There is another principle involved in jelly making in addition to the principle of preservation. Fruit contains a substance known as pectose, one of the carbohydrates, that partially solidifies the fruit juice when the water in the juice is partially evaporated. The addition of sugar helps in this process, but no amount of sugar will set the jelly if the pectose is not present. Some fruits have more than others, and also more when not overripe. Currants and firm apples are good jelly makers, and serve as a basis for other fruits that do not jelly well. Mellow summer apples do not set well. Crab apples are excellent for this purpose.
There is another step in this process, the straining out of the juice from the pulp. For this, prepare a jelly bag from firm cotton cloth which has been boiled and washed. This bag must be hung in such a way that the juice drops from the point of the bag into a bowl below. It may be hung upon a stick between two chairs, or upon the rod of a strong towel rack over a table.