Things You'll Need
Tart, green apples
Cheesecloth (or a jelly bag)
8-oz. canning jars or 1-pint freezer bags
Pectin is the component in fruit that "gels" fruit or juice into jam or jelly. Pectin is a polysaccharide, a form of carbohydrate structure. Most people who make jams and jellies at home probably don't realize that store-bought pectin is a relatively modern innovation. Your great-grandmother made her own, and so can you. Apple pectin is actually just unsweetened apple jelly. Making pectin is not hard to do and it's cheaper than the manufactured versions. Sometimes it's even more reliable. The finished pectin, or jelly, can also be used for other purposes, so it's a useful thing to have in your pantry or freezer.
Cut 4 lbs. of tart, green apples into large chunks and put them into a large (5 qt. size) saucepan. Cut a lemon into 1/3-inch slices and add the slices to the apples. Leave the peels, seeds and cores; you need the whole apple and lemon. Barely cover with water. Place over medium heat and bring to a boil, stirring every 10 minutes or so.
Check your saucepan after 45 minutes; your fruit should be quite soft and your liquid should have reduced by about half. Place three or four layers of cheesecloth into a large bowl or pan and secure it to the rim, to form a bag. Or, place your jelly bag into a bowl that fits.
Pour your fruit and liquid into the bag. Allow it to drain until the fruit is mostly solid and dry. Press only lightly on the fruit, just allow it to drain naturally. Mashing or pressing your fruit in the bag will cloud your pectin. Remove the bag and fruit and add the fruit to your compost pile or discard it.
Pour your juice back into the saucepan and continue cooking over medium heat, stirring frequently. Reduce the volume to about 50 percent of what you started with. Watch it so it doesn't boil over; stirring will prevent this. However, it should boil. Remove any foam or residue that comes to the surface. Remove the foam as it collects to keep your pectin clear. This step may take as little as 20 minutes.
Remove the pan from the heat. Drop a spoonful of your pectin onto a cool saucer and allow it to cool. If it forms a jelly, you are done. If not, keep cooking, testing every 20 minutes. Once your jelly is forming well on the saucer, remove from the heat. If using canning jars, go to Step 6. If using freezer bags, skip Step 6 and proceed to Step 7.
Pour your hot pectin into 8-oz. canning jars and seal with the lids. Invert onto a dish towel to allow the heat to sterilize the interior of the jar and lid. When slightly cooled, turn jars upright to finish cooling. One 8 oz. jar is enough pectin to gel about 3 lbs. of low-pectin fruit or juice.
Allow your pectin to cool. Place 8 oz. of the cooled jelly into a freezer bag and remove excess air. Press the bag flat and freeze flat on its side for easy storage. Allow to thaw before using.
Use a bamboo skewer to measure the height of your liquid. This makes it easy to judge when you've reduced by 50 percent. Add herbs such as rosemary and thyme to your jelly while boiling. Instead of just apple pectin you will have a beautiful glaze for pork chops or chicken. Warm your apple pectin and use it to glaze a cake or a fruit tart. Add equal parts of sugar and reboil and turn it into apple jelly. It isn't exact, but equal parts of fruit and sugar (by weight) should produce a good jelly or jam. When in doubt, stir. Stirring your juice/pectin constantly is okay. Less ripe or unripe apples will work much better. Crabapples also work very well.
Watch your juice once it starts to foam. It can boil up and out of the pan easily.