This section is from the book "A Book Of Recipes For The Cooking School", by Carrie Alberta Lyford. Also available from Amazon: A book of recipes for the cooking school.
The art of preparing jellies from the juice of fruits is one in which every housewife is eager to perfect herself. One can point with justifiable pride to the shelf of well-made jellies that are going to increase the attractiveness of the family table and add to the palatability of a diet which might otherwise be monotonously simple. A few fundamental principles must be borne in mind if one is to become expert in jelly making.
Pectin, the jelly-making property of fruits - The substance in fruit juice that causes the juice to thicken and form a jelly after it has cooked is called pectin. This substance is present in different quantities in various fruits; hence some fruits can readily be made into a jelly, while a jelly cannot be successfully made from other fruits. The amount of pectin varies at different stages of ripeness of the fruit. Pectin is present in largest quantity in fruits that are slightly under ripe. Overripe fruits contain less pectin. Pectin is found near the skin or around the core of fruit; hence clean parings and sound cores should be cooked with the fruit when making jellies.
Acid necessary in making jelly - In addition to pectin, it is necessarv that fruits contain acid or that acid be added to the fruit juice when jelly is being made. Most fruits which are good for jelly-making contain sufficient acid, but occasionally quinces and berries do not have sufficient acid present. If the juice of these fruits is then combined with the juice of apples or other tart fruit, they can be made into good jellies.
Sugar in jellies - While sugar is not necessary to the solidifying of fruit juice, it is necessary to add sugar to render the jelly palatable. The amount of sugar may be varied within certain limits according to the flavor of the fruit. The quantity of sugar used will vary from one-half as much sugar as juice to equal parts of sugar and juice. More sugar than juice is apt to interfere with the formation of the jelly and a syrupy substance result.
Cooking the jelly - The juice is best extracted from the fruit by cooking. With soft fruits practically no water needs be added. With apples and quinces it is necessary to cover the fruit with water in order that the pectin may be drawn out. After the fruit has been cooked sufficiently to soften the pulp and draw out the juices, it should be strained through a flannel or muslin bag or a bag made of two thicknesses of cheese-cloth. Two or three hours will be sufficient time to allow for straining, then the juice should be reheated. If much water has been added when extracting the juice, or if the fruit contains much water, it should be boiled for fifteen or twenty minutes to evaporate excess of water before adding the sugar. After the sugar has been added, it should not be necessary to cook the juice more than eight or ten minutes before the jellying point is reached. The best jelly results when only a small amount of juice is cooked at one time (not more than 3 cups).
Test for jelly - The juice is ready to form a jelly when it begins to sheet from the spoon, that is, when the spoon is dipped into the jelly and raised above it two drops will come from the edge of the spoon at the same time. The jelly should then be quickly removed from the fire and poured into sterilized glasses.
Fruit best for jelly making - The fruits which make the best jellies are apples, crab-apples, quinces, green grapes, red currants, plums, gooseberries, and firm, hard berries such as blackberries, dewberries, and logan berries. These same fruits do not make a good jelly if they are not sour; therefore it is often necessary to combine one fruit with another fruit that contains more acid.