The temperature and storage effect on the microstructure of the milk fat globule membrane
Résumé
Milk is an emulsion of fat droplets stabilized by a biological membrane called the milk fat globule membrane. The MFGM is mainly composed of polar lipids that have different melting temperatures (Tm), 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 [1]. This microstructure and the phase separation of lipids induced by the formation of domains are still poorly known despite its importance in dairy processing and in nutrition. 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?
In this work,we characterized the evolution of the MFGM microstructure in situ, in milk as function of temperature and time by using confocal microscopy and Rh-DOPE as fluorescent probe.
The dynamic of lipid domains was investigated on wide range of temperature (4-60°C) which includes the temperature of MFGM phase transition (gel/fluid; Tm = 36.4°C). The cooling kinetics from 60°C and storage duration at low temperature (4 or 20°C) affect the shape, the size and the number of lipid domains, through the mechanisms of nucleation and growth. On the other hand, heating kinetics show several responses of lipid domains such as diffusion, coalescence, reduction in size or disappearance at the surface of the fat globules [2]. We suggest also that the reaction of lipid domains to temperature changes can indicate its phase state (gel or ordered liquid) and the composition (presence of cholesterol). These structural changes can affect the interfacial properties of the MFGM with consequences on the functional properties of fat globules and on the digestion mechanisms.