Cow free cheese: Characterization of recombinant casein micell size and microstructure

Purpose and goal:

The aim of the project was to generate an increased understanding for how Cassius AB recombinant casein build micelles, and further how they build a cheese type structure. We characterized recombinant casein using Small Angle X-ray Scattering at MAX IV laboratory, and Dynamic Light Scattering .

By varying the ratio between alpha- and kappa casein, as well as pH and ion strength, we received quality information on what affects the size of Cassius recombinant casein micelles, as well as the size and ordering of the calcium-casein nanoparticles that build up the micelle microstructure. Expected results and effects:

The size of recombinant casein micelles decreased with increasing NaCl concentration, while pH has little or no effect on the micelle size. Increased ratio of kappa to alpha casein led to larger micelles, and also increased their sensitivity towards the solution´s ionic strength. After renneting and lowering of pH we found a pronounced peak in the SAXS data with a length scale of ~6 nm.

We assign this to a high degree of order of casein-calcium nanoparticles. The same peak was not found in cow casein handled in the same way, instead a much broader signal was found at a length scale of ~10 nm. Approach and implementation:

SAXS was measured on three different mixtures of recombinant alpha and kappa casein. The solutions had two different pH and four different ionic strengths. By combining spherical and gaussian coil models we could characterize the overall size of the micell as well as the size of the casein-calcium nanoparticles inside the micelle.

After renneting coagulation and lowering of pH we again measured SAXS. This time the data was harder to model without the risk of overfitting. Therefore, to be verify that our model is correct we need more information from e.g. electron microscopy.