Lipid domains in the milk fat globule membrane: Dynamics investigated in situ in milk in relation to temperature and time

Abstract

The microstructure of the milk fat globule membrane (MFGM) is still poorly understood. The aim of this study was to investigate the dynamics of the MFGM at the surface of milk fat globules in relation to temperature and time, and in relation to the respective lipid compositions of the MFGM from bovine, goat and sheep milks. In-situ structural investigations were performed using confocal microscopy. Lipid domains were observed over a wide range of temperatures (4-60°C). We demonstrated that rapid cooling of milk enhances the mechanisms of nucleation and that extended storage induces lipid reorganization within the MFGM with growth, leading to circular lipid domains. Diffusion of the lipid domains, coalescence and reduction in domain size were observed upon heating. Different MFGM features could be related to the respective cholesterol/sphingomyelin molar ratio in the three milk species. These structural changes may affect the interfacial properties of the MFGM, with consequences for the functional properties of fat globules and the mechanisms of their digestion.

Young modulus of supported lipid membranes containing milk sphingomyelin in the gel, fluid or liquid-ordered phase, determined using AFM force spectroscopy

“…In the native MFGM, cholesterol may be involved in the formation of liquid-ordered (Lo) phase domains that have been observed at as elevated temperature as 60°C, e.g. above the Tm of the MFGM polar lipids or of the milk SM [15]. The Young modulus values of these Lo phase domains, mainly composed of milk-SM and cholesterol, need further investigation to provide information useful in a biological context.…”

“…Meanwhile, the human erythrocyte membrane contains up to 45 mol% of cholesterol [82,83]. Even though the distribution and fluxes of cholesterol in cells are complex [84], this indicates that gel phase is probably inhibited in many biologically relevant situations, even though indirect observations pointed to some gel phase domains in the MFGM [15], not to mention the role of other compounds like ceramides [76]. The high amounts of cholesterol in plasmatic membranes, compared to those used in model vesicle or supported membranes, may account for the lower Young modulus values generally found when directly probing cells or milk fat globules.…”

“…Meanwhile, the cholesterol represents 3-6 g kg −1 of the total fat in milk (among the mono-, di-, triglycerides, free fatty acids, polar lipids etc. - [15,16]). It is mainly located in the MFGM [17] with a cholesterol:polar lipid mass ratio of~1:3.…”

“…In analogy with cellular "rafts" [18], these microdomains were interpreted as the lateral segregation of high Tm polar lipids, notably milk SM, dispersed in a continuous fluid phase matrix of unsaturated PE, PS, PI, and PC [19][20][21]. Individual milk SM-rich domains are expected to be in the gel or in the liquid-ordered phase in the absence or presence of cholesterol, respectively [15]. Therefore, the gel, liquid-ordered and fluid phases are expected to coexist in the MFGM, making it a most complex colloidal system in native milk.…”

“…Therefore, the gel, liquid-ordered and fluid phases are expected to coexist in the MFGM, making it a most complex colloidal system in native milk. Studies showed that the MFGM is indeed a dynamic system in which the organization of polar lipids [22][23][24] and the lipid domains are highly sensitive to temperature variations and time [15,25]. Increasing knowledge about the structure of the MFGM and its physical properties is therefore of primary interest to better control storage of human milk or to design bio-inspired lipid droplets e.g.…”