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

Protein particle gels, which consist of a continuous three-dimensional netw ork of flocculated particles, such as casein micelles, are the main constit uents of many biological and food colloids. These gels have an extremely sh ort region of linear elastic behaviour and small fracture strain, thus crea ting complications for their theological analysis. In the present paper we describe the application of low amplitude ultrasonic measurements for the m onitoring of casein particle gel formation in a suspension of casein partic les (a process similar to the formation of gel network in yoghurt). Combina tion of high-resolution longitudinal and shear wave measurements allowed us to make a complete analysis of the shear and volume viscoelasticity of cas ein particle gels in the frequency range 5-25 MHz. Both shear moduli, the s torage (G ' (Gel) similar to 30 kPa, at 7 MHz) and the loss moduli (G " (Ge l) similar to 100 kPa, at 7 MHz) of the gel in the megahertz frequency rang e are several orders higher than those determined previously by dynamic rhe ology at low frequency (0.1 Hz), indicating different contributions of the gel network to the changes in the viscoelastic parameters at low and high f requencies. The volume storage (K ' (Gel) similar to 75 kPa, at 7 MHz) and loss (K " (Gel) similar to 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 hig h-resolution ultrasonic measurements for analysis of these systems. (C) 200 1 Elsevier Science B.V. All rights reserved.