# Rheological Study of an Extruded Fish Diet with the Addition of Hydrolyzed Protein Flour
## 3 Results
### Effect of Temperature  
Figure 2 shows the effect of temperature variation on the shear stress and apparent viscosity in the treatments studied with a moisture content of 22%. The shear stress and apparent density have an opposite behavior compared to the shear rate, and as the temperature increases, the shear stress and the evident viscosity decrease. It is known that an increase in temperature results in a reduction of the viscosity of the molten polymer [13; 14; 21]. During the extrusion process, polymers such as starch and protein can present different degrees of transformation and molecular degradation under a thermomechanical treatment, such as extrusion cooking [33]. It is reported that the increase in speed can reduce viscosity, and although the average residence time is reduced, the intensity of mechanical treatment, transformation, and macromolecular degradation increases, and therefore, results in lower viscosity in a molten state, being more critical at a lower temperature [15].  
Increasing the temperature increases the molecular degradation, decreasing its size and growing mobility by increasing kinetic energy and reducing the volume occupied by each molecule [34]. Similarly, an increase in temperature generates an increase in the gelatinization of starch [10]. Usually, this would lead to an increase in viscosity. However,  
Figure 2: Effect of temperature variation on shear stress (**a**) and apparent viscosity (**b**) in the treatments studied with a moisture content of 22% and 35% HPF.  
in the analyzed treatments, it is observed that the viscosity decreases, suggesting that the effect of molecular degradation and the increase in kinetic energy are more significant than gelatinization on the change in thickness, as shown in Figure 2.  
The temperatures used in the extrusion process led to denaturation of proteins due to the irreversible alteration of non-covalent interactions (hydrophobic and electrostatic), wherein the secondary, tertiary, and quaternary structures of the proteins will weaken due to temperature and shear forces [34]. The proteins will unfold and align as the material travels through the extruder screw and the head [27]. As shown in Table 7, the value of \(n\) approaches one at a higher temperature, behavior attributed to a decrease in the size of starch and protein molecules generated by fractionation at high temperatures [11].  
Similar studies identified the effect of two different extrusion cooking (CE) conditions, milder and more severe, on the gelatinization of starch, the dough's rheological properties, and the techno-functional parameters of lentil flour. The results showed that the extruded flours also showed a lower degree of starch retrogradation than the native flour, in which they reported that the high speed of the screw has a negative effect on gelatinization, decreasing the initial viscosity and increasing the maximum viscosity of the flours extruded into a viscoamylographic profile. Therefore, the high shear causes starch degradation with the melting of the crystals of amylopectin molecules and dextrinization [12].