Cécile Le Floch-Fouéré

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

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.

To What Extent Do Whey and Casein Micelle Proteins Influence the Morphology and Properties of the Resulting Powder

How the type of protein influences particle morphology remains a hot topic of debate. In this study we focused on the drying behavior of two major milk protein types; that is, whey protein and native micellar caseins. To improve understanding of the role of each protein in the particle-forming mechanisms, seven mixtures containing different whey proteins to caseins ratios were investigated. A monodisperse spray dryer (MDSD) was used to produce uniform particles by drying monodispersed droplets in a hot, dry air flow. Single particles were also obtained from the same material using single droplet drying in a pendant configuration. Powders were characterized according to their physical characteristics and their rehydration properties. It was demonstrated that particle morphology was mainly governed by the type of protein matrix, almost regardless of the drying kinetics, which differed considerably between MDSD and single droplet drying. Controlling product formulation thus represents a potential means by which to tune particle morphology and therefore the functional properties of powder.

Changes in Functional Properties of Milk Protein Powders: Effects of Vacuum Concentration and Drying

Vacuum concentration and dehydration by spray drying are valuable techniques for the evaporation of water and are used to stabilize most dairy ingredients. In view of the increasing development of filtration processes, there is a need for the dairy industry to improve their understanding of the effects of controlling these processes on the concentration and spray drying and hence on the quality of protein powders. Several authors have reported that proteins have an important role in the mechanisms of water transfer during drying and rehydration. The residence time of the droplet and the powder in the spray dryer is so short that it is very difficult to implement studies on the mechanisms of the structural changes in the protein without fundamental research into the process/product interactions. However, many studies have reported that the main structural and functional modifications occur before spray drying; that is, during concentration by vacuum evaporation and heat treatment. Following an extensive introduction focusing on dairy powders, this article covers three significant areas: an overview of the main dairy powder processes, the properties of these powders, and the effects of the processes on powder quality.

Unexpected differences in the behavior of ovotransferrin at the air-water interface at pH 6.5 and 8.0

Adsorption of purified apo-ovotransferrin at the air-water interface was studied by ellipsometry, surface tension, polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and shear elastic constant measurements. No significant difference was observed between pH 6.5 and 8.0 as regards the final value of surface concentration and surface pressure. However at low concentration, a weak barrier to adsorption is evidenced at pH 6.5 and confirmed by PM-IRRAS measurements. At a pH where the protein net charge is negative (pH 8.0), the behavior of ovotransferrin at the air-water interface is more influenced by charge effects rather than bulk concentration effects. At this pH, the interface exhibits a low shear elastic constant and a spectral signature not usual for globular proteins.

Investigation of secondary structure evolution of micellar casein powder upon aging by FTIR and SRCD: consequences on solubility: Secondary structure of micellar casein powder upon aging

BACKGROUND Synchrotron radiation circular dichroism (SRCD) and Fourier transform infrared (FTIR) spectroscopy were used to examine the conformation evolution of micellar casein (MC) powder during storage and to determine whether the spectral changes could be related to their solubility evolution. RESULTS A loss in intensity of SRCD spectra as a function of storage time has been observed. Quantification of secondary structures revealed losses of α-helix content during storage. Moreover, a redshift of the amide I band in the FTIR spectrum was demonstrated during the storage and was interpreted as a rearrangement of the secondary structure of the protein, which is in line with the SRCD results. The qualitative results obtained by FTIR clearly support the quantitative evolution of the secondary structure obtained by the analysis of SRCD spectra. Principal component analysis (PCA) of FTIR spectra permits a good separation of samples according to the storage time. PCA shows that the evolution of secondary structures and solubility loss are closely linked. CONCLUSION With the quantitative data provided by SRCD spectra, it was established that, whatever the storage conditions, a unique curve exists between loss of α-helix content and loss in solubility, showing that loss of α-helix content is a marker of solubility loss for the MC powders studied.

Moderate conformational impact of citrate on ovotransferrin considerably increases its capacity to self-assemble at the interface.

We have compared the behavior of ovotransferrin at the air-solution interface in the presence of a monovalent ion (acetate), or a divalent ion (citrate), the latter being known to induce conformational changes of this protein upon interaction with its iron-binding sites. We have characterised the adsorption layer at the air-water interface in terms of homogeneity, surface concentration excess and rheological properties at pH 4.0. Besides we have investigated the bulk conformation in the presence of the two anions. In the presence of citrate only, interfacial layers display well-defined domains of higher overall surface concentration suggesting multilayers adsorption. Citrate also induces higher helical content and stabilizes the protein against thermal denaturation. Hence we propose that these changes are involved in the propensity of ovotransferrin to self-assemble at the air-water interface resulting in thick and heterogeneous interfacial layer.

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

Abstract.The drying of complex fluids provides a powerful insight into phenomena that take place on time and length scales not normally accessible. An important feature of complex fluids, colloidal dispersions and polymer solutions is their high sensitivity to weak external actions. Thus, the drying of complex fluids involves a large number of physical and chemical processes. The scope of this review is the capacity to tune such systems to reproduce and explore specific properties in a physics laboratory. A wide variety of systems are presented, ranging from functional coatings, food science, cosmetology, medical diagnostics and forensics to geophysics and art.Graphical abstract

Steam-jet agglomeration of skim milk powders: Influence of process parameters

ABSTRACT Steam-jet agglomeration consists in steam–wetting of the surface of fine primary particles, colliding the sticky particles and consolidating the agglomerates by drying in order to obtain the desired properties. The aim of this work was to evaluate the influence of the steam/powder ratio and drying time on growth mechanisms and skim milk agglomerate properties. We demonstrated a predominant influence of the steam/powder ratio on the growth mechanisms and agglomerate properties due to the contribution of the steam flow rate. Moreover, the drying time was identified as a key process parameter to control the final water content, rehydration and mechanical properties of the agglomerates. Highlights A steam-jet agglomeration pilot plant was developed to study the agglomeration of skim-milk powders. The growth mechanisms and agglomerates properties depended on the steam/powder ratio, through the contribution of the steam flow rate. The drying time was identified as a key process parameter to control the final water content, rehydration and mechanical properties of the agglomerates.