Making jam

Pectin, sugar, and acid, together with proper cooking, are what make fruit gel. Fruit supplies the pectin and the natural acid, and sugar is what lets the gel form.

The National Center for Home Food Preservation outlines the plain method: use ripe fruit, cut and crush it, then add sugar and lemon juice and boil while stirring constantly until the mixture thickens. The cook then tests for doneness. At sea level, the temperature test is 220 degrees F with a jelly or candy thermometer. In the refrigerator test, a little jam goes on a cold plate in the freezer for a few minutes, and if it gels it is done.

The foam is skimmed off, and for storage the jam is packed into sterile jars and processed in a boiling-water canner. Overcooking can destroy the pectin's ability to gel.