Mechanical properties of milk protein skins layers after drying: understanding the mechanisms of particle formation from whey protein isolate and native phosphocaseinate
Résumé
The spray drying of milk proteins usually leads to dry particles of which the final shape can influences
physical and functional properties of powders. The aim of this study was to understand the mechanisms
of particle formation by considering the mechanical properties of materials making up the two main
classes of milk proteins: whey proteins and casein micelles. The progressive solidification of the interface
of the droplet during drying time was studied by high speed camera and fluorescence microscopy, in
different experimental conditions. The mechanical properties of the final protein materials were then
characterized by micro indentation testing. The drying dynamics of whey protein and casein micelle
droplets showed different timescales and mechanical lengths, whatever the drying conditions and the
droplet configurations, leading to typical mechanical instability at the surface i.e. buckling and fracture.
The interface of casein micelles reached solegel transition earlier estimated at around 156 g.L 1
following by elastic and plastic regimes in which the shell distorted and buckled to form a final wrinkled
particle. In contrast, the interface of whey proteins became elastic at only half the drying time estimated
at around 414 g.L 1, retaining a spherical shape, which finally fractured at the end of drying. The mechanical
difference between the two plastic shells might be explained by the behaviour of proteins in
jamming conditions. Analogous behaviour could be discussed between the casein micelles and soft and
deformable colloids on the one hand, and between whey proteins and hard spheres on the other.