Structural Properties of Casein Micelles in Milk, the effect of salt, temperature, and pH

Milk is a complex liquid, which contains many different species, for example proteins, fat, minerals etc. It is the primary source of nutrition for young mammals before they are able to digest other types of food. The proteins can be divided into two groups: caseins and whey. Whey proteins are about 20 wt % of the total protein amount in milk, whereas the caseins corresponds 80 wt % of the total protein content in milk. The largest structures in the fluid portion of the milk are “casein micelles” which are aggregates of several thousands of protein molecules. The micelle is considered to be spherical and the diameter is in the micrometer size. The caseins can be divided in four types: αs1casein, αs2 casein, β casein, and k casein. ß casein is one of the most abundant caseins and it also self assembles to larger aggregates. In this thesis we have used a simple model to try to capture how electrostatic interactions affects the structure of β casein micelles in milk. The micelles have been modeled as hard spheres, with a central net charge of --140e, and a radius of 75 Å. These parameters have been taken from experimental data published in the literature. The structure of the solution has been studied by comparing the radial distribution functions for different solution conditions, such as the salt concentration and valency, pH, and the temperature. Popular scientific description Milk is a complex liquid, which contains many different species, for example proteins, fat, minerals etc. It is the primary source of nutrition for young mammals before they are able to digest other types of food. The proteins can be divided into two groups: caseins and whey. Whey proteins are about 20 wt % of the total protein amount in milk, whereas the caseins corresponds 80 wt % of the total protein content in milk. The largest structures in the fluid portion of the milk are “casein micelles” which are aggregates of several thousands of protein molecules. The micelle is considered to be spherical and the diameter is in the micrometer size. In this thesis we have used a simple model and computer simulations to try to capture how electrostatic interactions affects the structure of β casein micelles in milk. The micelles have been modeled as hard spheres, with a central net charge of 140e, and a radius of 75 Å. The volume fraction of was set to 5%, which is the actual volume fraction in the real product. The structure of the solution has been studied by comparing the radial distribution functions for different solution conditions, such as the salt concentration and valency, pH, and the temperature. It was noticed that due to the fact that the micellar charge is very large, the electrostatic repulsive interaction dominates, and the mean distance between the micelles are almost always obtained. Moreover, an increase of the temperature does not affect the structure at all i.e. the entropic contribution due to increased temperature can be neglected in comparison with electrostatic repulsion between the micelles. Also, there might also be an influence of the electric permittivity since it was kept constant during the simulations, When the salt concentration was increased to 80 mM, which corresponds to the ionic strength in milk, the structure of the β casein micelles resembles the structure of an ideal gas i.e. the electrostatic repulsive interactions are screened