Rheology of stirred acidified skim milk gels with different particle interactions

Steady-shear rheology as well as small and large amplitude oscillatory shear rheology was used to quantify particle interactions in stirred, acidified skim milk gels. Gels were made from milk using either high-pressure treatment up to 600 MPa or heat treatment at 90 °C and subsequent acidification. This allowed gels with different particle interactions to be studied. Upon increased rate of steady-shear, a marked decrease in viscosity occurred at a certain stress level, which was considered as an apparent yield stress. At this stress level the rheological properties of the stirred gels changed from a behaviour characteristic for a gel to behaviour of a dispersion of interacting particles. A gel to solution transition was also assessed in stirred gels by using large amplitude oscillatory shear. Stirred acidified milk gels with different particle interactions could be distinguished by quantification of the apparent yield stress or the shear stress where elastic and viscous moduli coincided. Particle interactions in the stirred gels were gradually enhanced by high-pressure treatment up to 600 MPa shown by gradually increased shear viscosity and apparent yield stress, as observed by steady-shear. Oscillatory shear measurements revealed that high-pressure treatment up to 600 MPa increased moduli in stirred gels and enhanced the shear stress recorded at the gel to solution transition. However, similar rheological properties were found in stirred gels when these were made either from milk subjected to high-pressure treatment at 600 MPa or heat treatment at 90 °C.