Making jelly at home is requires only a few ingredients and a couple of hours of your time, but is a project that pays off big in both satisfaction and compliments to the cook. While many fruits contain enough acid to complete the gelling process, a few require the addition of lemon juice. Follow jam and jelly recipes exactly if you want to get satisfactory, safe results.
A Little Food Chemistry
Fruits contain a substance in their skins known as pectin, one of the key components in making jelly. Sugar and fruit acid are also needed in this process, with white cane sugar being the most commonly used sweetener. Many fruits contain enough acid in their juices to react favorably with pectin and sugar to form an acceptable gel, while others require the addition of acid. Jelly recipes often call for the addition of lemon juice to raise fruits’ acidity.
Blueberries, strawberries, raspberries, grapes, figs, apricots, peaches and Italian-type plums are the most common fruits that aren’t acidic enough to set up on their own. These fruits always require the addition of lemon juice to raise their acidity high enough to reaction favorably with the pectin and sugar. Without it, the jelly has a too-soft texture and may spoil before it is used.
Fruits that contain a borderline amount of acid may or may not require the addition of lemon juice and pectin. Sweet varieties of ripe apples, grapefruit, oranges, ripe blackberries, elderberries, sour cherries, bottled grape juice, elderberries and chokecherries fall into this group.
Slightly underripe fruits do not need to have lemon juice added to them. One bite of a green apple will tell you why. Cranberries, crabapples, underripe blackberries, cranberries, lemons, gooseberries, currants, Eastern Concord grapes and non Italian-type plums fall into this category.
While slightly overripe fruit can be used in making jelly, throw out any pieces that are moldy or rotten. Bruises and blemishes on larger fruit can be cut out and discarded, leaving the sound fruit for your jelly pan. Other acidic items, such as vinegar or citric acid, can be used in place of lemon juice, but their stronger flavors may overpower the fruit flavors in the finished product.
References and ResourcesClemson University: Basics of Jelly Making
Plant Physiology: Biosynthesis of Pectin, Pectin Structure