This section is from the book "Everywoman's Canning Book", by Mary B. Hughes. Also available from Amazon: Everywomans canning book; the A B C of safe home canning and preserving.
Jelly making is simple enough, if a few general rules are observed. Those who wish to know the scientific principles underlying the art are referred to "Successful Canning and Preserving," by Ola Powell, and to "The Principles of Jelly Making," by N. E. Goldthwaite, Ph.D. The latter is a free bulletin (No. 31), issued by the University of Illinois, Urbana, Illinois.
Jellies are made by cooking together certain fruit juices and sugar in the right proportion. A good jelly will have certain essential qualities. It should be of good color, with sparkling transparency, of decided flavor, and firm enough, without being tough or gummy, to hold its shape when slipped out of the glass.
Fruit juice is composed mostly of water, and also of small amounts of flavoring substance, sugar, acids, and a most important substance called pectin. Without pectin present in the fruit juice, no jelly can be made unless artificial means are employed. Some fruits are very rich in pectin, while others have very little; hence the importance of choosing fruit rich in pectin, or of combining two fruits when a fruit is known to have but little pectin. Sour apples, unripe grapes, currants, and quinces have a large proportion of pectin; while pears, peaches, strawberries, and cherries have smaller amounts. Over-ripe fruit is almost entirely deficient in pectin, and it is impossible to make jelly from it. Always cook fruit before straining out the juice, for heat is essential in developing the pectin. Uncooked fruit is often found lacking in pectin, while the same fruit cooked is found to have a great deal.
In using another fruit juice to supply pectin, use equal measure of the two juices. Apples are ordinarily used for this purpose, since apple juice is mild, and will not obscure the desired flavor. Grapes and quinces have too decided a flavor of their own to be used as a pectin supply.
To discover whether fruit juice has pectin, take a little of the juice on a saucer and add to it a small amount of grain alcohol, ninety-five per cent pure. If a gelatinous mass forms, there is enough pectin in the juice to make a good jelly. If there is no pectin, the juice must be cooked again with apple parings, apple juice, or the white inner skin of lemons, oranges, or grapefruit. Continue the cooking of the fruit juice until it responds to the alcohol or pectin test.
Use of over-ripe fruit; use of too much sugar; cooking too large a quantity of fruit juice at a time; failure to add a fruit rich in pectin when using fruits known to be lacking in pectin, as, for example, using ripe cherries without combining with their juice some apple juice, to supply pectin.