By Harold McGee
Fruit preserves are a kind of physical structure called a gel: a mixture of water and other molecules that is solid because the other molecules bond together into a continuous, sponge-like network that traps the water in many separate little pockets. The key to creating a fruit gel is pectin, long chains of several hundred sugar-like subunits, which seems to have been designed to help form a highly concentrated, organized gel in plant cell walls. When fruit is cut up and heated near the boil, the pectin chains are shaken loose from the cell walls and dissolve 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.