While some fruits have sufficient acid and pectin to
gel into jams and jellies (blackberries and quince), but there are a lot of fruits that require additional pectin to gel.
Apricots,
figs,
Concord grapes,
guavas,
peaches,
pears,
prunes,
raspberries and
strawberries all need extra pectin before they become
jams and
jellies. Additionally, depending on how ripe they are, other fruits like
cherries,
apples, and
blackberries need it, too.
There's a pretty simple way to test for pectin. Mix one teaspoon of cooked fruit with one tablespoon of rubbing alcohol. If the mixture coagulates into a sticky blob, you're good to go. It should also be noted that unless you are Kitty Dukakis, it is not a good idea to taste this mixture.
Since jamming is one of the best ways of preserving fruit, pectin is an important ingredient in the manufacturing process. And while pectin is commercially available as a powder or a liquid, in the event that commercial pectin is not available, you can make it from apples.
If you've got about ten pounds of spare, tart apples and some extra time on your hands, here's what to do. Wash the apples, remove the stems, and quarter them, but don't core them. Put them in a fat-ass kettle, cover them with cold water, and bring the kettle to a boil over a moderate flame. Cover the kettle and let it simmer for about 30 minutes (wait until the fruit is soft). Drain the fruit in a jelly bag overnight and collect the juice (should be about three quarts). Boiling this liquid down yields about 1 ½ to 2 cups of pectin.
While it's usually not necessary to make your own pectin, it's certainly a skill that will increase your worth to the tribe after the impending apocalypse, or even the run-of-the-mill Lord Of The Flies type situation.