and change rapidly. Apple and pear juices turn
brown, for example, thanks to the action of
browning enzymes and oxygen (p. 269). If not
used immediately, they’re best kept chilled or
frozen, perhaps after a heat treatment just
short of the boil to inactivate enzymes and kill
microbes. Modern juicing machines can apply
very strong forces, and make it possible to
extract juice from any fruit or vegetable, not
just the traditional ones.
Foams and Emulsions The cell-wall
carbohydrates in purees and juices can be used
to stabilize two otherwise fleeting physical
structures, a foam of air bubbles and an
emulsion of oil droplets (pp. 638, 625), which
are especially easy to prepare with modern
electrical blenders and mixers. If a puree or
juice is whipped to fill it with air bubbles, the
cell-wall carbohydrates slow the flow of water
out of the bubble walls, so the bubbles take
longer to collapse. This allows the cook to