Raspberries, blueberries, lingonberries, highbush cranberries, wild currants, cloudberries, and rose hips, not to mention the strawberries from our gardens, are transformed in quantity each year into spreads to put on next winter's toast. The process of making jam or jelly involves a substance called pectin. Pectin is a carbohydrate, made up mostly of carbon, hydrogen, and oxygen. It is formed from substances found in the cell walls of fruit, and particularly concentrated in skins and cores. The conversion of these precursor substances to pectin occurs naturally during ripening, and can also be forced by long cooking, as in the traditional methods of making jellies without added pectin.
Fruits vary a great deal in how much pectin or pectin precursors are present.
Raspberries and blueberries, for instance, are low in pectin, while apples, citrus fruits, cranberries, and currants are high. Commercial pectin today is usually extracted from the white inner part of citrus skins. Pectin produces structure and a kind of stiffness in jellies and jams by forming a water-holding network within the juice or crushed fruit. Before jelling starts, individual molecules of pectin are surrounded and isolated from each other by water molecules. If the surrounding solution is acid enough, the pectin loses some of its attraction for these isolating water molecules.