Casein Proteins in Milk

Analysis of bimodal particle size distributions of casein micelles in milk

Milk is a natural product originating from animals and is used as a basic food and raw material for a variety of dairy products. Various types of milk and cheeses, cream, butter or sour milk products are used for direct consumption. Milk is an aqueous, white, cloudy emulsion (colloidal dispersion) consisting of partly dissolved minerals, vitamins, lactose, proteins and fat droplets. The milk fat contains unsaturated fatty acids and the fat-soluble vitamins A, D, E and K. Colloidal casein calcium transport complexes, known as casein micelles, are at the heart of the milk system. The main function of the micelle is to swirl the casein molecules and to dissolve calcium and phosphate.

Creaming behaviour, stability, texture and the processing properties of milk depend on the fat content and the particle size of the fat droplets and casein micelles. Homogenisation takes place for various types of milk in order to break up the fat droplets, spread them evenly and thus improve the stability of the emulsion. The fat droplets of the raw milk originally have a broad size distribution between 1 and 10 microns. The homogenisation process results in the fat droplets being cut up to a size of less than 1 to 2 microns under the influence of high pressure. Casein micelles act as stabilisers responsible for the longevity of the milk and to prevent the phase separation of fat and water. The size, shape and structure of the casein micelles are very important in the dairy industry, particularly for cheese production. Differences in the particle size of the casein micelles can influence milk processing, and also cheese production. The size of the casein micelles is an important feature of the raw milk and determines the yield of dairy products.

Dynamic light scattering with photon cross-correlation spectroscopy (PCCS) is recommended for the detection of fat droplets and casein micelles because this method reliably identifies the slightest differences in size down to the nanometre range, even in cloudy samples. The results obtained can be used to improve the homogenisation process, the function of the casein micelles and thus to increase the milk quality.

In the analysis, two different types of milk with low and high fat content (skimmed milk with 0.3% and whole milk with 3.9% fat content) illustrate the existence of two particle populations – those of the coarser fat droplets and those of the finer casein micelles.

The skimmed, high-heat-treated UHT milk with a fat content of only 0.3% has a mean particle size of 205 nm. The fat content is negligibly small – predominantly casein micelles are demonstrated (red curve). The whole milk with a fat content of 3.9% has two modes, one at 250 nm and one at 1,300 nm (blue curve). The fine peak represents the population of the casein micelles and those of the coarse fat droplets. After filtration at 450 nm, the concentration of the fat droplets is negligibly small. Only the casein micelles with an average particle size of 200 nm are visible in the particle size distribution (green curve).