Influence of internal interfacial area on nanosecond relaxation of wheat gluten proteins as probed by broadband ultrasonic spectroscopy

Understanding interactions between interfaces and biopolymers in complex industrially processed materials of plant origin will allow for their better utilization. Wheat flour doughs are one such material whose industrial use strongly depends on such interactions due to their effect on the mechanical properties of the dough. To date, mechanical characterizations of dough have been limited to a narrow range of frequencies. Here, ultrasonic spectroscopy measurements over a very broad frequency range are used to show that a fast volumetric relaxation occurs in dough; the nanosecond timescale of the relaxation is associated with ultrasonic stress-induced changes in the secondary structure of gluten proteins. Interestingly, there is a four-fold difference in the speed of this relaxation phenomenon in doughs mixed in air (where substantial internal interfacial area exists) compared to those mixed under vacuum (where bubbles are absent). Given the large internal interfacial area in dough, the amphiphilic proteins residing at gas bubble interfaces significantly alter the high-frequency mechanical response of this important material.