This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
Putting Food By so lovingly know as PFB in RFP has the answers here.
If you are serious about preserving you do owe yourself a copy of
PFB. If you can afford only one preserving book this is it. It is
also known in rec.food.preserving with good reason as "the Bible of
"Pectin is highest in lightly underripe fruit, and diminishes as the
fruit becomes ripe; overripe fruit, lacking adequate pectin of its
own, is responsible for a good deal of runny jams and jelly.
This natural pectin in the fruit can be activated only by cooking -- but
COOKING QUICKLY, both in heating the fruit to help start the juice, and
later when juice or pulp is boiled together with the sugar. And TOO-SLOW
COOKING or BOILING TOO LONG, can reduce the gelling properties of the
pectin, whether natural or not.
Testing for pectin content. There are several tests, but the simplest
one uses ready-to-hand materials. In a cup, stir together 1 teaspoon
cooked fruit juice with 1 tablespoon non-methyl alcohol. No extra pectin
is needed if the juice forms one big clot that can be picked up with a fork.
If the fruit is too low in pectin, it will make several small daubs that do
not clump together. DON'T EVER TASTE THE SAMPLES.
Homemade Liquid Pectin
Liquid pectin is especially helpful in making peach, pear, strawberry, or
those other jellies whose fruit is low in pectin.
Four to 6 tablespoons of homemade pectin for every 1 cup of prepared juice
should give a good gel: but experiment! These pectins can be frozen or canned
for future use. To can, ladle hot into hot [ sterile - ED ] jars, leaving 1/2
inch of headroom; process at a simmer, 185F/85C, for 15 minutes. remove from
canner, cool upright and naturally.
Crab Apple Pectin
2 pounds sliced unpeeled crabapples
3 cups water
Simmer, stirring, for 30-40 minutes adding water as needed. Plop into colander
lined with one layer of cheesecloth [ or muslim - ED] and set over a bowl; press
to force the juices. To clear, heat the collected juice and pour through a stout
jelly bag that has been moistened in hot water. The result is the pectin you
can, or freeze, or use right away.
Tart Apple Pectin
4 pounds sliced apples with peels and cores.
8 cups water
Simmer, little stirring needed, for three (3) minutes. Press apples through a
sieve to remove cores, etc. Return liquid to a heavy kettle [ or use a heavy
wide mouth pot to enhance reduction ] to cook briskly, [ and quickly ] stirring,
until volume is reduced to one-half. Clarify by pouring though a stout jelly
bag that has been moistened. Use, can, or freeze as above.