## 3 Results and Discussion
### Moisture Content, \(a_{w}\), and pH Value  
The variations in the moisture content, a\({}_{w}\), and pH are shown in Figure 2. Moisture loss is one of the factors that cause dough staling, while moisture retention promotes chemical or microbial interactions. Therefore, the moisture content is a factor affecting bread quality. In Figure 2A, the moisture content of the control bread on the first day of storage was 37.85 \(\pm\) 0.17%, while the moisture contents of the 2%, 4%, 6%, and 8% SPI-fortified breads were 37.87 \(\pm\) 0.08%, 37.67 \(\pm\) 0.15%, 37.62 \(\pm\) 0.08%, and 37.50 \(\pm\) 0.36%, respectively. On the fifth day of storage, the moisture content of the control had decreased to 36.88 \(\pm\) 0.26%, while the moisture contents of the 2%, 4%, 6%, and 8% SPI-fortified bread had decreased to 37.17 \(\pm\) 0.63%, 36.98 \(\pm\) 0.81%, 37.32 \(\pm\) 0.18%, and 37.30 \(\pm\) 0.48%, respectively. There were no significant changes in the water content of the central bread structure, mainly because, as the dough stales, the water migrates closer to the crust. Thus, there were no significant changes in the moisture content in the center of the bread. This finding is consistent with that of [PERSON] et al. [52]. However, during the sixth day of storage, the moisture content of the control (36.67 \(\pm\) 1.01%) was significantly lower (\(p\) < 0.05) than those of the SPI-fortified breads (37.83 \(\pm\) 0.29-37.30 \(\pm\) 0.36%). During the seventh day of storage, the moisture content of the control (36.37 \(\pm\) 0.75%) was also significantly lower (\(p\) < 0.05) than those of the SPI-fortified breads (37.70 \(\pm\) 0.01-37.43 \(\pm\) 0.43%). This is because soy protein is more hydrophilic than wheat protein, due to \(\beta\)-conglycinin and glycinin, which have higher water-binding capacities. Furthermore, during the preparation of SPI, many proteins had partially denatured or expanded, which increased their water retention capacities. The use of SPI promotes the water retention capacity of bread, which was observed from the sixth day of storage onwards.  
Water activity is closely associated with microbial growth. Most foods become moldy and spoil when the a\({}_{w}\) exceeds 0.80. In Figure 2B, except for the control in which the a\({}_{w}\) had decreased to 0.87 on the seventh day, no significant differences were observed in the a\({}_{w}\) of the control and SPI-fortified breads during the storage period, which were all greater than 0.88. The a\({}_{w}\) of the control and SPI-fortified breads had decreased on the second day because the rearrangement of water molecules within the bread had altered the bound water/free water ratio when the water migrated from the internal bread structure to the crust. This finding is consistent with that of [PERSON] and [PERSON] [53].  
The variation in bread pH during the storage period is shown in Figure 2C. The pHs of all the breads significantly increased with increasing SPI incorporation (\(p<0.05\)), likely because of the higher pH of the SPI raw ingredient (8.05), but were not significantly different over time. On the first day, the mean pH values of the control, 2%, 4%, 6%, and 8% SPI-fortified breads were 5.12, 5.34, 5.52, 5.70, and 5.83, respectively.