Heterogeneity of milk fat globule membrane structure and composition as observed using fluorescence microscopy techniques

Fluorescence microscopy studies of fat globules in milk and milk products using fat-soluble stains (Nile Blue or Nile Red) allow visualisation of the neutral fat (core) of the globules, but provide no information about the milk fat globule membrane (MFGM). We applied the lipophilic probes 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine-5,5′-disulfonic acid (DiIC18(3)-DS) and N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylamino)phenyl)hexatrienyl)pyridinium dibromide (FM4-64), as well as fluorescent conjugates of the lectin wheat germ agglutinin (WGA488, WGA594 and WGA647), to milk to stain specifically the MFGM in its native environment. Using fluorescence microscopy, we observed various distribution patterns of the probes. This demonstrates that the MFGM of fat globules in harvested milk is structurally and chemically heterogeneous both within and among globules from the same species and probably between species. We propose that the use of membrane-specific fluorescent probes has significant potential for providing real time structural and chemical information about the MFGM in matrices such as mammary gland tissue, harvested milk, and milk products.