## 3 Results and Discussion
### Functional Properties  
The functional properties of fish proteins are presented in Figure 6. Protein solubility in water (Figure 6A) results follow the consensus that protein hydrolys are extremely soluble due their low molecular mass and high concentration of ionizable groups [55] whereas protein concentrates are less soluble due to protein denaturation caused by heating during processing [56; 57]. However, if only the middle soluble part of heated fish-water suspension is collected in protein concentrate production, the proteins have higher solubility ranging from 63.4% to 87% as demonstrated by [PERSON] et al. [58]. Solubility is a crucial functionality in proteins as it affects proteins' WHC, gelation, foaming, and emulsification abilities and thus the usability of the product in foodstuffs [55; 59]. FBC is an important function in a protein since the fat-binding ability affects the mouthfeel and texture of the food [60]. FBCs were low in all samples (Figure 6B) compared to previous literature with hydrolyses and concentrates [58; 61]. In comparison, the hydrolyzed samples had higher FBC than concentrated samples, since with enzymatic hydrolysis more hydrophobic peptides are available [55]. Protein ingredients need to possess some water holding capacity to function in a food matrix since it fundamentally affects the texture properties such as mouthfeel and juiciness [60]. WHC of the food-grade samples is presented in Figure 6C. The WHC of CONC samples was good compared to plant and soy protein and very similar to whey protein [60]. Fish meals have been reported to have even higher WHC [56]. Hydrolyzed samples had poor WHC compared to previous research [62]. Hydrolysates were expected to have higher WHC due to hydrolyses' low molecular weight and high protein-water interactions, but the solubilization of hydrophilic protein groups might affect WHC [55]. Foaming properties of proteins affects the body, smoothness, and lightness of the food [60]. Hydrolyzed samples showed better FC than the protein concentrates (Figure 6D). Enzymatic hydrolysis has been shown to improve FC in various matrices [55]. The FC of the hydrolyzed samples was over 200%, which is quite high compared to previous research [8; 63]. However, good FC indicates a suitable degree of hydrolysis and high solubility [60]. High solubility and hydrophobic amino acids have been associated with better FS [55]. The effect of hydrophobic amino acids does not seem to be significant with  
Figure 5: Malondialdehydedehyde (MDA) contents (nmol MDA/g powder “as is”) of commercial fish protein concentrates, hydrolyses, and hydrolyzed collagen. Fish protein concentrates = CONC1–3, fish protein hydrolyses = HYDR1–2, hydrolyzed fish collagen = COLL1. Different letters indicate a statistically significant difference with ANOVA post hoc pair comparison (\(p<0.05\)). fish proteins, since HYDR1 has the most hydrophobic amino acids but has the poorest FS (Figure 6E). Figure 6: Functional properties of commercial fish protein concentrates, hydrolysates, and hydrolyzed collagen. (**A**) Protein solubility, (**B**) water holding capacity (WHC), (**C**) fat binding capacity (FBC), (**D**) foaming capacity (FC), and (**E**) foaming stability (FS). In figure (**E**), the results are compared by timepoints between the samples. Different letters indicate a statistically significant difference with ANOVA post hoc pair comparison (\(p<0.05\)). Fish protein concentrates = CONC1–3, fish protein hydrolysates = HYDR1–2, hydrolyzed fish collagen = COLL1. Gelling properties of proteins affects the texture of food, mainly juiciness, body, and mouthfeel [60]. Unfortunately, none of the samples showed any gelation properties at 15% concentration. To form a rigid gel network, proteins need to form covalent bonds and aggregate between denaturized protein strands [55]. The lack of gelling properties can be a result of a too-high degree of hydrolysis [8]. With concentrates, the low solubility and higher fat content can be factors behind lack of gelation properties.