Solubility and hydration capacity of soy protein isolates as influenced by salt and sugar in selected simple and food systems

Three commercial soy protein isolates, Pro-Fam 90/Ls, Promine-D, and Supro 62o, were studied. Part A of the study was an investigation of protein solubility and hydration capacity of the soy isolates with levels of salt and sugar at ambient temperature (22-25°c) and 40°C. Part B involved the study of farinograph water absorption of the three soy isolates in flour-soy mixtures. Part C of the study involved the evaluation of the soy protein isolates in a food system. Yeast bread was made from the three soy isolates, and subjected to sensory and objective evaluation. The consumer texture profile technique was used for sensory evaluation of different parameters of the bread. Objective measurements were loaf volume, specific volume, hardness, elasticity, cohesiveness, gumminess, and chewiness.

Nitrogen solubility index (NsI) was affected significantly (P-0.000l) by the soy protein isolates, temperature, level of salt, and their interaction.

The effects of soy isolate, temperature, level of salt, and their interactions on water absorption were significant at the level p<0.oo0l. Sugar did not have a significant effect on water absorption (P>0.05). The interaction among temperature, salt,and sugar and the interaction between soy isolate and sugar were significant at the level pzo.0l.

The overall effect of the flour-isolate on farinograph water absorption was significant at the level of Pz0.000l. Adding the soy isolates to wheat flour increased the percentage of water absorption; the water iii

iv absorption of flour alone was significantly lower (P<0.05) than that of flour in combination with the soy isolates. The effect of increasing salt level on farinograph water absorption was significant overall at P<0.0001. Sugar level did not have a significant effect on the farinograph water absorption. Addition of soy isolate Supro 620 resulted in the shortest development time, whereas wheat flour alone showed the longest development time. Doughs containing soy isolates were less stable than those of wheat flour alone.

Evaluation of the yeast bread made from wheat flour with the three soy isolates by the consumer texture profile technique showed that bread made with Pro-Fam 90/Ls tended to be closer to the ideal product than those containing the other isolates. However, bread made from Promine-D appeared to have more uniform cell distribution than the other breads. The breads also differed significantly in chewiness, greasiness, gumminess (p<0.05), smoothness (P<0.0l), uniformity of cell distribution, and crumbliness (P<0.0ol). Objective evaluation of the breads showed significant differences in their volume, specific volume, hardness, gumminess, chewiness (P<0.000l), elasticity (P<0.0l), and cohesiveness (P<0.05). Bread made with Pro-Fam 90/Ls had the highest volume, whereas that with Promine-D had the lowest volume.

Correlation of the sensory parameters with the objective ones showed that the sensory parameter chewiness was correlated with all of the objective parameters, while greasiness, crumbliness, and gumminess were related to most of the objective parameters.