(211c) Stick-Slip in Dairy Powders During Shear Testing

In an investigation of the flow properties of skim milk powder, whole milk powder and cream powder, shear test measurements were made at 25°C, 30°C and 37°C, using an annular cell. In all experiments, both for angle of wall friction where a sample of wall material was sheared against a powder bed, and in measurements for powder flow parameters where a bed of powder was sheared until it failed, the trace, after failure, exhibited the oscillations characteristic of stick slip friction. Observed amplitudes were highest at the higher normal stresses used, and temperature had little effect on the behaviour of the skim milk powder, which had a fat content of 1%, compared to 26% and 55% for the whole milk powder and cream powder respectively. There appears to be a strong correlation between amplitude and period of oscillation. Some implications arising from different ways the raw data might be interpreted are discussed.