Impact of Bran Addition on Water Properties and Gluten Secondary Structure in Wheat Flour Doughs Studied by Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy

The aim of this work was to elucidate the underlying physical mechanism(s) by which bran influences whole grain dough properties by monitoring the state of water and gluten secondary structure in wheat flour and bran doughs containing 35-50% moisture and 0-10% added bran. The system was studied with attenuated total reflectance (ATR) FTIR spectroscopy. Comparison of the OH stretch band of water in flour dough with that in H2O-D2O mixtures having the same water content revealed the formation of two distinct water populations in flour dough corresponding to IR absorption frequencies at 3,600 and 3,200 cm(-1). The band intensity at 3,200 cm(-1), which is related to water bound to the dough matrix, decreased and shifted to lower frequencies with increasing moisture content of the dough. Addition of bran to the dough caused redistribution of water in the flour and bran dough system, as evidenced by shifts in OH stretch frequency in the 3,200 cm(-1) region to higher frequencies and a reduction in monomeric water (free water). This water redistribution affected the secondary structure of gluten in the dough, as evidenced by changes in the second-derivative ATR-FTIR difference spectra in the amide I region. Bran addition caused an increase in beta-sheet content and a decrease in beta-turn (beta-spiral) content. However, this bran-induced transconformational change in gluten was more significant in the 2137 flour dough than in Overley flour dough. This study revealed that when bran is added to flour dough, water redistribution among dough components promotes partial dehydration of gluten and collapse of beta-spirals into beta-sheet structures. This transconformational change may be the physical basis for the poor quality of bread containing added bran.