# Rheological Study of an Extruded Fish Diet with the Addition of Hydrolyzed Protein Flour
## 3 Results
### Effect of Hydrolyzed Protein Flour  
Enzymatic hydrolysis in the starches is important as it offers the environment more favorable breakdown conditions and does not involve solvents or chemical reagents. It leads to significant changes in the functional properties of starches, which is essential in defining the industrial use of starches. The enzymatically hydrolyzed starches have lower  
Figure 3: Effect of the moisture content variation on the shear stress (**a**) and apparent viscosity (**b**) in the treatments studied with a protein flour content of 35% and 115 \({}^{\circ}\)C.  
paste viscosities and swelling power, thus influencing the functional characteristics of the flour [39]. Figure 4 shows the effect of the variation of the hydrolyzed protein flour content on the shear stress and apparent viscosity at 125 \({}^{\circ}\)C. Therefore, as the shear rate increases, the shear stress increases, and the viscosity decreases. Furthermore, as the amount of hydrolyzed flour in the formulation increases, the shear stress and viscosity values tend to grow. Due to the low molecular weight of the peptides from hydrolyzed protein flour, it could be inferred that they tend to be located between the starch chains, acting both as plasticizers of the matrix and as crosslinking agents between the different extrudate fractions (free sugars, dextrins, peptides of various molecular weights) [30]. Starch, for its part, as the main component in the evaluated matrix (52%) interacts directly with proteins since they contain several hydrophilic groups such as -OH, -NH2, -COOH, and -SH in the side alkyl chains, which can form bonds with starch, affecting the rheological properties of the material [40]. Generally, an increase in hydrolyzed protein slightly increases the viscosity, as shown in Figure 4.  
Figure 4: Effect of variation in hydrolyzed protein flour content on shear stress (**a**) and apparent viscosity (**b**) at 125 \({}^{\circ}\)C and 22% moisture. Within hydrolyzed protein flour content, as shown in Figure 4, peptides that promote the sliding of starch chains would act as plasticizers, while peptides with exposed amino acid residues with the ability to react with dextrin and sugar end groups would act as crosslinking agents, reducing molecular mobility and therefore generating a viscosity increase [30], according to the results obtained. The increase in viscosity of the treatments studied suggests that crosslinking has a more significant effect on the viscosity of the treatments than the possible plasticizing effect of the low molecular weight peptides, which would also be consistent with the increase in shear stress promoted by the rise [40; 41; 42; 43]. Similar studies reported the increase in viscosity due to gel formation under extrusion conditions, related to the denaturation of myosin, mainly responsible for gelation, aggregation, and various forms of interaction phenomena [44; 42], forming a three-dimensional network that the terminal groups of the molecules can establish via hydrophobic interactions, as well as disulfide bridges. The exposed hydrophilic amino acid residues are free to interact and react with the other components, such as sugar residues and the aqueous medium. In contrast, the hydrophobic amino acid residues align, forming intermolecular hydrophobic bonds, trapping apolar molecules, such as lipids or air present in formulations, inside [27]. The protein-starch interactions due to the formation of amylose-protein complexes [45; 46], the number of insoluble aggregates formed [46], as well as the crosslinking of the peptides during Maillard reactions [41], could affect increasing the viscosity and resistance to the flow of the material, requiring greater torque during the extrusion process with the addition of hydrolyzed protein flour.