Evaluation of time-dependent rheological property in the coagulation of skimmed milk by ultrasonic velocity profiler

Abstract

We propose a method to evaluate time-dependent rheological properties, shear stress, effective viscosity and shear rate, of coagulating curd of the skimmed milk as an analog of cheese products. Ultrasonic spinning rheometry was adopted for the rheological evaluation, where the rheological properties were calculated via the equation of motion from spatio-temporal velocity information of oscillatory shear flow in a cylindrical vessel measured by ultrasonic velocity profiler (UVP). Dispersed small gels formed immediately after the addition of rennet to the skimmed milk allow UVP measurements without seeding tracer particles for ultrasonic wave. Shear distribution in the vessel due to the oscillatory rotation of the cylinder wall formed two distinct regions. In the inner region, the gel structure was developed because of the sufficiently weak shear. Consequently, in the near-wall region, sol-like state in which small gels were floating in the whey dehydrated from the developing gel in the inner region was maintained. The rheological evaluation performed on the near-wall region elucidated a time-dependent trend of the decreasing effective viscosity representing the state that whey gradually accumulated in the region. The increase of amount of whey reduces the volume fraction of floating gels in the near-wall region. The results suggested that we can estimate the physical states of developed curd in the inner region from the information of rheological property changes in the near-wall region.

Keywords

• effective viscosity
• milk-coagulation
• transient behavior
• rheometry
• ultrasonic velocimetry