The flow properties of 30% milk fat O/W emulsions containing monoglyceride and sorbitan ester were determined by a coaxial cylinder viscometer at 10, 20, 30, and 40°C.
All the emulsions examined had a flow curve with hysteresis loop and yield values, and showed pseudoplastic flow during increasing shear rate and Bingham flow during decreasing shear rate. Since the logarithmic plot of subtracting yield stress from shear stress versus shear rate showed a linear relationship, the power law was applicable. The flow behavior index
n approached unity as the temperature of the emulsions approached 40°C. As plastic viscosity (η
t) at shear rate 230s
-1 decreased with increasing shearing time (
t) and the curve of ln (η
t-η
∞)
vs (
t) showed linearity, the relaxation time in time thinning behavior was obtained from the slope of the curve. Although the curve of ln η
t vs 1/
T did not show perfect linearity, Andrade's equation appeared to be satisfactory within a rather narrow range of temperature in this study. A slight flection on the curve appeared at 20°C. This might be caused by melting of dispersed phase. Activation energy for flow was obtained from Andrade's equation. The flow behavior index was found to be proportional to the activation energy for flow when
n>0.75.
The influence of hydrocarbon chain length of sorbitan ester on flow properties and the influence of the flow properties on emulsion stability were also discussed.
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