An acid, like lemon juice, works with natural fruit pectin to form insoluble fibers, which promote gel formation by absorbing juice. Fruit cannot turn to jelly without adequate amounts of acid. Too little acid and gel will not form. Too much acid and the gel that does form loses liquidity. Fruits that do not contain enough acid to form jelly require additional acids to get the job done.

Most jelly recipes call for lemon juice as the standard, but you may substitute citric acid -- usually sold in powdered form -- in place of it. The two acids are not equal in volume, so you must adjust the amount of ingredients when substituting. Use 1/8 teaspoon of citric acid for every tablespoon of lemon juice. You may also use vinegar -- 2 tablespoons in place of every tablespoon of lemon juice -- but this may alter the taste of the jelly.

A simple test helps to determine if lemon juice is needed. Add 1 teaspoon of bottled lemon juice to a small cup. Add 3 tablespoon of water and 1/2 teaspoon sugar to the lemon juice and stir. Take a sip of the lemon juice, then take a sip of the fruit juice. The fruit juice you are using for jelly should taste as tart as the lemon juice. If it doesn’t, more acid is needed.

Just as too little acid makes jelly-making close to impossible, too much acid results in instability. Proper ratios of acid to fruit juice are important. Don’t overdo it with the lemon juice. For every 1 cup of fruit juice, add 1 tablespoon lemon juice Avoid using more than 8 cups of juice when making a single batch of jelly. Larger amounts of fruit juice rarely yield well-formed jelly.