Citrus peel and juice are frequently the basis of marmalade. Conserves are jams made from a mixture of fruits. They usually contain citrus fruit, nuts, and raisins. Pectin is needed to provide thickening or gel formation. Pectin is formed from a parent compound, protopectin, during the ripening of fruit and during the cooking of underripe fruit.
All fruits contain some pectin.
Apples, crabapples, gooseberries, some plums, highbush cranberries, and citrus peel contain large amounts of pectin. Sufficient acid must be present for gel formation in marmalades and thickening in jams and conserves. For fruits lacking in natural acid, like strawberries, it is provided by lemon juice or other citrus fruit. Commercial pectin products contain organic acids, like fumaric acid, which increase the acid content of fruits which lack acid. Sugar aids in gel formation, develops flavor by adding sweetness, and acts as a preservative in jams, marmalades, preserves, and conserves.