Almost all modern jelly recipes call for commercial pectin as a gelling agent. It's the pectin in the mixture that turns juice to jelly, no matter the flavor. While pectin may seem like a mysterious chemical to novice jelly makers, it is little more than apples that are cooked and treated a certain way. Pectin is a naturally occurring substance in many fruits, but apples have the most of the substance, compared with other fruits, and are readily available. The process is uncomplicated enough for beginners to be successful at making homemade pectin. As with all preserving activities, follow directions and keep tools clean and sterile to prevent mold.
Wash the apples under warm water to rinse off any dust or debris. Make sure that the apples are somewhat under-ripe, to get the most pectin from them. Choose apples that are mostly green, have no soft spots or bruises and are smaller in size. Pick apples yourself for the best apple choice. Cut the apples into quarters and place them in a heavy stockpot. Cut up enough apples to fill the pot about halfway.
Add enough water to the pot to barely cover the apples. Place the pot on the stove and turn on the burner. Heat until the water comes to a boil, then lower the heat until the water is slowly simmering.
Simmer the water and apples until the apples are very soft and mushy. This should take 20 to 30 minutes, depending on your apples. Turn the heat off after the apples have finished cooking.
Line a colander with cotton cloth like cheesecloth or a clean piece of T-shirt. Heat water in a teapot until it is boiling, then pour the boiling water over the cloth to sterilize it. Set the colander over a large bowl or pot to catch the juice.
Pour the mushy apple mixture into the colander and allow the juice to seep out and into the pot below. Allow the apples to drain for several hours, up to overnight. Do not squeeze the cloth with the apples in it, as this will cause the pectin to be cloudy.
Pour the drained juice back into the heavy pot and return it to the stove. Turn on the heat and boil the juice until it is about half the volume that you started with.
Test the strength of your pectin after boiling by placing 2 tbsp. of rubbing alcohol into a saucer or small bowl. Add 1 tsp. of cooled pectin. Stir the pectin in the alcohol gently until it congeals into a clump. If you can pick up the pectin lump with a fork and the lump doesn't slump down between the fork tines, your pectin is strong enough. If the pectin is still too loose, boil it some more to concentrate it even further. Discard the test materials.
Place your finished pectin into clean freezer containers, leaving 1 inch of head space. Cover the containers tightly and freeze until solid. Thaw the pectin before using it in jelly recipes. Your pectin will remain good in the freezer for up to six months, but use it within four days after it is thawed.