Rheology of Powdered Milk
The shelf life of milk can be increased extensively by drying it. Variations in raw material (e.g. whole milk, skimmed milk) as well as process variations enable a large variety of finished products. This includes, amongst others, infant formula, skim milk powder, casein powder, whey protein powder and lactose powder.
Powdered milk is produced by several processing steps, which all present different technological challenges. Usually, the milk is first heated under a vacuum, allowing evaporation of the water at low temperatures and resulting in milk concentrate. This is generally followed by a spray drying step, in which the milk concentrate is atomized into fine droplets under a flow of hot air in a large spray drying tower. After recovery from the tower, a final drying step might take place in a fluid bed, in which hot air is blown through a layer of fluidized powder  .
Because milk powder is cohesive, problems often arise when the powder discharges from spray drying towers or storage containers, resulting in undesired bridging or rat-holing. Cohesiveness is also critical for the final drying step, as it influences flowability and, thus, the amount of energy needed to fluidize the milk powder.
The Anton Paar Powder Flow Cell enables different measurements that are applicable in quality control as well as in research in order to anticipate the problems mentioned above. In the present study, methods for determining the Warren Spring cohesion and for measuring the wall-friction angle of powdered milk are presented and discussed.
 Pearce, K.N. (2000). Milk Powder. Food Science Section, New Zealand Dairy Research Institute.
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