Many recipes for preserves, including jams and most especially jellies, call for an ingredient called pectin. Without pectin, jellies and jams won't gel, so it's an essential ingredient—but it's not something you're likely to find in your cupboard unless you make jellies and jams on a regular basis.
Pectin is a starch called a heteropolysaccharide, that occurs naturally in the cell walls of fruits and vegetables. It is, in fact, the very thing that gives them structure. When cooked to a high temperature (220 F) in combination with acid and sugar, it forms a gel. Pectin, in fact, is what makes jams and jellies develop a semi-solid texture when they cool. Pectin can be used in other dishes that require food to gel or thicken. It's also used as a fat substitute in some baked goods.
Pectin's Origins and Uses
Unlike gelatin, which is made from animal parts, pectin comes from fruit. As a result, any food with pectin listed as an ingredient is both vegetarian and vegan.
Some fruits, like apples and quince, are naturally very high in pectin; this is why they are very firm. The rinds, seeds, and membranes of citrus fruit are also very high in pectin—up to 30% by weight. This is why marmalades are made from citrus; in fact, the word "marmalade" comes from the Portuguese marmelada, for quince paste, derived from marmelo, for quince. Commercial pectins are usually made from citrus rinds.
Other fruits, especially very ripe ones, have relatively little pectin. Think of strawberries and raspberries, which are easily squashed. For these fruits, without added pectin, making a properly-set jelly or jam may require adding lots of sugar, cooking for excessively long times, or both. If you're looking to make a jelly from fruits like strawberries, adding some pectin is a healthier alternative to adding more sugar. Adding pectin shouldn't noticeably change the flavor.
To find out how much pectin is in your fruit, try this test. Combine one tablespoon of grain alcohol and one teaspoon of your fruit juice. If it sets up firm, it's high in pectin. If it becomes a loose, gelatinous mass, it's medium on the pectin scale. If it doesn't set at all, or forms slivers of gel, it's low in pectin.
Forms of Pectin
There are several different types of pectin, and each behaves differently. Dry pectin comes in multiple forms, tailored to the amount of sugar in a recipe. Liquid pectin is similar to the regular dry pectin but pre-dissolved to avoid clumping. High methoxyl (HM) pectin comes in fast-set and low-set varieties; fast set HM is best for chunky jams and marmalades while slow set HM works well for clear jellies. Low methoxyl (LM) pectin, which combines with calcium instead of sugar to create a set, is good for low- or no-sugar preserves. You can even make your own pectin, using citrus or apples.
Because different types of pectins behave differently, it's best to follow the recipe you are using. If you find the set too hard or too soft, you can always adjust the amounts accordingly.