Studying Fat Globules in Milk

Milk

The following article, based on application information available via the Beckman Coulter website, gives an example of how measurements of particle size and zeta potential may both be obtained using the same instrument. In this case the subject is fat globules in homogenised milk.

Within raw, fresh, whole milk, fat takes the form of emulsified globules, each enveloped by a milk fat globule membrane. Composed of phospholipids, glycolipids, proteins, lipoproteins and enzymes, the membrane becomes damaged in the process of homogenisation. As a result, the fat globules decrease in size. Meanwhile, the increased interfacial area allows the casein micelles and plasma proteins to be adsorbed onto the surface of the fat globule. With the decrease in fat globule size, creaming rate reduces, susceptibility to cold agglutination is lowered and heat stability in the concentrated milk is increased.

It is important to determine and understand the changes that the homogenisation process brings about in fat globules. In the past this was done using fatty acid titration, but there was much inconsistency in results between users. An alternative was to compare size distribution before and after homogenisation, but if the milk had already been subjected to partial creaming or microfiltration this was not possible.

Today, measurement of zeta potential can be used as a way of characterising homogenised milk. When plasma proteins are adsorbed onto the globule surface during homogenisation they alter its surface charges. Zeta potential gives a measurement of this effect.

Use of instruments developed by Beckman Coulter allows analysts to measure zeta potential changes in cream samples homogenised at different pressures and to confirm the results by making size measurements.

As you may have read elsewhere on this site, dynamic light scattering or photon correlation spectroscopy is a very effective technology for measuring particle size. In a number of Beckman Coulter’s analysers, there is also the option of measuring zeta potential.

Zeta potential can be defined as the potential at the shear plane, located close to the boundary and the diffuse layer. When an electric field is applied, charged particles in a fluid will move. This movement is called electrophoresis. The analyser uses the principle of electrophoretic light scattering to detect this electrophoretic movement and then computes the zeta potential from it.

In this application, it can be seen that as homogenisation pressure increases, the mean fat globule size decreases and zeta potential increases. That decrease in size can be attributed to the increasing zeta potential, which results in greater adsorption of plasma proteins onto the globule surface. The conclusion is that measurement of zeta potential gives a better quantitative way of characterising the effect of homogenisation on fat globules.