On Food and Cooking

fifth of the water in the mix remains
unfrozen even at 0ºF/–18ºC. The result is
a very thick fluid that’s about equal
portions of liquid water, milk fat, milk
proteins, and sugar. This fluid coats each
of the many millions of ice crystals, and
sticks them together — but not too
strongly.
Air cells are trapped in the ice cream mix
when it’s agitated during the freezing.
They interrupt and weaken the matrix of
ice crystals and cream, making that
matrix lighter and easier to scoop and
bite into. The air cells inflate the volume
of the ice cream over the volume of the
original mix. The increase is called
overrun, and in a fluffy ice cream can be
as much as 100%: that is, the final ice
cream volume is half mix and half air.
The lower the overrun, the denser the ice
cream.