Shelf life of jam and marmalade
Sugar increases the keeping qualities of jam and marmalade.
When sugar is added to jam, water activity is reduced because the sugar binds water. The lower the water activity, the better the keeping qualities.
Microorganisms can only grow in "free water" in food products. If water activity is sufficiently low, microorganisms cannot decompose the food.
A sugar content of 500 g per kg of berries or fruit gives water activity of 0.97. A sugar content of 1 kg per kg of berries or stewed fruit gives water activity of 0.92.
Combined with boiling, this sugar quantity can prevent the growth of mould.