If you are relatively new to canning, you might not know the meaning of the term “canning headspace,” and its importance in the canning process.
Canning headspace is the empty space that is left in your jar after you add the contents and before you put the mason jar lid on. No matter what you are canning, leaving the correct amount of space specified before processing the jar is extremely important.
The heat applied to a mason jar during processing causes the contents inside the jar to expand. As air escapes around the lid, canning headspace will decrease. If you did not leave enough canning headspace, the contents of the jar could also seep under the lid and create a problem with the seal.
Leaving too much canning headspace can also be a problem. Cooling jars naturally contract and pull the lid down tight to seal the jar completely. If there is too much canning headspace, the processing time called for in the recipe may not have been long enough to drive out the air in the jar. More air in the mason jar means more oxygen is present to discolor the food and promote rancidity in fats. This can lead to an improper seal as well.
Canning headspace is needed when food begins to expand during the canning process and for forming vacuums upon cooling. Air expands when heated to high temperatures; the higher the temperature the greater the expansion. Foods expand less than air when heated.
Before you begin a canning recipe, be sure it specifies how much canning headspace to leave as every recipe is different. The best recipes will tell you how much to leave. Some canners follow the general rule of 1″ for foods like vegetables, 1/2″ for fruits and 1/4″ for jams and jellies. When canning meats, poultry, and fish it is best to have a headspace of 1 – 1-1/4″.