Flow Behavior of Isolate Protein from Soybeans var. Grobogan and Whey Protein Isolate at Acidic Condition under Various Heating Times

Flow behavior of Soy Protein Isolate (SPI) suspension and Whey Protein Isolate (WPI) solution at pH 2.0 under various heating times were studied using steady shear viscosity measurements. Shear rate sweeps with increasing shear rates (up ramp) was performed to investigate the structural breakdown of the proteins during shearing. Down ramp shear rates were performed to check structural recovery of the proteins. The results showed that unheated SPI suspension has Newtonian flow; meanwhile, unheated WPI solution was slightly shear thickening. Heating the proteins at 80ºC for 4, 8, 12, and 16 h changed flow behavior of the proteins. Flow curve of SPI suspension heated for 12 h and 16, fitted Ostwald model with flow behavior index (n) of 0.625 and 0.264, respectively. This index indicates pseudoplastic (shear thinning) behavior, which also observed in heated WPI solution. The changes in flow behavior was attributed by the changes in protein structures, i.e., globular structures into fibrillar structures under prolonged heating at acidic condition. This conversion also increased the apparent viscosities of the proteins. SPI fibrils have higher apparent viscosities than WPI fibrils. This difference might be attributed to the detail fibril structures. SPI fibrils have branched and curvy structures; meanwhile, WPI fibrils are long and straigh