Buckling and collapse during drying of a single aqueous dispersion of casein micelle droplet

The drying process for dairy products is based on rapid removal of water, quickly concentrating milk components to finally form dry particles. The change in concentration may have a considerable effect on the product structure and influence the final characteristics of the dry material. The aim of this study was to investigate the drying behaviour of a major milk protein, i.e. caseins. The droplet – particle conversion was investigated through a single droplet system and by combining complementary methods. Our results showed that the drying process for caseins comprised three stages involving different evaporation rates and droplet dynamics. A thin layer was formed in the early stage of drying that underwent surface instabilities such as buckling and invagination of the droplet. The drying of caseins led to the formation of a deflated and wrinkled particle shape. These results suggest particular mechanical properties of the casein shell leading to a predictable and a characteristic particle shape

Mechanical properties of milk protein skins layers after drying: understanding the mechanisms of particle formation from whey protein isolate and native phosphocaseinate

The spray drying of milk proteins usually leads to dry particles of which the final shape can influencesphysical and functional properties of powders. The aim of this study was to understand the mechanismsof particle formation by considering the mechanical properties of materials making up the two mainclasses of milk proteins: whey proteins and casein micelles. The progressive solidification of the interfaceof the droplet during drying time was studied by high speed camera and fluorescence microscopy, indifferent experimental conditions. The mechanical properties of the final protein materials were thencharacterized by micro indentation testing. The drying dynamics of whey protein and casein micelledroplets showed different timescales and mechanical lengths, whatever the drying conditions and thedroplet 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 1following by elastic and plastic regimes in which the shell distorted and buckled to form a final wrinkledparticle. In contrast, the interface of whey proteins became elastic at only half the drying time estimatedat around 414 g.L 1, retaining a spherical shape, which finally fractured at the end of drying. The mechanicaldifference between the two plastic shells might be explained by the behaviour of proteins injamming conditions. Analogous behaviour could be discussed between the casein micelles and soft anddeformable colloids on the one hand, and between whey proteins and hard spheres on the other

Shape, shell, and vacuole formation during the drying of a single concentrated whey protein droplet

International audienceThe drying of milk concentrate droplets usually leads to specific particle morphology influencing their properties and their functionality. Understanding how the final shape of the particle is formed therefore represents a key issue for industrial applications. In this study, a new approach to the investigation of droplet−particle conversion is proposed. A single droplet of concentrated globular proteins extracted from milk was deposited onto a hydrophobic substrate and placed in a dry environment. Complementary methods (high-speed camera, confocal microscopy, and microbalance) were used to record the drying behavior of the concentrated protein droplets. Our results showed that whatever the initial concentration, particle formation included three dynamic stages clearly defined by the loss of mass and the evolution of the internal and external shapes of the droplet. A new and reproducible particle shape was related in this study. It was observed after drying a smooth, hemispherical cap-shaped particle, including a uniform protein shell and the nucleation of an internal vacuole. The particle morphology was strongly influenced by the drying environment, the contact angle, and the initial protein concentration, all of which governed the duration of the droplet shrinkage, the degree of buckling, and the shell thickness. These results are discussed in terms of specific protein behaviors in forming a predictable and a characteristic particle shape. The way the shell is formed may be the starting point in shaping particle distortion and thus represents a potential means of tuning the particle morphology.Anglai