## 3 Results and Discussion
### The Influence of SPI Addition on the Thermo-Mechanical Properties of the Gluten-Free Mixtures
Figure 3: Mixolab curve of dough prepared mixtures of quinoa flour (QF), rice flour (RF) and (**a**) soluble fraction of soy protein hydrolyses (sSPIs), (**b**) insoluble fraction of soy protein hydrolyses (rSPIs) and (**c**) total soy protein hydrolyses (hSPIs) obtained with bromelain (B), Neutrase (N) and trypsin (T). The Influence of Soluble Soy-Derived Peptides' Addition on the Thermo-Mechanical Properties of the Gluten-Free Mixtures  
Regardless of the exogenous enzyme used for SPI hydrolysis, the addition of sSPI to the composite flour notably reduced the consistency of the dough during Ph I and Ph II (Figure 3). It is known that the protein from quinoa contains mainly globulins and albums, stabilized by disulfide bonds, the cysteine residues from these fractions playing, therefore, an important role in protein contribution to the overall behavior of the dough matrix [32]. In the composite flour, this behavior is balanced by the presence of high-molecular-mass glutelins, which are the proteins prevailing in the rice flour [33]. The addition of sSPI, containing low-molecular-mass peptides (Figure 1), disturbed the protein network of the dough obtained from the composite flour. [PERSON] et al. [34] reported a deterioration of the protein network in the dough prepared from wheat flour supplemented with soy protein hydrolysates. The authors speculated that the interactions established between the soy peptides and the low-molecular-weight glutenin fractions and gliadin involved disulfide bonds, therefore interfering with the formation of the typical protein network within the dough. Although the number of hydrogen and disulfide bonds was reported to increase, the addition of soy protein hydrolysate weakened the gluten network. [PERSON] et al. [35] noted that soy protein hydrolysates could modify ionic and hydrophobic interactions and covalent and hydrogen bonding, preventing the complete hydration of proteins during dough formation. Another possible explanation for the decrease in dough consistency in Ph I and Ph II could be related to the fact that peptides with low molecular weight have a low water binding capacity. [PERSON] et al. [36] reported the decrease in the apparent viscosity of the soy protein hydrolysate obtained with bromelin as a result of the high solubility of the low-molecular-weight peptides formed. Similar observations were collected by [PERSON] et al. [37] for the soy protein hydrolysate obtained with papain. Taking into account that all Mixolab measurements were carried out at the same WA of 65%, the excess of the water in the system significantly reduced the consistency of the soy peptide-rich dough samples. The minimum consistency of the dough samples enriched in sSPI ranged between 0 and 0.03 Nm and was registered after 1080 s at temperature of 55 \({}^{\circ}\)C. When compared with the control and the dough supplemented with SPI, one can observe that sSPI addition delayed the start of starch gelatinization (Figure 3). Thus, after 1380 s of mixing and reaching the maximum heating temperature specific to the Chopin+ protocol, the C-II of the soy peptide-enriched doughs was 0.32, 0.27 and 0.26 Nm, for samples supplemented with sSPI-B, sSPI-N and sSPI-T, respectively (Table 2). The gelatinization maximum was reached only after 1620 s, at a temperature of 83 \({}^{\circ}\)C, the C4 of the samples ranging between 1.15 and 1.17 Nm (Table 3). At the end of Ph III, the dough reached a temperature of 87 \({}^{\circ}\)C, and a C-III of 1.23 Nm was recorded for the samples with sSPI-B and sSPI-N and of 1.29 Nm for the sample with sSPI-T (Table 2). [PERSON] et al. [38] investigated the possibility of improving the quality of steamed bread by supplementing flour with soy protein hydrolysate obtained with pepsin. They observed that the addition of soy protein hydrolysate interfered with the orderly structure of the starch granules, therefore affecting the swelling properties and causing the increase in the gelatinization temperature and the decrease in the gelatinization enthalpy.