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

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Lipid domains in the milk fat globule membrane: the dynamics investigated in situ in milk in relation to

temperature and time

Oumaima Et Thakafy, Fanny Guyomarc’H, Christelle Lopez

To cite this version:

Oumaima Et Thakafy, Fanny Guyomarc’H, Christelle Lopez. Lipid domains in the milk fat globule membrane: the dynamics investigated in situ in milk in relation to temperature and time. IDF World Dairy Summit, Oct 2016, Rotterdam, Netherlands. 2016. �hal-02800559�

Lipid domains in the milk fat globule membrane:

the dynamics investigated in situ in milk in relation to temperature and time

INRA - Agrocampus Ouest

Science and Technology of Milk and Eggs Rennes, FRANCE

www.rennes.inra.fr/stlo

O. Et Thakafy, F. Guyomarc’h, C. Lopez

Science and Technology of Milk and Eggs (STLO), UMR 1253, INRA, Agrocampus Ouest, Rennes, France Christelle.Lopez@inra.fr

The milk fat globule membrane (MFGM)

Conclusion & Outlooks:

Milk is an emulsion of fat droplets stabilized by a biological membrane called the milk fat globule membrane (MFGM; Figure 1). The MFGM is mainly composed of polar lipids that have different melting temperatures (T_m), cholesterol, glycoproteins, and enzymes. The microstructure of the MFGM depends on the packing of polar lipids in the outer bilayer of the membrane that can lead to phase separation with formation of ordered domains (Figure 2). Milk is usually submitted to changes in temperature e.g. storage (4-7°C), churning (10-12°C) and heat treatments (e.g. pasteurization). Do these temperature variations and thermal history of milk affect the packing of polar lipids in the plane of the MFGM ?

Experiments were performed by confocal laser scanning microscopy (CLSM) under temperature controlled conditions, using the head-labelled phospholipid fluorescent probe Rh-DOPE. Only the fluid phase of MFGM was labelled.

Rapid cooling of milk from 60°C favors the appearance of small elongated domains while extended storage induces

lipid reorganizations within the MFGM

with growth leading to large circular lipid domains Heating of milk favors the diffusion of the lipid domains,

coalescence, reduction in size

or complete melting of the polar lipids (disappearance of domains)

Dynamics and reorganization of MFGM polar lipids investigated upon milk heating or cooling

This study demonstrates that the packing of polar lipids within the MFGM strongly depends on the temperature and the thermal history of milk. Changes in the morphology and physical properties of the lipid domains induced by temperature and time could affect many interfacial mechanisms involved in e.g. lipid digestion, adsorption of enzymes, bacteria or viruses, localization of proteins, and could also alter the physical stability of fat globules during milk processing.

10 µm 2 µm

Rh-DOPE

Membrane proteins Fluid

phospholipid phase

Milk: oil/water emulsion Milk fat globules MFGM enveloping fat globules

Ordered lipid domain

Ordered lipid domain (Lo phase): high T_m

polar lipids and cholesterol Ordered lipid domain

(gel phase):

high T_m polar lipids

Fig.2 2D representation of the MFGM

The disappearance of domains at 60°C was interpreted as gel to fluid phase transition of the saturated high T_m polar lipids. Meanwhile, the domains present at 60°C must be in liquid ordered (Lo) phase due to the involvement of cholesterol.

After cooling from 60°C, the formation of small elongated domains at the surface of milk fat globules was interpreted as fluid to gel phase transition of the saturated high T_m polar lipids.

Cooling from 20°C had no effect on lipid domains (circular shape & number), which was probably due to the fact that the temperature quench of 20°C did not encompass the T_m of the MFGM polar lipids.

Fluid phase : low T_m polar lipids

Fig.1 confocal laser scanning

microscopy Differential interference contrast (DIC)

microscopy

Outer bilayer Inner

monolayer

Heating at 10°C/min

T < Tm T > Tm

Heating

60°C

20°C

Red matrix of unsaturated low

T_m polar lipids in disordered

phase

5 µm 5 µm

60°C

4°C

Saturated high T_m polar lipids in ordered phase

5 µm

to 4 °C to 20 °C to 4°C

From T=60°C > Tm=36.4°C From T=20°C < Tm

Cooling

4°C 20°C 4°C

4°C 20°C

4°C Observations

after storage for 16h

Observations directly after

cooling

(>100°C/min)

Acknowledgements for co-funding the PhD thesis

IDF World Dairy Summit, 16-21 October 2016 Rotterdam, The Netherlands

Et Thakafy, O., F. Guyomarc'h & Lopez, C. Lipid domains in the milk fat globule membrane: the dynamics investigated in situ in milk in relation to temperature and time. Food Chemistry. DOI.10.1016/j.foodchem.2016.10.017