The effect of ageing fresh milk for 24 h at 4°C on the composition of 4 fractions of milk fat globule membrane (FGM) – microsomal pellet membrane (MPM), deoxycholate soluble membrane (DOCM), high density membrane (HDM) and low density membrane (LDM) – prepared from washed cream treated with sodium deoxycholate (DOC), was studied for milk of individual cows. Total FGM and its fractions were solubilized by treatment with sodium dodecyl sulphate (SDS), EDTA and β-mercaptoethanol, and the dissociated FGM proteins were separated by polyacrylamide gel electrophoresis in the presence of SDS.
Ageing resulted in a greater loss of phospholipid during cream preparation than was found with fresh milk, and also in a reduction in the total amount of FGM isolated. Loss of FGM material was confined to MPM, DOCM and LDM and was accompanied by compositional changes in DOCM and LDM, quantitatively the most significant fractions. Ageing also produced changes in the gel electrophoresis patterns of DOCM, where the band of greatest mobility (mol. wt about 16000) was partially lost, and of LDM, where there was an increase observed in the major protein band.
The implication of the results on the proposed models of FGM structure is discussed.