Jam problem solving

Mould

Mould is often caused by a failure to cover the jam with a waxed disc while it is very hot. Alternatively, jars may have been damp or cold when used, weren't filled right to the top or have been stored in a damp place.
Possibly, also, insufficient evaporation of water during preliminary cooking and , or too short a period of boiling after the sugar has been added.
Jam with a good set is less likely to go mouldy, while a softer-set jam will be more inclined to spoil.
Mould is not harmful to the jam but it may affect the taste slightly. If it is removed, the jam can be boiled up again and re-potted in clean, sterilized jars.

Bubbles

Bubbles indicate fermentation, which is usually the result of too little sugar in relation to the quantity of fruit. When jam is not reduced sufficiently, this can also affect the proportion of sugar.

Fruit rising in the jam

When the fruit is in big pieces or used whole, such as strawberries, the pieces tend to rise in the jam after potting. To keep them dispensed throughout the preserve, leave the jam in the pan for 10-15 minutes after setting point is reached to thicken slightly, then stir before pouring it into the pots. The syrupy consistency of softer-set jams means that fruit will invariably rise. The same problem may occur with rind in marmalade. The solution of waiting and stirring is the same.

Crystallizing

Too much sugar or too little acid are usually the cause. Low-acid fruits benefit from addition of acid in the form of lemon juice. Making sure that the sugar has dissolved completely before bringing the jam to a fast boil also helps. Sometimes over-riped fruit is responsible or storing the jam in a too warm place.

Jam won't set

Low levels of pectin due to using fruits containing very little pectin or over-ripe fruit make it difficult to reach setting point. Other reasons include under-boiling the fruit, so the pectin is not extracted, or insufficient evaporation of the water before the sugar has been added, for which there is no remedy.
For fruits low in pectin, such as strawberries and cherries, you can add more pectin in the form of fruits, such as lemon juice, apples or redcurrants, bottled pectin or preserving sugar, which has added pectin.

Jam shrinks in the jar

Shrinking is caused by the jam being inadequately covered or sealed or failure to store it in a cool, dark and dry place.