Introduction

Many natural systems are aqueous solutions or suspensions, the latter exhibiting a wide variety of structures. Among them, colloidal particules formed by proteins associations form very well controlled structures. This is the case of colloids resulting from the assembly of caseins, which are the main components of milk : their structure is very monodisperse, repeatable from one micelle to another and they can be manipulated by chemistry or physical modification. 

They may thus be considered as a model of suspensions of particles with attractive interactions, that may be tuned by the modification of the proteins that compose the casein micelle. In turn, these modifications may result in the change of the aggregation kinetics and or structure.
The goal of this collaboration is to study the dynamics of floculation of the caseins, using light scattering, rheology and microrheology techniques (IPCMS partner) as a function of biochemical modifications of casein proteins.

Main objectives of research

Milk is a ubiquitous liquid and is used either by itself or in various elaborate food products. Dairy processing includes milk evaporation, spray drying, membrane filtration and floculation leading to yogurt or cheese production. This changes the mechanical properties, such as elasticity, or texture that are a consequence of the attractive interactions between its elementary constituents. Whey proteins, minerals and fatty droplets are constitutive components of milk, but casein micelles, consisting in the aggregation of casein proteins, are its major component (e.g. that represent 80% of bovine milk proteins) and play a key role in the structural properties of dairy processed products. 

Casein micelles are constituted of proteins (four main different casein molecules are present), minerals, and water, which is a major component of casein micelles (≈ 4 g of water per g of protein). Caseins proteins moreover possess extremely open and flexible conformations. As a result, casein micelles may be seen as sticky swollen microgels whose properties strongly depend on the composition of the solvent. In particular, the casein tails of the assemblies interact through weak interactions, leading to a three-dimensional aggregated network. The microstructure of dairy products may be controlled by various physico-chemical parameters, such as temperature, shear, pH, ionic strength, and water content. The understanding and tuning of this microstructure is important as it determines not only their processability but also the sensory properties of the final products, such as their texture, firmness or mouthfeel.

Network activities and expected results

In this project, we wish to tackle two specific questions that occur during the process of dairy systems : 

• How do the structural and dynamical properties of networks of dilute casein micelles depend on their interaction?
• How do the dynamics of concentrated casein micelles depend on the composition of the system? In particular, we will seek to describe the properties of systems that do not evolve with time, but also aim at
  following the evolution of their properties with time after a change of their interactions. 

Institutions and laboratories involved

France

IPCMS/CNRS, UMR 7504, 23 rue du Loess 67034 Strasbourg 

Australia

University of Melbourne, 03, 313, Chemical Engineering #1, Parkville 

Distribution of relaxation times during the floculation of a casein solution, during rennet gelation. The images represents the time relaxation distribution P(tau) as a function of the time after the addition of rennet.  One observes a divergence of the relaxation time (blue continuous line) at 110000s, that is characteristic of the gel transition.