Use good quality seasonal fruit, ideally freshly picked, dry and unblemished.
If required, wash well. Strawberries and raspberries may not need to be washed.
If the fruit is picked when wet the jam is likely to go mouldy.
Make jam in small quantities for best results.
If you use too little sugar the jam may ferment and too much sugar may cause crystallisation.
The sugar must be completely dissolved before boiling to avoid crystallisation.
Ideally use a preserving pan for rapid boiling and evaporation. If you don’t have one use your widest pan with a heavy base.
For the initial cooking of the fruit, cook at a simmer , after adding the sugar always boil rapidly ( at the highest temperature ) until setting point is reached.
Over boiling will reduce the fresh flavour and appearance of the jam.
Test for setting point frequently. It should read 105C / 220F on a sugar thermometer.
Make sure jars, lids and all equipment are spotlessly clean.
Only use sterilised jars. Place in an oven at 180C for 10 minutes or ideally wash in a dishwasher and then warm in the oven while making the jam.
Fill the jars almost to the top and seal while still hot.
Pectin is the natural setting agent in jams. It is found in the skin, seeds and core of the fruit.
The levels of pectin vary in different fruits. Raspberries, redcurrants, blackcurrants, cooking apples and plums all have high levels of pectin.
It is best to use fruit slightly underripe as the levels of pectin will be higher.
Strawberries, blackberries, blueberries and cherries have low levels of pectin and often lemon juice ( which is high in pectin ) is added to aid setting of these jams.