We found that brown rice flour produced using a jet mill after soaking for more than 12 h yielded a better formulation of rice/gluten bread, giving an equivalent specific loaf volume (SLV) as bread made with white rice flour. Quality analysis of the brown rice flour showed that soaking for more than 12 h resulted in a lower damaged starch content. There was a significant inverse correlation between damaged starch content and SLV. Substitution of 10% white rice flour with rice bran had little effect on final SLV. Furthermore, endogenous α-amylase activity in brown rice flour produced after soaking was approximately 5 to 12 times higher than that of white rice flour, but there was no correlation with SLV. These results indicate that the improved SLV in brown rice/gluten bread was predominantly due to the decrease in damaged starch content, which depends on soaking time.Keywords: rice bread, brown rice flour, water soaking, damaged starch *To whom correspondence should be addressed. E-mail: sty564@affrc.go.jp
IntroductionRice (Oryza sativa L.) is an important source of nutrition and energy, and is one of the most suitable cereal grains used for preparing foods. Rice flour can be used as a wheat flour substitute and processed into bread. However, rice storage proteins do not possess the viscoelastic properties that are typical of wheat gluten. In wheat, gliadins (prolamins) are responsible for the dough's cohesiveness, while the glutenins (glutelins) are apparently responsible for the dough's resistance to extension (Hoseney, 1994). The gluten network constructed by these two proteins provides the dough with unique viscoelastic properties and the ability to retain gases, making it a good quality raw material for the production of yeast-leavened products. However, rice flour lacks these properties; hence, the qualities of gluten-free rice bread and wheat-rice bread with a high proportion of rice flour are generally inferior.Rice flour is used for bread-making in three flour mixtures: rice/wheat flour; rice flour/vital gluten; and gluten free. It has been reported that in the case of rice/wheat flour bread, substitution of wheat flour with up to 30% rice flour is acceptable without compromising the sensory qualities (Watanabe et al., 2004;Nakamura et al., 2009). There have been few reports on bread-making quality with the addition of