Fourier transform infrared (FTIR) spectroscopy as a tool for wheat gluten secondary structure analysis and for probing moisture distribution in whole grain dough system

Access restricted to the OSU CommunityFourier transform infrared (FTIR) spectroscopy is the best method for analyzing protein secondary structure in viscoelastic dough samples, but the significant contribution of water in the amide I spectral region results in errors that cannot be corrected by subtraction of a pure water spectrum. Use of deuterated water instead may result in more accurate results without the need for extensive spectral processing as D₂O does not absorb in the amide I spectral region. The objectives of this study were to evaluate spectral analyses of H₂O and D₂O hydrated gluten samples with the appropriate spectral processing to develop a more accurate method of determining protein secondary structure in aqueous systems, and then to investigate the underlying physical mechanism(s) by which bran influences whole grain dough properties by monitoring the state of water and gluten secondary structure in model gluten - bran and flour - bran systems. Spectral analyses from H₂O and D₂O samples showed no agreement in overall spectral intensity or secondary structure estimates between H₂O and D₂O samples due to the spectral processing methods. Subtraction of H₂O led to a large underestimation of secondary structures between 1650 - 1700 cm⁻¹ and concurrent overestimation between 1600 - 1650 cm⁻¹. The average differences between D₂O and H₂O secondary structure estimates varied little and so were used as correction factors in subsequent studies involving dough systems. The correction factors were: (+) 13.97% sheet, (-) 3.58% aperiodic, and (-) 10.38% P-turn structures. A series of model gluten - bran and flour - bran doughs were studied over a range of moisture and bran contents and 3 different bran particle sizes. A series of H₂O - D₂O blanks were created at various ratios to more accurately correct for the original water contribution. Analyses of the OH stretch band revealed two water states in the dough. The primary OH stretch peak (3150 cm⁻¹) shifted toward lower frequencies as moisture content increased with concurrent decreases in peak intensity and area. On the other hand, the peak intensity and area of the secondary OH stretch (-3575 cm⁻¹) increased with moisture content, but the frequency remained constant. Addition of bran decreased the peak intensity and area of both peaks, while the frequency of the primary peak shifted to slightly greater values. Analysis of absorption in the amide I region revealed that the content of hydrated P-turns increased with increase of moisture content at the expense of pseudo- p -sheets, but subsequently decreased with bran addition. These FTIR results indicated that two separate populations of water exist in dough systems: bound (primary peak) and free (secondary peak) water. Addition of bran reduces the amount of free water in the system, corroborated by the decrease in secondary OH stretch peak absorbance and area. Additionally, bran increases the overall energy requirement to excite bound water, which may correlate to stronger water - bran interactions. Flour type had an impact on the ability of bran to effect a redistribution of water in dough systems with better bread-making flours demonstrating less redistribution of water on bran addition. Additionally, flour type influenced the secondary structural changes observed on bran addition with better bread-making flours demonstrating increases in aperiodic rather sheet structures. Bran particle size had no influence on any of the parameters studied. These data indicated that water re-distribution in the presence of bran may drive changes in gluten secondary structure, which may in turn influence bread and dough properties in whole grain systems