The effect of ultrasound on the physical and functional properties of skim milk

Pasteurized homogenized skim milk was treated with 20 kHz ultrasound at 20 and 41 W under controlled temperature conditions for different time intervals up to 60 min. There was a reduction in the turbidity, but viscosity of milk was not affected. The size of the casein micelles, fat globules and soluble particles, respectively, after 60 min of sonication were 174 nm, 157 nm and 89 nm at 20 W and 172 nm, 175 nm and 84 nm at 41 W in comparison to 178 nm, 170 nm and 136 nm for unsonicated samples. The whey proteins in the milk were denatured and formed soluble whey–whey/whey–casein aggregates, which further interacted with casein micelles to form micellar aggregates during the initial 30 min of sonication. Prolonged sonication resulted in the partial disruption of some whey proteins from these aggregates.Industrial relevanceUltrasonic processing is a promising technique, which could be adapted in food processing sector. The minor changes to the milk, which are imparted by the shear forces of the acoustic cavitation, foresee the potential for optimizing this technique for industrial applications. The optimization process needed, depends on the functionality and the end products, such as food emulsions using bioactives. The optimization can be achieved by fine tuning the power density, temperature, processing time, etc. This study reports on the processing effect of ultrasound on a simple food matrix (milk) in view of establishing a base to extend the investigation to other complex foods.

► Sonication of skim milk was performed at 20 kHz. ► The integrity of the casein micelles was preserved during sonication. ► The whey proteins and their aggregates were largely affected upon sonication. ► Fat globules were disrupted by sonication whereas viscosity was not affected. ► Shear forces generated by acoustic cavitation are responsible for the changes.