On Food and Cooking

into the released cell fluids and any added
water. They can’t simply re-form their gel for
a couple of reasons. Pectin molecules in water
accumulate a negative electrical charge, so
they repel each other rather than bond to each
other; and they’re now so diluted by water
molecules that even if they did bond, they
couldn’t form a continuous network. They
need help to find each other again.
The cook does three things to cooked fruit
to bring pectin molecules back together into a
continuous gel. First, he adds a large dose of
sugar, whose molecules attract water
molecules to themselves, thus pulling the
water away from the pectin chains and leaving
them more exposed to each other. Second, he
boils the mixture of fruit and sugar to
evaporate some of the water away and bring
the pectin chains even closer together. Finally,
he increases the acidity, which neutralizes the
electrical charge and allows the aloof pectin
chains to bond to each other into a gel. Food