TIME-DEPENDENT RHEOLOGICAL CHARACTERIZATION OF BUTTERMILK AT 5-DEGREES-C

Rheological data at 5-degrees-C is presented for buttermilk, a cultured skim milk product. The material exhibits time dependent rheological behavior. The time dependent behaviour of the product can be characterised using a simple Herschel Bulkley model, modified to include a structural parameter lambda to account for time dependent effects. The resulting equation of state takes the form: tau = lambda (tau(y) + K(gamma)n)). The second order rate constant for the decay of the structural parameter with time was found to be relatively independent of shear rate in the range 10-500 s-1, and for periods of shear up to 30 min. By taking the rate constant to be independent of shear rate, the rate equation for the decay of the structural parameter lambda was integrated to yield an equation of state for shear stress that depends only on shear rate and time shear. This equation was used to successfully predict the behavioir of buttermilk in constant and varying shear rate conditions. Such information can be used for the analysis of buttermilk flow in pipelines during start-up and steady state conditions. This type of analysis is valuable for the proper design of pipes and pumps in buttermilk processing plants.