High-frequency shear and volume viscoelastic moduli of casein particle gel

Protein particle gels, which consist of a continuous three-dimensional network of flocculated particles, such as casein micelles, are the main constituents of many biological and food colloids. These gels have an extremely short region of linear elastic behaviour and small fracture strain, thus creating complications for their rheological analysis. In the present paper we describe the application of low amplitude ultrasonic measurements for the monitoring of casein particle gel formation in a suspension of casein particles (a process similar to the formation of gel network in yoghurt). Combination of high-resolution longitudinal and shear wave measurements allowed us to make a complete analysis of the shear and volume viscoelasticity of casein particle gels in the frequency range 5–25 MHz. Both shear moduli, the storage (G′Gel∼30 kPa, at 7 MHz) and the loss moduli (G″Gel∼100 kPa, at 7 MHz) of the gel in the megahertz frequency range are several orders higher than those determined previously by dynamic rheology at low frequency (0.1 Hz), indicating different contributions of the gel network to the changes in the viscoelastic parameters at low and high frequencies. The volume storage (K′Gel∼75 kPa, at 7 MHz) and loss (K″Gel∼15 kPa, at 7 MHz) moduli of the casein particle gel are of the same order as the shear modili of the gel. The contribution of the gel to the volume storage modulus of the whole suspension of casein particles is very small, about 0.001%. This indicates the importance of high-resolution ultrasonic measurements for analysis of these systems.