a creamy mouth-feel to the milk as opposed to a dry texture. The fat phase in milk also has an
important role in determining flavour - it acts as the reservoir for many of the milk flavour compounds
that are found in the fat phase of the milk. The fat phase is found in the form of small fat droplets that
are evenly dispersed in the water phase, which is called an emulsion.
Why do the fat phase and the water phase not separate out?
The fat in milk exists as droplets, each of which is surrounded by a “membrane” containing
phospholipids. As explained previously, phospholipids are made of two parts: hydrophobic and
This membrane therefore helps the dispersion of the fat droplets in the water phase because the
hydrophobic parts of the molecules in this membrane stay in contact with the fat droplets that it
surrounds, holding it together, while the hydrophilic parts contact the surrounding water molecules,
keeping the droplets temporarily dispersed in the water phase.
The milk proteins
Milk also contains many other proteins, which can be classified into two groups – the caseins and the
The caseins exist in the milk as big bundles of proteins, glued together by the calcium and phosphate
ions present in milk, to form a large structure known as a “micelle”. These bundles are surrounded by
a negative charge that keeps them repelled from each other.
The whey proteins, in contrast, are much less common and exist dissolved in the liquid.
Milk has a cloudy appearance because both the fat droplets and the casein agregates are sufficiently
large to deflect light. As light is usually white, milk appears white, but let’s look at milk under red or
blue light: because we can see only reflected light in this case, milk appears respectively red or blue.
Usually the separation of phases is undesirable. Milk to be sold is usually subjected to the two
processes of homogenisation and pasteurisation. Pasteurisation involves heating the milk, which
denatures these proteins and prevents them from helping to separate the emulsion. (Pasteurisation