The preserving jar
Atmospheric pressure is used in the kitchen in connection with the preservation of fruit which is unsuitable for deepfreezing.
The preserving jar, is a glass jar covered with a metal cap seated on a flat rubber ring (Fig. 11.5). Clean fruit and water are placed in the jar, leaving a small air space at the top. Several of these jars are placed in a large vessel of cold water, which is
then slowly brought to the boil. During this process the metal caps with their rubber rings are loosely held in position by a metal screw cap. About 10 minutes’ boiling is generally sufficient to sterilize the fruit and to cause air to be driven from the jars by steam from the water inside. The screw caps are then tightened and the jars removed from the water.
After cooling, the space at the top of the jars contains only water vapour at low pressure. As a result, the metal cap is then firmly pressed down by atmospheric pressure. No bacteria-laden air can afterwards enter, and so the contents remain in
good condition for a long period. It is important to notice that, when the jars have cooled the presence of the metal screw cap is not strictly necessary, as the seal i now maintained by atmospheric pressure.