Since we have fruit trees, I make jam during the summer reasonably regularly. Sometimes Simon does too, and although we hadn't done much jam making before we came to live here, we really haven't had the sort of problems other people seem to have, particularly with jam not setting.
Now that I have played around with various methods and settled on a basic technique that works for me, I want to work on improving the fruity flavour and reducing the sweetness of our jams, so I've been reading up on jam making.
Why is a low sugar jam not a jam then? It's mainly because the sugar acts as a preservative, binding the water content and preventing moulds from developing then causing the jam to ferment, which alters its flavour and texture (it becomes more liquid). When heated together the sugar attracts the water from the fruit. The cell walls in the fruit burst and release pectin too. The pectin is only released at high temperature, and the presence of the sugar means that boiling point is above 100°C. (Beware - the pectin will irreversibly break down if you go more than a few degrees over jam temperature for more than a few minutes.) Dispersed amongst the fruit water, the pectin molecules cannot join together, but once the sugar has bound the water, the pectin can form a gel. A certain amount of evaporation takes place as well, which helps thicken the mix. The ideal ratio for this transfer of molecules to take place turns out to be a sugar content of between 50 and 65% (remember that the fruit itself contains sugar). Apparently too much sugar can cause crystalization. The final player in the mix is acidity, which aids the attraction of the pectin molecules to one another. Once the jam cools, all the elements should have formed a network resulting in perfectly set jam.
Many people will tell you to add lemon juice or half a lemon to any and all jams. This is presumably on the understanding that the pectin needs that hit of acid to reassemble and that the lemon itself contains pectin. My personal preference is for no lemon - I find the flavour overpowers the fruit, and I can't see why the acid naturally present in your fruit of choice wouldn't be sufficient to activate the pectin.
Many people will tell you to make sure you mix fruit that is rich in pectin (citrus, apples, redcurrants) with fruit that has less (apricots, peaches, strawberries, cherries, blackberries), but we never have any problem with our cherries, so I am not convinced this is necessary. However, I have this year made a batch of apple pectin and put it in the freezer for adding to jams next year. This so I can reduce the sugar a bit but still get a good set without cooking the jam for too long and losing flavour.
Apple pectin is easy to make. I use my imperfect apples, cut out the bad bits, chuck them in a big boiler, cover with water and boil for 20 minutes. The apples are well and truly mushed by then. I tip the lot into a muslin which we suspend from the back of a chair over a bowl. The theory is that the resulting already slightly gelled liquid is pectin. I rinse out my muslin in very hot water and hang it on the line to dry, but if you were worried about it not being sterile, you could simply iron it before using.
Many people recommend using copper pans for jam making as the metal ions link to the pectin and aid gelling. However, the tarnish on copper pans is toxic, so I don't fancy it, and, like beating egg whites in a copper bowl, I've never found it necessary. If you do use a copper pan, apparently calcium citrate, available at the pharmacy, neutralises the vert-de-gris.
Many people swear they can tell when their jam is ready by putting a teaspoon of hot jam on a cold plate, waiting a few minutes then pushing the jam. If it wrinkles it's ready. This technique never worked for me, although Simon uses it. I always use a thermometer and make sure the jam has reached 105°C and stays there for 5 minutes.
Pectin is released rather slowly, so a relatively long cooking period is required, but this means that many volatile aromatic flavours are lost. There is no reason you can't cook the fruit first to release the pectin, then add the sugar and leave it to dissolve and bind up the water. This is useful when making jellies, as you can boil the fruit, strain through the muslin overnight, then add the sugar and bring slowly to the boil to ensure it is all dissolved, then boil hard to reach jamming temperature. Making apple pectin should also solve this problem, in that you are adding it already activated to another fruit jam. It doesn't matter that the pectin has lost its apple flavour by being cooked hard earlier.
To help prevent loss of aromatics, don't stir the jam too much either. Having said that, though, it is wise to stir every now and then, just to check it's not burning on the bottom.
The foam that forms on top of the boiling jam must be removed because it leaves aerated trails through the jam which can allow moulds to grow. However, there is no reason you can't eat it, and added to yoghurt, or spread on bread still warm as an after jam-making snack, it is delicious.
To sum up, you have two approaches to choose from when jam making:
Rapid - the current trend. Chop your fruit and macerate in an equal quantity of sugar first and the fruit will break down quickly and release its water and pectin.
Long - unavoidable if you want to reduce your sugar content or increase your fruit content. The advantages are that water will evaporate and you will get a thicker syrup form, the fruits burst, the pectin is liberated, the house smells fantastic...but you risk losing flavour with the aromas.