Why is headspace so important?
Headspace is the space at the top of the canning jar between the underside of the lid and the top of the food or liquid in the jar.
Quite simply, it's important because having the correct amount of headspace in the jar is important in order to get a good vacuum seal during processing.
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According to the NCHFP (The National Center for Home Food Preservation), this headspace should be completely empty space. There should not be any food sticking up into that headspace.
As the canning jars are heated, food will expand. The amount of expansion depends on the amount of air in the food, its starch content and the processing temperature. The higher the temperature, the greater the expansion.
Foods that are packed into jars hot may shrink when cooled. Air spaces in raw packed foods rise to the top of the jar when heat processed and may increase headspace.
In addition, starchy foods such as corn, potatoes, lima beans and rice expand and absorb water during heat processing.
These fluctuations are why the headspace you started with may not be the same as when you finish and why it is so important to follow the instructions as to headspace in each recipe.
General Canning Headspace Recommendations
The United States Department of Agriculture has given the following recommendations:
- Jams and jellies - 1/4 inch.
- Tomatoes, fruits and pickles that are processed via water bath - 1/2 inch.
- Most low-acid foods that will be pressure canned -- 1".
- In addition, some vegetables and meats require 1¼ to 1½ inches headspace.
The Debubbling Process
Debubbling the jar of food goes hand-in-hand with headspace.
Debubbling is needed to release the air bubbles from the jars after the food and liquid are added. Any plastic or wooden utensil will work without leaving small scratches or scars in the jar like something metal may do. I prefer the one pictured above as it also has a guide for measuring headspace at the top.
It's a simple process. The debubbler is placed into one side of the jar and gently pulled towards the center. You will see air bubbles rise to the top and release. Continue to do this until you no longer see air bubbles release.
After debubbling you may notice the fill level has dropped visibly because of the air that escaped. You then need to recheck the headspace and add more liquid or food as needed to get back to the appropriate level.
If you leave those air bubbles trapped somewhere in the middle of the jar, they may try to escape during processing and can end up pushing some of your liquid out of the jar.
So what happens if the headspace isn't correct?
Too Little Headspace
If too little headspace is allowed, the food may expand and bubble out as the air is being forced out from under the lid during processing. The bubbling food may leave a deposit on the rim of the jar or the seal of the lid and prevent the jar from sealing properly.
Too Much Headspace
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, which can lead to an improper seal.
Other Tips
As not all canning jars are shaped the same, you may want to occasionally use a ruler to check the accuracy of the headspace. The picture in the heading is one of the most common type of Ball jars.
Tattler (Reusable) Lids and Headspace
The Tattler reusable lids work differently than disposable lids and, in my experience, seem to need slightly more headspace in order to work properly.
More about using Tattler Lids
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So if you're a new canner, you will have much more success by being very vigilant in having the recommended amount of headspace after debubbling!
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