The Preserving Jar
Fruit is generally unsuitable for deep freezing. Fruit is mostly water, and to deep freeze it would turn the water to ice crystals. The ice crystals would destroy the cell structure. Instead, fruit can be preserved using a preserving jar. This consists of a glass jar covered with a metal cap seated on a flat rubber ring. Fruit and water are placed in the jar with a small air gap at the top.
The jar is placed in a large container of water and slowly brought to the boil, with the cap being held firmly in place with a screw cap.
During boiling air is driven from the jar, and the screw caps are tightened. Water vapour condenses so pressure inside the jar is much less than pressure outside. Atmospheric pressure closes the lid tight
No air – containing bacteria – can enter, so the fruit can be kept fresh for an extendsed period of time.