Shape, Shell, and Vacuole Formation during the Drying of a Single Concentrated Whey Protein Droplet

Abstract

The 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.

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

“…At Stage 3, the profile of the caseins droplet seemed not to evolve further: constant shape indicators were measured whereas the pure water droplet evaporated rapidly. Such droplet dynamics were previously observed during the drying of whey proteins (Sadek et al, 2013), which showed three similar phases corresponding to evaporation of the free water at the surface, buckling instability and in that case solidification into a spherical particle. However, the occurrence of a second phase of surface instability in the drying of caseins at t I clearly distinguished the behaviour of these two milk proteins.…”

“…A droplet of 0.48 mg was formed with a micro-syringe (0.5 mL syringe, SGE Analytical Science) and deposited onto a hydrophobic substrate made of cylindrical pillars and described in Sadek et al (2013) (Fig. 1a,b).…”

“…This experimental approach allows direct and accurate observation of the droplet-particle conversion. In a previous work, we developed such an approach combining complementary methods including a microbalance, a high-speed camera, and confocal microscopy in order to investigate the shape dynamics and final structure of a milk protein droplet in a dry environment (low relative humidity) on a nonwetting surface by using a hydrophobic substrate (Sadek et al, 2013). In order to overcome the difficulties of the fast-drying kinetics, we lowered the drying temperature at 20 C to slow down the drying kinetics.…”

Drying colloidal systems: Laboratory models for a wide range of applications

“…Depending on the chemical and physical properties of the solute and the external conditions, later on different surface instabilities occur such as buckling, invagination, delamination and cracks [25]. Sadek et al [26][27][28] investigated on the drying dynamics of pendant droplets made of the two main milk proteins, whey proteins and casein micelles. For that purpose, the concentrated protein solution drop was hanged to a micropatterned substrate and exposed to stressful conditions (relative humidity, RH, below 5%).…”

Formation process of core‐shell microparticles by solute migration during drying of homogenous composite droplets

“…For example, certain solids undergo crystallization during particle formation, 36,37 and solutes that form a rigid non-shrinkable shell could produce dry particles with an internal vacuole. 10,38 During drying of the RS/silica composite droplets, an insoluble incipient shell consisting of both constitutes with dominating RS is produced by all compositions. Evaluating the chemical structure and properties of each constitute, we propose the following mechanism for the formation of the insoluble complex.…”