## 3 Results and Discussion
### Sensory Evaluation, Odor-Active Compounds, and Other Explanatory Factors  
The color of the samples is presented in Table 6. [PERSON] et al. [8] found hydrolyzed herring and roach proteins whiter than pH-shifted alternatives. However, [PERSON] et al. [58] found even higher whiteness values when studying herring and arrowtooth flounder (_Atheesthes stomias_) protein powder produced by heat treatment. [PERSON] et al. [66] determined that lipid oxidation plays a role in yellow discoloration. This negative correlation was also seen in the present study: HYDR1 had the highest TBARS values (100.5 nmol MDA/g) and the lowest whiteness value of all the hydrolyzed samples (66.4). In addition, HYDR1 had the highest yellow-blue value (27.0) of all samples. CONC1 had a significantly higher peroxide value (15.3 meq/kg oil) and TBARS (33.1 nmol MDA/g) values of all CONC samples and the lowest whiteness value (62.5) of all the samples. The appearance of the fish proteins could possibly be improved if lipid oxidation is prevented. The intensities of the sensory properties are found in Table 7, and the spider plot of mean values is found in the supplementary materials (Figure S1). The sensory evaluation clustered the proteins according to their processing method, which has been observed previously by [PERSON] et al. [8]. However, all the proteins were equally intense in flavor and odor except for COLL1. The PLS correlation loadings plot of explanatory factors for the sensory properties of fish proteins is presented in Figure 7. Ummi taste was strongly associated with HYDR1, which could be due to relatively large amounts of free glutamic acid and aspartic acid compared to other samples [67; 68]. Additionally, complex flavor formation has been associated before with a large number of free amino acids [67], which could be affecting the taste of HYDR samples. In GDA, hydrolyses were found to be more bitter and more metallic than the concentrates. The results were in accordance with previous studies that demonstrated bitterness to be the main sensory challenge with fish hydrolyses [9; 68; 69; 70]. The peptide size analysis showed that the HYDR 1-2 samples consisted mostly of peptides under 10 kDa, which have \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **L*** & **a*** & **b*** & **Whiteness** \\ \hline CONC1 & 68.4 b \(\pm\) 0.44 & 3.3 b \(\pm\) 0.06 & 19.9 b \(\pm\) 0.19 & 62.5 b \(\pm\) 0.30 \\ CONC2 & 73.8 c \(\pm\) 0.26 & 2.3 c \(\pm\) 0.07 & 17.4 c \(\pm\) 0.27 & 68.4 c \(\pm\) 0.28 \\ CONC3 & 84.6 a \(\pm\) 0.15 & 0.1 a \(\pm\) 0.03 & 13.3 a \(\pm\) 0.05 & 79.6 a \(\pm\) 0.12 \\ HYDR1 & 80.2 d \(\pm\) 0.10 & 2.7 d \(\pm\) 0.26 & 27.0 d \(\pm\) 0.23 & 66.4 d \(\pm\) 0.17 \\ HYDR2 & 87.5 e \(\pm\) 0.21 & 1.7 e \(\pm\) 0.04 & 21.1 e \(\pm\) 0.11 & 75.4 e \(\pm\) 0.14 \\ COLL1 & 93.2 f \(\pm\) 0.18 & \(-\)1.8 f \(\pm\) 0.04 & 13 a \(\pm\) 0.14 & 85.2 f \(\pm\) 0.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The color measurement of the commercial fish protein concentrates, hydrolyses, and hydrolyzed collagen with mean and standard deviation of five replicate measurements. Different letters within the same row indicate a statistically significant difference with ANOVA post hoc pair comparison (_p_ < 0.05). L* = lightness, a* = red-green, b* = yellow-blue. previously been associated with a bitter taste [68]. Peptides with exposed hydrophobic side chains are associated with a bitter taste [71], and with smaller peptides, more side chains are exposed leading to a more bitter taste. Contrary to the results of [PERSON] et al. [70], the secondary lipid oxidation products did not seem to affect the intensity of bitterness in this study: HYDR2 was perceived as more bitter than HYDR1, which had significantly higher TBARS values. Hydrophobic amino acids (Trp, Phe, Val, Leu, Iso, Try) are a key factor affecting the bitterness of the protein [72].