Cook the fruit to extract the pectin. This preliminary cooking should be brief and as gentle as possible since heat and acid eventually break pectin chains. If a clear jelly is desired, the cooked fruit is strained to remove solid particles and cell debris.
Add sugar and supplemental pectin if needed. Bring the mixture to a rapid boil to concentrate the ingredients. Continue to boil until the mix reaches a temperature of 217-221 degrees F/103 - 105 degrees C at sea level. If not at sea level, adjust the temperature by lowering 2 degrees F/1 degree C per every 1000 ft/305 m elevation. At the noted temperature, the sugar concentration reaches around 65%.
Use a wide pot with a large surface area for evaporation and more gentle cooking.
Add acid in the late stages of cooking to avoid breaking the pectin molecules.
Test the readiness of the mix by placing a drop on a cold spoon and see whether it gels.
Pour into sterilized jars. The mix sets as it cools below 180 degrees F/80 degrees C. It sets most rapidly at warm room temperature (around 86 degrees F/ 30 degrees C), and continous to get firmer with time.
Failure of the mix to set can be attributed to inadequate amounts of good-quality pectin, prolonged cooking that damages the pectin, or inadequate amounts of acid. To remedy, addition of commercial liquid pectin and cream of tartar or lemon juice, and a brief reboiling should work. Too much acid causes weeping of fluid from an overfirm gel.
Uncooked and unsweeted preserves can be made by using concentrated pectin. "Freezer" jams are made by loading crushed fruit with concentrated pectin and sugar and allowed to set for a day. They are preserved in the refrigerator or freezer. Low-calorie and low-sugar jams are made by adding a modified pectin that uses cross-linking calcium to gel instead of sugar. The sugar is replaced by artificial sweeteners.
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