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

The objectives of this study were to evaluate spectral analyses of H ₂O 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 were used as correction factors in subsequent studies. The correction factors were: (+) 13.97% sheet, (–) 3.58% aperiodic, and (–) 10.38% β-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 – D₂0O blanks were created at various ratios to more accurately correct for the original water contribution. The 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 and protein content had an impact on the ability of bran to effect a redistribution of water in dough systems. 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. These data indicated that water redistribution 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.