By Harold McGee
Preserve making begins with cooking the fruit to extract its pectin. Quince, apples, and citrus fruits are especially rich in pectin and often included to supplement other pectin-poor fruits, including most berries. The combination of heat and acid will eventually break pectin chains into pieces too small to form a network, so this preliminary cooking should be as brief and gentle as possible. (If a sparkling, clear jelly is desired, then the cooked fruit is gently strained to remove all solid particles of cell debris.) Then sugar is added, supplemental pectin if necessary, and the mixture rapidly brought to the boil to remove water and concentrate the other ingredients. The boiling is continued until the temperature of the mix reaches 217–221°F/103–105°C (at sea level; 2°F/1°C lower for every 1000ft/305m elevation), which indicates that the sugar concentration has reached 65% (for the relationship between sugar content and boiling point). A fresher flavor results when this cooking is done at a gentle simmer in a wide pot with a large surface area for evaporation. (Industrial manufacturers cook the water out under a vacuum at much lower temperatures, 100–140°F/38–60°C, to maintain as much fresh flavor and color as possible.) Now supplemental acid is added (late in the process, to avoid breaking down the pectin chains), and the readiness of the mix is tested by placing a drop on a cold spoon or saucer to see whether it gels. Finally, the mix is poured into sterilized jars. The mix sets as it cools below about 180°F/80°C, but firms most rapidly at 86°F/30°C and continues to get firmer for some days or weeks.