The use of low intensity ultrasound to investigate the effect of mixing time and ingredients on the mechanical properties of bread dough

Mixing is the vital stage of breadmaking as it transforms a mixture of flour, water, salt and other ingredients (bread improvers) into a viscoelastic dough for breadmaking. The nucleation of gas cells in the dough also occurs during dough mixing. The matrix and the gas cells in dough affect the mechanical properties of dough which in turn affects the final product quality. There are many conventional methods available to investigate the properties of the dough as a whole, but studies to demonstrate the contribution of the gas cells to the mechanical properties of dough are few. The use of ultrasonic techniques in assessing the quality of food products has great potential in this regard.; The aim of the thesis research was to use low intensity ultrasound (at 50 kHz) as a tool to investigate the effects of mixing time and bread improvers (shortening and surfactant) on the mechanical properties of dough. This approach is based on measuring the velocity and the attenuation coefficient of the longitudinal waves as they propagate through the dough sample, giving information on the mechanical properties of dough. To accomplish our aim, we have performed three separate sets of experiments. In first set of experiments, the doughs were prepared using CWRS wheat flour (good breadmaking flour) and shortening as a bread improver. The mixing time, head space pressure of the mixing bowl during dough mixing and level of shortening in the dough were the variables used to control the amount of air occluded in the dough. In the second set of experiments, SWS wheat flour (poor breadmaking flour) and surfactant as the bread improver were used to prepare the doughs. The mixing time, head space pressure of the mixing bowl during dough mixing and level of surfactant in the dough were the variables used to control the amount of air occluded in the dough. To relate the first and the second set of experiments, the third set of experiments comprised of using CWRS flour along with different levels of surfactant to prepare the doughs.; By relating the ultrasonic velocity and attenuation coefficient results to the void fraction (calculated from the dough density and the matrix density), the effects of voids on the elastic properties of the dough were revealed. For the first set of experiments (for doughs prepared from CWRS flour and shortening as a bread improver), based on the capabilities of the mixer and vacuum pump to create vacuum in the headspace of the mixer during dough mixing, a void fraction (&phgr;) approximately ranging from 0.003<&phgr;<0.14 was achieved. For lower void fractions (0.003<&phgr;<0.005), with an increase in &phgr; the attenuation coefficient increased from 600 to 1100 m-1 for the control doughs. But in contrast to this the ultrasonic velocity dramatically decreased from 3000 to 167 ms-1. At higher &phgr; values the increase in attenuation coefficient and decrease in ultrasonic velocity (from 165 to 105 ms-1) was less rapid. The effects of void fraction on the ultrasonic parameters (ultrasonic velocity and attenuation coefficient) were similar when additional bread improver (shortening) was added to the control doughs (0% shortening) except a decrease in ultrasonic velocity and an increase in attenuation coefficient was observed.; For the second set of experiments (doughs prepared from SWS flour and surfactant) the effect of gas cells on ultrasonic parameters (ultrasonic velocity and attenuation coefficient) were similar to those observed in the first set of experiments. But addition of surfactant to the dough formula (0.5, 1, 2% w/w) increased the velocity (160 ms-1 for control dough and 200 ms-1 for doughs having 2% surfactant for 1 min mixed doughs), while the attenuation coefficient decreased (2100 m-1 for control dough and 1500 m-1 for doughs having 2% surfactant for 1 min mixed doughs) for doughs when compared to the control doughs (doughs prepared from SWS flour with 0% surfactant). For doughs prepared with CWRS...