Lipid domains in the biological membrane surrounding milk fat globules: role of temperature and cholesterol on their morphology and nanomechanical properties

The biological membrane surrounding fat globules in milk, the MFGM, is poorly known despite its importance in the functional properties of many dairy products and in the mechanisms of milk lipids digestion occurring in the gastrointestinal tract of mammal newborns. Studies recently revealed the formation of lipid domains in the MFGM that have been interpreted as the lateral segregation of high phase transition temperature (Tm) lipids, mainly milk sphingomyelin (MSM; Tm 34°C) [1]. The role played by these MSM-rich domains in the MFGM is currently unknown and needs further investigation. In this context, the aim of this study was to investigate the role of temperature and cholesterol in the morphology and nanomechanical properties of the MFGM. Accessing the biophysical properties of biological membranes at nanoscale is possible using Atomic Force Microscopy (AFM). In the complex biomimetic membranes formed using a MFGM lipid extract, topographical AFM images revealed for T < 35°C the formation of µm-large domains that protruded above the continuous phase by ~1 nm [2]. The resistance of the MSM-rich gel phase domains to rupture was significantly higher than that of the liquid disordered continuous phase, i.e. breakthrough force FB of 23 nN vs. 6 nN at 25°C, respectively. The key role played by cholesterol in the lateral segregation of high Tm lipids, by decreasing the size of the lipid domains, their height and by fluidizing the MFGM has been demonstrated [3]. Such work on the heterogeneous distribution of polar lipids in the MFGM may open perspectives in dairy technology and infant nutrition, e.g. optimisation of the interface of processed lipid droplets in infant milk formulas to mimick the biological MFGM in breast milk