Soy peroxidase enzyme obtained from isoelectic precipitation procedures was added to all-purpose flour (APF) to assess its effects on the rheological properties and consumer acceptability of yeast bread. A pH 4.8 isoelectrically precipitated fraction from soybeans was used because it produced the most precipitate and had about the same peroxidase activity as the other fractions. Gluten strength was determined using a farinograph for seven treatment groups: control (all-purpose flour), bread flour, all-purpose flour + soy flour, bread flour + soy flour, all purpose flour + pH 4.8 precipitate, all-purpose flour + 15 mg soybean peroxidase, and all-purpose flour + 25 mg soybean peroxidase. Four types of yeast bread were baked for loaf volume determination, texture analysis, and consumer acceptability: a control loaf using only all-purpose flour, a reference loaf using all bread flour, a loaf with all purpose flour + whole soy flour, and a loaf with all-purpose flour + pH 4.8 soy precipitate.

The APF+soy flour, bread flour, bread flour + soy flour, and the APF + pH 4.8 precipitate produced an improvement in the gluten strength and mixing tolerance compared to the control (p<0.05). However, the improvement by the addition of the pH 4.8 precipitate cannot be attributed to the peroxidase enzyme because peroxidase needs hydrogen peroxide as a substrate and no hydrogen peroxidase could be added to the farinogragh; therefore, it was concluded that the increase in gluten strength produced by the pH 4.8 soy precipitate was due to an unknown component present in the pH 4.8 fraction. No significant differences (p<0.05) were found in crumb or crust texture for any of the treatment groups. The addition of pH 4.8 precipitate to APF significantly decreased (p<0.05) loaf volume compared to bread made from bread flour. The results from sensory analysis showed there was no difference in preference for any of the breads. This study showed no conclusive evidence that peroxidase enzyme improved gluten strength or loaf volume of yeast bread, but further research is warranted.