Effects of soymilk residue (okara) or soy flour on staling and keeping quality of bread

Abstract

This research aimed to investigate the effects of partial substitution of wheat flour by full-fat soy flour or soymilk residue (okara) on staling and keeping quality of white bread. Wheat flour in the bread recipe was substituted by soy flour or okara at 5, 10 and 15% by weight. Upon substitution of soy flour and okara, bread samples generally exhibited an increase in crude protein, crude fat, crude fiber and ash contents with a decrease in available carbohydrate content. Moisture and freezable water contents of freshly baked soy flour- and okara-containing breads were significantly higher than those of the control wheat bread (p≤0.05). Both moisture and freezable water contents became decreasing with increasing storage time, with a sharper reduction in the control. Meanwhile, unfreezable water content was not significantly different among the samples (p>0.05) and remained relatively constant throughout 7-day storage. Amylopectin recrystallization was observed to be most pronounced in the stored control bread. The okara-containing breads possessed slightly lower amylopectin recrystallization as compared to the soy flour-containing sample of the same substitution level. The increase in amylopectin recrystallization was consistent with the decrease in water soluble starch content. These changes were found to occur rapidly in the first two days of storage. Significant decreases in loaf volume, specific loaf volume and loaf height, and a significant increase in loaf weight, were observed in those breads containing okara. An increase in hardness and a decrease in cohesiveness and springiness were found to be especially prominent in the samples with higher levels of okara substitution. In spite of this, the control exhibited the greatest changes in those textural parameters during storage. As revealed under a stereomicroscope, crumb alveoli became smaller and varying in shape and size upon substitution of soy ingredients. Hue angle of all crumb samples was approximately 90˚, representing yellow color. The intensity of yellowness increased upon soy substitution which was more pronounced in the soy flour-containing crumbs. Bread crusts exhibited hue angle in the 60˚ range, representing orange-yellow color. There appeared to be no definite trend in crust chroma changes upon soy substitution. From sensory evaluation, it was indicated that beany flavor significantly increased in soy flour- and okara-containing samples. From hedonic evaluation of freshly baked breads, the panel preferred the control over the soy-containing samples. However, as shown through overall preference score, the 5, 10 and 15% soy flour-substituted and 5% okara-substituted samples were still considered acceptable.