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What is headspace? Headspace is the unfilled space above the food in a jar and below its lid. This space is needed for expansion of food as jars are processed, and it is also used for forming vacuums in cooled jars.
The extent of expansion is determined by the air content in the food and by the processing temperature. Air expands a lot when heated to high temperatures. The higher the temperature, the greater the more expansion that occurs. Foods expand less than air when they are heated.
You should leave 1/4-inch headspace for jams and jellies, 1/2 inch for fruits and tomatoes, and from 1 to 1 1/4 inches for low acid foods. Low acid foods should be processed in a pressure canner. You can easily measure headspace with this inexpensive canning funnel.
This document incorporates information from the “Complete Guide to Home Canning,” Agriculture Information Bulletin No. 539, USDA (Revised 2009) and information available from the National Center for Home Food Preservation.
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