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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 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 greatly when heated to high temperatures; the higher the temperature, the greater the expansion. Foods expand less than air when heated.
When canning it is recommended to leave 1/4-inch headspace for jams and jellies, 1/2-inch headspace for fruits and tomatoes to be processed in boiling water, and from 1- to 1-1/4-inches headspace in low acid foods to be processed in a pressure canner.
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.