Ultrasonic Investigation of the Effect of Mixing Under Reduced Pressure on the Mechanical Properties of Bread Dough

Mixing is critical to attainment of a desirable gas cell distribution in dough. By varying mixer headspace pressure, changes in the mechanical properties of dough were investigated as a function of the dough's void concentration using low frequency (50 kHz) ultrasonic techniques. For the mixer used, this allowed the volume fraction of voids (Φ) to be varied from ≈0.01 to 0.08. The ultrasonic attenuation of longitudinal waves increased linearly with increases in Φ. If, as reported, pressure reductions during mixing decrease the number density of the voids but do not affect void size, the change in attenuation is proportional to the number of voids. By contrast, the velocity of longitudinal ultrasonic waves decreased dramatically with increasing Φ in the range 0.012 < Φ < 0.03, dropping from a value near that of water to values well below the velocity of sound in air. At higher Φ, the velocity decrease was less rapid. The longitudinal elastic modulus determined from these ultrasonic results shows that the mechanical properties of the dough are sensitive to the presence of gas bubbles. At low void fractions, the elastic behavior of dough is also influenced by changes in dough matrix properties.