Application of ultrasound to reduce viscosity and control the rate of age thickening of concentrated skim milk

Concentrated skim milk was treated with high intensity low frequency ultrasound (20 kHz) to lower viscosity through a process of acoustic cavitation. Batch sonication for 1 min at 40-80 W, and continuous treatment delivering an applied energy density of 4-7 J mL-1, reduced the viscosity of medium-heat skim milk concentrates containing 50-60% solids. Viscosity was reduced by approximately 10%, but this improved to >17% in highly viscous age thickened material. Sonication also changed the shear thinning behaviour at shear rates below 150 s-1. Although ultrasound lowered the viscosity of skim milk concentrated to ≥50% solids, the treatment could only delay the rate of thickening once the ageing process was established. It was only when ultrasound was activated during concentration that sonication prevented the viscosity of skim milk concentrates from increasing rapidly.