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An emulsion is a smooth mixture of two
liquids, such as oil and water, that normally do not mix. On a microscopic
level, an emulsion consists of tiny particles of one liquid (or
occasionally a solid) suspended in another. These tiny particles are kept
separate from one another by an extremely fine coating of an emulsifier
that surrounds each particle, keeping them from touching one another and
from clumping into larger globules, which would float or sink.
Mayonnaise is an example of an emulsion in
which microscopic globules of oil are suspended in a relatively small
amount of water-based liquid. The suspension is held in place by the
emulsifier, which, in this case is egg yolk. Other familiar emulsions are
beurre blanc (globules of butter fat suspended in a medium of white wine
and vinegar, emulsified by the milk solids in the butter), hollandaise
(emulsified with egg yolks, like mayonnaise), cream sauces (milk solids in
the cream keep the butterfat and liquid in it in a stable emulsion),
vinaigrettes (mustard holds the oil and water in an emulsion), and
bechamel sauce (the flour in the roux stabilizes the milk into an emulsion
that can withstand heat).
Cooks rely on various emulsions to make
sauces, custards, souffles, cakes and pastries. But some emulsions are
undesirable. If a meat stock or jus is allowed to boil, the fat that is
slowly being released by the meat is churned back into the liquid and will
eventually become emulsified, resulting in a cloudy, greasy, muddy-tasting
stock. (simmering allows the fat to float to the surface, where it can be
skimmed off). For the same reason, the first step in making gravy from pan
juices is to skim off the fat floating in the roasting pan. Otherwise, the
thickener in the gravy, usually flour, will emulsify the fat into the
gravy.
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