When initially cooking the fruit for a jam it is important to simmer it gently until it is very tender before adding the sugar, as cooking afterwards will not soften the fruit. Those with tough skins, such as plums, will take about 30 minutes, while soft-skinned fruit, such as strawberries, will take about 20 minutes or less. It is this cooking that releases the pectin from the fruit.
To make about 5kg (10lb) plum jam you will need 3kg (6lb) plums, 900ml (1½pints) water, 3kg (6lb) sugar and aknob of butter.
Put the plums and water in a preserving pan and simmer gently, stirring occasionally, for about 30 minutes until the fruit is soft and the contents reduced. Remove the pan from the heat. Meanwhile warm the sugar slightly in a low oven so it dissolves more quickly.
Stir in the warmed sugar until dissolved, then add a knob of butter to reduce foaming. Return the pan to the heat, bring the jam to boiling point as quickly as possible and boil rapidly for about 10—15 minutes.
Remove from the heat and test for a set. Remove plum stones and carefully skim off any scum. It is not necessary to skim while the jam is boiling as too much clear jam would be lost. Pot and cover.