Jam and Preserves
Whole fruit rises to the top
Crystallizes, forms a mould or
Crystalises in refrigerator:
Ferments and goes fizzy:
The keeping quality of Jam is largely determined by the set and the correct proportion of sugar, but even with high proportion of sugar jam may go mouldy if kept in a damp store. The type of cover used is also an important factor. A waxed disc pressed evenly on the surface of the jam while still hot, certainly prevents mould growth to a large extent and even if mould develops it rarely penetrates through the paper into the Jam.
Jams covered with parchment or cellulose paper keep better than those with metal covers lined with cardboard. Home-made covers such are best put on the jam at once when it is very hot.
Most commercial jams are sealed with airtight covers made of metal. Home made jam can be sealed in the same way but they must be sealed as soon as the jam is potted. A wax disc is unneccessary under seal and is in fact more important to cover the jam quickly.
Seals are a time saving method of storing jam but involve extra cost. Plastic seals press on lids are generally reusable and again must be placed immediately on the hot jam. They have the advantage in reducing shrinkage that might occur if the jam is stored in a centrally heated house.
Sealing the jars in a boiling water canner produces a strong seal that should last at least one year. Sealing the jars by turning them upside down (as recommended by some pectin manufacturers) saves time on jelly-making day but results in a weaker seal that may fail during storage. Sealing the jars with paraffin wax is no longer recommended because too often the wax seperates from the side of the jars, allowing air to touch the jelly and mold is possible. Mistakes melting the paraffin also caused many kitchen fires and severe arm burns.