Etske Bijl

The influence of genetic variants, degree of phosphorylation and environmental conditions on casein micelle characteristics

Etske Bijl



Prof. A.C.M. van Hooijdonk, PDQ, WU

Ir. H.J.F. van Valenberg, PDQ, WU

Project term:

September 2009- September 2013



Melk op Maat


This project is part of the Milk Genomics Initiative. One of the objectives of the milk genomics initiative is to improve milk quality by breeding strategies. To reach this aim an accurate description is needed of the desired milk composition to improve product properties. This project will focus on casein micelle composition and its physical and chemical characteristics in milk. Casein micelles are colloidal complexes which consist of caseins (alpha-s1, alpha-s2, beta and kappa-casein), which make up for 80% of the milk protein, and water and salts. Large variation exists in casein micelle composition due to difference in genetic variants, post-translational modifications and interaction with the milk environment. Although statistical relations have been found between variation in genetic variants, protein composition and product properties, knowledge of the underlying mechanism is missing. It is this missing step, which contains information how variation in composition influences casein micelle characteristics such as size, charge and structure, that could help us to understand product properties of cheese and yoghurt.


The project has two main objectives. First of all to determine the variation in genetic variants, degree of phosphorylation, and environmental conditions (especially the mineral composition) in raw milk. Secondly to determine the influence of these variations on casein micelle characteristics.


The objective will be approached from two sides. Research will focus on the one  

side on genetic variants and phosphorylation of caseins and their influence on casein micelle composition. On the other side focus will be on the variation in ion composition of the milk environment in relation to casein micelle composition. Analysis of these factors should result in a complete overview of the variation in casein micelle composition and its environment. To reach the objective, at the beginning of the project the seasonal change in mineral composition of bulk milk and individual milk was measured to gain insight in the variation, with special attention to free calcium concentration and calcium activity. Measurements were performed with the use of  Donan Membrane Technique and ICP-AES. Simultaneously a start was made to determine the variation in ratio of the four caseins and degree of phosphorylation in milk samples of individual cows taken at three moments during their lactation period. Samples were selected from the milk genomics database and analyzed with the use of CZE method.

Future research

The influence of casein micelle composition and environmental conditions on micelle size, charge and amount of colloidal calcium phosphate clusters will be measured by techniques such as dynamic light scattering, neutron scattering, cryo-TEM and zeta potential analysis. Furthermore a theoretical approach will be sought to understand the influence of variation in composition and environment on casein micelle stability and interactions in milk.