associated with the COLL1 sample, which could be affecting its mild odor, and thus the odor activity of methanethiol should be taken into consideration. Methanethiol has been previously found in fish sauce [78]. Methanethiol, hydroxyacetone, and 2-heptanone were found in fish proteins for the first time in this study. Figure 7: The Partial Least Squares (PLS) correlation loading of commercial fish protein concentrates, hydrolysates, and hydrolyzed collagen, where the free amino acids, lipid oxidation, peptide profile, odor-active volatile compounds identified with GC-MS-O, color, solubility, water holding capacity (WHC), fat binding capacity (FBC), and foaming capacity (FC) are the predictors (blue) of sensory properties (black). Fish protein concentrates = CONC1–3, fish protein hydrolysates = HYDR1–2, hydrolyzed fish collagen = COLL1. \begin{table}
\begin{tabular}{l l c c c c c c c c c} \hline \hline \multirow{2}{*}{\#} & \multirow{2}{*}{**Volatile Compound**} & \multirow{2}{*}{**RI**} & \multirow{2}{*}{**Identification**} & \multirow{2}{*}{**Oder Description**} & \multicolumn{6}{c}{**Relative Concentration (RC) \({}^{\dagger}\) or Detection Frequency (DP) \({}^{2}\)**} \\ \cline{5-10}  & & & & & **CONC1** & **CONC2** & **CONC3** & **HYDR1** & **HYDR2** & **COLL1** \\ \hline
1 & Trimethylamine & -600 & R1t, MS, O & Fishy, fatty & \(146\pm 16\) & \(100\pm 57\) & \(160\pm 79\) & \(350\pm 75\) & \(57\pm