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

Rheological data at 5-degrees-C is presented for buttermilk, a culture d skim milk product. The material exhibits time dependent rheological behavior. The time dependent behaviour of the product can be character ised using a simple Herschel Bulkley model, modified to include a stru ctural parameter lambda to account for time dependent effects. The res ulting equation of state takes the form: tau = lambda (tau(y) + K(gamm a)n)). The second order rate constant for the decay of the structural parameter with time was found to be relatively independent of shear ra te 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 equ ation for the decay of the structural parameter lambda was integrated to yield an equation of state for shear stress that depends only on sh ear rate and time shear. This equation was used to successfully predic t the behavioir of buttermilk in constant and varying shear rate condi tions. Such information can be used for the analysis of buttermilk flo w in pipelines during start-up and steady state conditions. This type of analysis is valuable for the proper design of pipes and pumps in bu ttermilk processing plants.