Pour some sugar over berries that you are freezing or place the berries in some sugar syrup. Remember that the syrup must be cold before it is poured over the berries. Unsugared berries keep for 3-4 months in the freezer, while sugared berries keep for at least 1 year.
How do you check whether jam and marmalade have the right consistency? Place a dish in the fridge when you begin boiling jam or marmalade. Pour a little jam or marmalade into the dish and place it in the fridge again until it has cooled. Then drag a spoon through the jam or marmalade. If the track made by the spoon doesn't close in on itself, the jam/marmalade is ready.
This is due to too little pectin in the fruit and/or insufficient cooking time. So what do you do? The simplest thing is to use jam sugar, which contains pectin and gives a perfect result with most berries and fruits. Alternatively, you can boil the jam or marmalade again and add extra pectin (gelling agent).
When you boil juice and jam, you must skim it extremely carefully. The scum contains bacteria and mould fungus that can have disastrous consequences for juice, jam or marmalade.