Monitoring of acidified milk gel formation by ultrasonic shear wave measurements. High-frequency viscoelastic moduli of milk and acidified milk gel

In the present paper we describe the application of the thickness shear mod e resonator technique for the measurement of viscoelastic parameters of mil k and acidified milk gels in the megahertz frequency range. The technique p rovides information on the viscoelasticity of milk and milk gels in the tim e scale of 10(-8) to 10(-7) s. The length scale of the measurements, determ ined by the depth of penetration and the wavelength of the shear wave, fall s between the submicron and micron range. In milk acidified by glucono-delt a -lactone we observed an increase in the high- and low-frequency loss modu lus, G", below pH 5.1, indicating aggregation of casein particles into clus ters. There was a sharp rise in high- and low-frequency storage (G') and lo ss (G") moduli between pH 4.85 and 4.7, as a result of gel network formatio n in acidified milk. Both G" and G' of milk gel in the megahertz frequency range are several orders higher than those we obtained at low frequencies ( 0.02 to 10 Hz) using dynamic rheology. The high-frequency (5 to 25 MHz) vis cosity of milk was found to be the same as at low frequencies. Overall, our results demonstrate the high sensitivity of the ultrasonic shear wave meas urements to the changes in the rheological parameters in acidified milk dur ing gelation.