Effect of Homogenization
|No Homogenization||15 MPa (2500 psig)|
|Av. diam. (µ m)||3.3||0.4|
|Max. diam. (µ m)||10||2|
|Surf. area (m2/ml of milk)||0.08||0.75|
|Number of globules (µ m-3)||0.02||12|
Surface layers The milk fat globule has a native membrane, picked up at the time of secretion, made of amphiphilic molecules with both hydrophilic and hydrophobic sections. This membrane lowers the interfacial tension resulting in a more stable emulsion. During homogenization, there is a tremendous increase in surface area and the native milk fat globule membrane (MFGM) is lost. However, there are many amphiphilic molecules present from the milk plasma that readily adsorb: casein micelles (partly spread) and whey proteins. The interfacial tension of raw milk is 1-2 mN/m, immediately after homogenization it is unstable at 15 mN/m, and shortly becomes stable (3-4 mN/m) as a result of the adsorption of protein. The transport of proteins is not by diffusion but mainly by convection. Rapid coverage is achieved in less than 10 sec but is subject to some rearrangement.
Surface excess is a measure of how much protein is adsorbed; for example 10 mg/m2 translates to a thickness of adsorbed layer of approximately 15 nm.