ABSTRACT
The unfilled space between the inside of the lid and the top of the food or liquid is termed the headspace. This space is needed for expansion of food, entrapped air, and moisture as the containers are processed, and for forming a vacuum in the cooled containers. The extent of expansion of the contents of the container is de termined by the air content in the food and by the processing temperature. Air ex pands greatly when heated to high temperatures, and moisture in the food is con verted to steam. The solid part of the food expands less than air when heated (18). The amount of headspace to leave at the top of the container is determined by the type of food being canned. If containers are overfilled (i. e., too little headspace), the contents may boil out during processing. When this happens, food material may be deposited under the sealing compound or on the rim of the jar, and this condition may prevent an airtight seal from forming. When too much headspace is left at the top of the jar, processing time may not be long enough to drive out all the extra air and may result in the formation of a poor vacuum and discoloration of the food at the top of the container owing to chemical reactions between the food and residual air present in the headspace. Follow the directions for headspace in the recipe being used. However, when specific directions for headspace are un available, the following general directions can be used: for jams and jellies, leave 1/4 in. (6 mm) headspace; leave 1/2 in. (12 mm) for fruits and tomatoes to be processed in boiling water, and 1-1 1/4 in. (2. 5-3. 2 cm) for low acid foods to be processed in a pressure canner.
