When making jams and jellies, pectin helps make the juice “jell.” A hundred years ago commercial liquid and powdered pectin was not available. Rather cooks used fruits with naturally occurring pectin – and often combined several fruits, including one with a lot of pectin, when making jelly. Here’s what it said in a hundred-year-old home economics textbook:
Fruit juice can be made into jelly when it contains two substances, (1) pectin and (2) acid. All fruits do not contain these in sufficient amounts to make good jelly, and often it is necessary to combine the juices of two fruits before the juice will “jell.” Sugar helps to make the juice form jelly, but unless pectin and acid are present, no amount of sugar will have that effect.
Fruits used for jelly should not be over-ripe, and sometimes it is better to use green fruits, because as fruit ripens it contains less pectin and acid. Tart apples, grapes, currants, crab apples and plums are good for making jelly. Sweet ripe apples, strawberries, blackberries, peaches and pears are poor fruits from which to make jelly.
Lemon and orange peel contain pectin in considerable amounts and are sometimes used to make fruit juices “jell.” Remove the yellow layers of the peel and put the white material that is left through the food grinder, cover with water and let stand for several hours, then cook slowly for two or three hours; strain the liquid and add it to the fruit juice that lacks pectin.
Elementary Home Economics (1921) by Mary Lockwood Matthews