## 3 Results and Discussion  
### Volume, Weight, and SV  
The post-cooling constant weight and subsequent specific volume of the entire loaf of bread were measured on an electronic balance using the rapeseed displacement method. The results are presented in Table 2. The bread volume significantly decreased (\(p\) < 0.05) with increasing SPI use, and the SV significantly decreased (\(p\) < 0.05) with increasing weight. The SVs of the bread in all groups were within the standard range of 3.5-6 mL g\({}^{-1}\). The external appearances of the breads in all groups are shown in Figure 1. Based on the external appearance, irregular shapes were formed on the bread surfaces when SPI use exceeded 6%. This indicates that different proteins were not completely homogenized during dough fermentation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Group** & **Volume (mL)** & **Weight (g)** & **Specific Volume (mL g\({}^{-1}\))** \\ \hline Control & 3226.00 \(\pm\) 64.09  & 5.22 \(\pm\) 0.10 [FOOTNOTE:]Footnote : *: Means within the same column values with different superscript letters are significantly different (\(p\) < 0.05).[ENDFOOTNOTE] \\
2\% & 3104.67 \(\pm\) 38.28  & 5.00 \(\pm\) 0.04 [FOOTNOTE:]Footnote : *: Means within the same column values with different superscript letters are significantly different (\(p\) < 0.05).[ENDFOOTNOTE] \\
4\% & 2860.67 \(\pm\) 10.02  & 4.60 \(\pm\) 0.03 [FOOTNOTE:]Footnote : *: Means within the same column values with different superscript letters are significantly different (\(p\) < 0.05).[ENDFOOTNOTE] \\
6\% & 2773.67 \(\pm\) 46.36  & 4.44 \(\pm\) 0.09 [FOOTNOTE:]Footnote : *: Means within the same column values with different superscript letters are significantly different (\(p\) < 0.05).[ENDFOOTNOTE] \\
8\% & 2390.00 \(\pm\) 39.74  & 3.82 \(\pm\) 0.08 [FOOTNOTE:]Footnote : *: Means within the same column values with different superscript letters are significantly different (\(p\) < 0.05).[ENDFOOTNOTE] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Effects of partially substituting high-gluten flour with soy protein isolate on the volume, weight, and specific volume of bread. Figure 1: The effects of partially substituting high-gluten flour with soy protein isolate on the external appearance and cross-section of bread. (**A**) External appearance. (**B**) Cross section. SPI affects the structure and specific volume of bread because soy protein and wheat protein form covalent, disulfide bonds. These bonds affect the arrangements of gluten proteins and other macromolecules within the dough, forming a dilution effect that causes the dough to lose its gas-retaining ability [17; 47]. Additionally, the substitution of soy protein reduces the size of the bread, which could be due to the variation in the denaturation temperature, foaming properties, and soy protein composition or properties. [PERSON] et al. [48] highlighted that gluten proteins are primary gluten networks formed by prolamin and glutenin. Since wheat protein has a lower denaturation temperature, it stabilizes the dough structure during the early phases of baking. Soy protein consists of \(\beta\)-conglycinin and glycinin, which have higher denaturation temperatures. Hence, the presence of soy protein affects the form and stability of the dough structure by weakening it. Meanwhile, [PERSON] et al. [49] proposed that, because of its higher foaming capacity and its influence on the stability of multiphase food systems, SPI affects the porosity of bread structure. The SPI-fortified bread was heavier, probably because of the increased water retention rate of soy protein. The \(\beta\)-conglycinin and glycinin in soy protein are spherical, and soy protein is more hydrophilic than wheat gluten because of its amino acid composition [50]. Additionally, soy protein retains water molecules and prevents them from evaporating during the baking process [51]. Thus, adding an equal ratio of water to the raw ingredients increases the weight of the SPI-fortified bread, which also has a lower baking loss.