ABSTRACT: Consistent reliable flow of milk powders out of hoppers and silos is very important in their handling and processing. Shear cell techniques were applied in this work to measure and compare the flow properties of a commercial skim-milk powder (SMP), a whole milk powder (WMP) and a 73% high fat milk powder (HFP), and to investigate how storage temperature and exposure to moisture in air affected the flowability of these powders. These techniques were also applied to investigate how powder particle size and free-fat content affected the flowability of a number of milk powders produced at pilot-scale. WMP and HFP were cohesive powders while SMP was easy flow, but SMP showed greater wall friction on the stainless steel material tested. Cohesion of SMP and WMP increased with storage temperature in the range of 5–25 °C. Likewise, the cohesion of HFP increased from 5 to 20 °C, but decreased at 30 and 40 °C although it became very sticky at 60 °C. Exposure of the powders to moisture in air at 46% relative humidity and 20 °C showed a major increase in the cohesion of SMP, but had little effect on WMP and HFP. Decreasing particle size from 240 to 59 μm produced a major increase in cohesion of 26% fat milk powders. A similar effect was found with 1% fat milk powders, however decreasing particle size from 199 to 96 μm had no effect on the cohesion of 50% fat milk powders. Varying free-fat content had no major effect on the cohesion of 26% fat milk powders at 20 °C.
Journal of Food Engineering.