Monitoring of acidified milk gel formation by ultrasonic shear wave measurements: high frequency viscoelectric moduli of milk and acidified milk gel

Kudryashov E.D., Hunt N. T., Arikainen E. O. and Buckin V. A.

J Dairy Sd, 84, pp. 375-388 (2001))

In the present paper we describe  the  application of  the thickness shear mode resonator technique for the measurement of viscoelastic parameters of milk  and  acidified  milk  gels  in  the  megahertz frequency range. The  technique  provides information  on  the  viscoelasticity  of  milk  and milk  gels  in  the  time  scale  of  10-8  to  10-7s. The  length scale of the measurements, determined by the depth of penetration and the  wavelength  of the shear wave, falls between the submicron and micron range. In milk acidified by  glucono-δ-lactone we observed an increase in the high- and low-frequency  loss  modulus, G"., below pH 5.1, indicating  aggregation  of  casein  particles  into clusters. There was a sharp rise in high- and low- frequency storage (G'.) and loss (G".) moduli between pH 4.85 and 4.7, as a result of  gel network formation 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) viscosity of milk was found to be the same as at  low frequencies. Overall, our results demonstrate the  high  sensitivity  of  the  ultrasonic  shear wave  measurements to the changes in the rheological parameters in acidified milk during gelation. 

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