Shear-sensitive casein micelle suspensions for future Rheo-SANS experiments

Caseins from cow milk are used as raw material for several food products such as yoghurt or cheese or as additives for protein enrichment. They form so-called “casein-micelles”, colloidal structures of spherical shape with many structural similarities to microgels (e.g. architectural versatility depending on conditions, permeability and deformability) [1]. Caseins are prone to self-assemble because they have structural features similar to diblock-copolymers. While hydrophobic blocks cross-link the caseins with themselves, phophoserin-rich patches interact with calcium-phosphate nanoparticles. Spherical casein micelles with a diameter of approx. 150 nm or recently produced casein micro-particles (10-30 µm) are examples of self-assembled, higher aggregated structures. The lack of secondary and tertiary structure confers high flexibility to the protein which might explain why supramolecular casein complexes behave like soft matter. This has consequences for a number of functional and processing properties. The flow behaviour of casein suspensions is quite interesting for a number of food engineering processes. During processing, caseins micelles are subjected to a wide range of shear rates and temperatures. For transportation through pipes comparatively low temperatures and shear rates (100 – 103 s-1) occur [2,3] while higher shear rates (103 – 106 s-1) and temperatures (50 – 75 °C) are used during homogenization [3,4]. Both variables affect the rheological properties of casein suspensions [5]. However, little is known about the relationships between flow behaviour and structures in casein micelle suspension. We will report in detail on X-ray, Raman- and light scattering experiments in combination with elongational flow and compression, which we used to probe the soft properties of casein micelles. Furthermore, we present viscosity curves of casein micelles measured over a broad temperature and concentration range and the corresponding analysis with regard to similarities between casein micelles and microgels [6,7,8].

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[8] S. Khanna (2017) master thesis, AVT.SMP, RWTH Aachen University