In the UK, at least, there is a considerable difference between jams and jellies. Where a jam is made with whole fruit (including skins and seeds) and a jelly is made solely from the juice strained from the fruit. Both are cooked with sugar and use natural (or added) pectin as the gelling agent. They are boiled until they reached the 'setting point' (when a spoonful of the jam or jelly, when placed on a cold plate forms a skin when pushed with a fingernail).
For a jam or a jelly to set you typically need 60% sugar and about 0.2 to 0.5% pectin. And it's the boiling process that concentrates the sugars added to the jam and the natural sugars in the fruit down to this level. Of these components, pectin is probably the most important.
Pectin itself is a complex sugar found in plants (technically called a heteropolysaccharide) and is the main gelling agent in jams and jellies. In the plant itself, pectin helps to bind the plant cells together and also regulates the water in the plant.
Indeed, it's pectin that leads to the noticeable changes on ripening of many fruit. Initially, many fruit are packed with pectin and this keeps the fruit firm and hard. On ripening, pectin is broken down into other sugars by the action of the enzymes pectinase and pectinesterase this makes the fruit noticeably softer and makes then taste sweeter.
Pectin is a critical component for the setting of jams and jellies, in that it's the pectin from the fruit that allows the preserve to set hard. When fruit is cooked the pectin from them is released into the liquid. When the solution cools the pectin molecules join together forming a mesh and it's this pectin mesh that allows the jam to set. This is why sugars specifically designed for jam making, the so-called 'jelling sugars' or 'sugar with pectin' contain pectin as an additive (but you typically need about 60% sugar for this to work).
However, many fruit contain natural pectin, with apples, quinces, plums, gooseberries and oranges (or other citrus fruit) containing the most. Apples and apricots typically contain 1% pectin, with crab apples containing up to 3% pectin. Oranges vary between 0.5% and 3.5%, with the zest containing most pectin. Soft fruit like cherries, grapes, strawberries and blackberries contain little pectin and either have to be concentrated down or need to be mixed with apples and other fruit to achieve the setting or gelling point. It should be noted, however that seeds often contain lots of pectin which is why grape jellies are always made with grape seeds and why plum seeds are often added in a muslin bag to plum jams. You can also add citrus peel to other jams to add more pectin to them.
One interesting thing is that carrots can contain up to 1.5% pectin so you can add a muslin bag of grated carrot to a fruit jam or jelly to help them set. The same is true of rhubarb which also contains about 1.2% pectin.
To show you how this process work, below is a recipe for jam made with concord grapes.
Concord Grape Jam
Grapes contain very little pectin and the majority of what little pectin is present can be fund in the seeds. As this is a jam, made with whole fruit (and not a strained jelly) additional pectin has to be added. You can substitute jelling sugar for the sugar in the recipe below or you can add about 45g powdered pectin, but I prefer the natural methods so I add the seeds back to the jam and include 2 chopped crab apples for their pectin.
Ingredients:
1.6kg ripe Concord grapes
250ml water
1.35kg sugar
2 crab apples (or 1 cooking apple), chopped
Method:
Squeeze the pulp from the grape skins into a large pot (reserve the skins). Add the water and bring the mixture to a boil. Reduce to a simmer and cook, covered, for 5 minutes. Transfer the mixture to a fine-meshed sieve and press through the sieve into a bowl with the back of a spoon (this removes the seeds).
Combine the strained seeds and the apple in a muslin or cheesecloth bag and tie tightly. Add the reserved skins from the grapes to a food processor along with a little of the juice and render to a fine paste. Add this paste, the strained pulp and the bag of seeds and apple to your pan and bring slowly to a rolling boil. Add the sugar, return to a boil and boil hard for about 10 minutes.
Test for setting by placing a plate in the fridge. Spoon a little of the jelly onto the plate, allow to cook then move it with your fingernail. If a crinkly skin forms then the jelly is ready. If not continue boiling for 5 minutes more and test again.
Skim the surface then ladle into sterilized jars that have been warmed in an oven set to 110°C for 15 minutes. Allow 1cm of head space then secure the lid, allow to cool and store.
For a Jelly
If you want to make a jelly jelly from the mixture then add the whole fruit and water to a saucepan, bring to a boil, reduce to a simmer then cover and cook for about 10 minutes, or until the fruit is soft. Place in a jelly bag or a fine-meshed sieve covered with a double layer of cheesecloth or muslin and allow to drain naturally in to a large bowl over night (do not be tempted to press down on the pulp with a spoon, as this will make your jelly cloudy).
The following day, measure the volume of liquid you have and add 75g of sugar per 100ml of liquid. Add the juice and sugar to a saucepan then follow the instructions given above for boiling and testing for setting.
These jam and jelly instructions work for just about any fruit. The only consideration is how much pectin lies in the fruit and how likely they are to set on their own.
