The jelly-making quality of fruit depends on the presence of a certain substance called "Pectin." When equal quantities of sugar and fruit juice are combined, and heated to boiling point for a short time, this pectin in the fruit gelatinises the mixture.
It is important that the jelly-maker should understand when this gelatinising agent is at its best Pectin is at its best when the fruit is just ripe, or when it begins to ripen. Soon after fruit is ripe the fermentative stage follows, when the pectin loses its gelatinising power, and the same results when the cooking of the jelly is continued too long. Therefore, it is of the greatest importance that the fruit should be just ripe or underripe, and that the boiling of the juice and sugar should not be continued too long. The shorter the time of boiling, the more pronounced will be the fruit flavour.
An acid fruit is the most suitable for jelly-making, although in some of the acid fruits the pectin is so small that it is difficult to make jelly from them. The fruits most desirable for jelly-making are the following, the best being given first: Quince, apple, Kaffir watermelon, orange, grape, guava, strawberry, blackberry, raspberry, plum, pineapple, pear, peach. Some of these, however, are deficient in acid, such as the guava and Kaffir watermelon, and so it is necessary to add a little lemon juice or citric acid. On the other hand, fruits deficient in pectin, such as the peach, strawberry, apricot, etc., can be combined with those containing a higher percentage, and a very satisfactory jelly made.