Dairy Chemistry And Biochemistry

MILK LIPIDS 103

Figure 3.18 Schematic representation of a trilaminar cell membrane which is derived from the

apical membrane of the mammary cell and forms the outer layer of the milk fat globule

membrane following expression from the mammary cell, but which is more or less extensively

lost on ageing. 1, phospholipid/glycolipid; 2, protein; 3, glycoprotein.

dark, electron-dense layers). The phospholipid molecules are arranged in a
bilayer structure (Figure 3.1 8); the non-polar hydrocarbon chains are
orientated inward where they ‘wriggle’ freely and form a continuous hydro-
carbon base; the hydrophilic regions are orientated outward and are
relatively rigid. In this bilayer, individual lipid molecules can move laterally,
endowing the bilayer with fluidity, flexibility, high electrical resistance and
low permeability to polar molecules. Some of the globular membrane
proteins are partially embedded in the membrane, penetrating into the lipid
phase from either side, others are completely buried within it, while others
transverse the membrane. The extent to which a protein penetrates into the
lipid phase is determined by its amino acid composition, sequence, second-
ary and tertiary structure. Thus, membrane proteins form a mosaic-like
structure in an otherwise fluid phospholipid bilayer, i.e. the fluid-mosaic
model (Figure 3.18).
Thus, the milk fat globules are surrounded and stabilized by a structure
which includes the trilaminar apical membrane (which is replaced by Golgi
membranes on secretion of proteins and lactose). The inner face of the
membrane has a dense proteinaceous layer, 10-50 nm thick, probably
acquired within the secretory cell during movement of the globule from the
rough endoplasmic reticulum at the base of the cell, where the triglycerides
are synthesized, to the apex of the cell. A layer of high melting triglycerides
may be present inside this proteinaceous layer. Much of the trilaminar
membrane is lost on ageing of the milk, especially if it is agitated; the
membrane thus shed is present in the skim milk as vesicles (or microsomes),
which explains the high proportion of phospholipids in skim milk.
McPherson and Kitchen (1983) proposed a detailed structural model of
the MFGM, which appears rather speculative. Keenan et aI. (1983), Keenan
and Dylewski (1995) and Keenan and Patton (1995) describe the current

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