The role of headspace in home canning (2024)

Headspace is the space at the top of the jar between the underside of the lid, and the top of the food or liquid in the jar.

Judy Harrison at the University of Georgia explains,

When filling the jars, it is important not to overfill. We call the space at the top of the jar, between the top of the food and the bottom of the lid, the headspace.” ^[1] Harrison, Judy A. Canning Foods at Home: The Basics. Powerpoint presentation. Cooperative Extension: University of Georgia. Accessed December 2014.

You never want the food coming up so high that the lid is pressing down onto and touching the food inside the jar. There needs to be some completely empty space under the lid. According to Elizabeth Andress, project director of the National Center for Home Food Preservation (NCHFP), there should not even be any food sticking up into that space even. ^[2] Andress, Elizabeth. “History, Science and Current Practice in Home Food Preservation.” Webinar. 27 February 2013. 1:03. Accessed January 2015.

Headspace is one of the key factors in determining whether you will get a good vacuum seal or not on the jar.

You’ve got to get the amount of space right, at least going into the processing the jar stage. You may end up with too much headspace after processing owing to loss of liquid or trapped air in food from the jar, but that’s okay — the starting headspace recommendations take that into account. If you’ve gone into it with the right headspace, you will still almost always get a good seal.

General rule of thumb for headspace

There’s a general rule of thumb for headspace that you should follow unless your recipe specifies otherwise.

• Jellies, jams, soft spreads, fruit juice: ¼ inch ( ½ cm)
• Fruits, tomatoes, pickles, relishes, chutneys, condiments: ½ inch (1 cm)
• Low-acids: 1 to 1 ¼ inches (2 to 3 cm)

Judy Harrison from Georgia says,

Check the directions for the correct amount of headspace called for in a recipe. For jelly, it is usually ¼ inch. For acid foods like fruits, tomatoes and pickles, it is ½ inch. For low acid foods like vegetables, meats and mixtures of acid and low acid foods, it is usually one inch.” ^[3] Harrison, Judy A. History, Science and Current Practice in Home Food Preservation. Powerpoint presentation. Cooperative Extension: University of Georgia. Accessed December 2014.

Bernardin adds this helpful two-stage headspace advice about pickles and fruit: “Pickles and fruit require ½ inch (1 cm) headspace. Because these foods are packed with solids and liquid, pack solids to within ¾ inch (2 cm) of the top rim, then add liquid to the ½ inch (1 cm) level.” ^[4] Bernardin Guide to Home Preserving. Toronto, Canada: Bernardin Ltd. 2013. Page 10.

Elizabeth Andress at the NCHFP expands a bit on the last headspace category, saying, “1 to 1 ¼ inch for low-acid, that’s to allow for temporary expansion and boiling during pressure canning.” ^[5] Andress, Elizabeth. “History, Science and Current Practice in Home Food Preservation.” Webinar. 27 February 2013. 1:03. Accessed January 2015.

What happens if you don’t get headspace right?

Getting headspace right isn’t a food safety issue per se right off the bat. But it can prevent you from getting a seal. And, it can prevent you from getting a seal that fails on the shelf, which might then cause a food safety issue. And, it can prevent discoloration of food at the time of the jar — which though still safe, is unsightly.

The NCHFP explains in a FAQ answer:

Do I really need to leave a certain amount of headspace in the jar? Yes, leaving the specified amount of headspace in a jar is important to assure a vacuum seal. If too little headspace is allowed the food may expand and bubble out when 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. If too much headspace is allowed, the food at the top is likely to discolor. Also, the jar may not seal properly because there will not be enough processing time to drive all the air out of the jar.” ^[6] National Center for Home Food Preservation. Frequently Asked Canning Questions. Accessed March 2015.

The Presto company lists improper headspace as one of the reasons that jars don’t seal after pressure canning:

Jars do not seal after pressure canning for the following reasons: Jars were packed too solidly with food or were overfilled. Allow ½-inch (1 cm) headspace for fruits and tomatoes, and 1-inch (2 cm) headspace for vegetables, meats, poultry, and seafood. This is necessary since food expands during canning.” ^[7] Presto. Frequently Asked Questions. Accessed March 2015.

De-bubbling and headspace

You have to de-bubble or you will end up with false headspace. The reason is that there is almost always air trapped in the food you pack into a jar: either air bubbles between pieces of food, or bubble pockets in thick things like chutneys, jams, etc. As you process a jar, this hidden air rises to the top of the jar, causing the food levels in the jar to sink, increasing the amount of headspace beyond what you’d thought you had.

After you de-bubble, the level of product in your jar always drops appreciably. You will be amazed at how much “false fill” you had in the jar owing to trapped air. That can affect the strength of your seal.

A Ball spokesperson, Jessica Piper, has pointed out why de-bubbling is important in protecting the headspace that you have carefully measured:

….headspace is quite important for the food expansion inside of your jar, as well as making sure you get the right vacuum, and a good strong vacuum…. you want to make sure that you remove your air bubbles, again if the headspace is incorrect, and if you don’t remove air bubbles, that also can affect your headspace, your total headspace, so that can cause the lid to buckle too, there’s no room for your food to expand, and there’s pressure building up in there. If you don’t remove air bubbles that can affect your total headspace and therefore the quality of the seal you end up with …. headspace is vital to getting a good vacuum seal.” ^[8] Jessica Piper. Video: Canning Lids 101. 2 October 2014. 21:49 Accessed March 2015.

Author Linda Ziedrich says,

After filling the jar, release air bubbles by inserting a narrow plastic spatula or similar tool between the pickles and the inner surface of the jar. Move the tool up and down while slowly turning the jar. Add more liquid if the level drops in this process, and then wipe the rim with a clean, damp cloth or paper towel.” ^[9] Ziedrich, Linda. The Joy of Pickling. Boston: Harvard Common Press. 2009. Page 22.

Further notes about headspace

Virginia Cooperative Extension says,

Foods that swell require more headspace when filling. If too little headspace is left, food may boil up and out of the jar, preventing a seal. If too much headspace is left, the processing time may be inadequate to drive off the oxygen within the jar, preventing the seal from forming.” ^[10] Boyer, Renee R. and Julie McKinney. Boiling Water Bath Canning: Including Jams, Jellies, and Pickled Products. Virginia Cooperative Extension. Publication 348-594. 2013. Page 5. Accessed March 2015.

Angela Fraser at Clemson University says,

…pack acid foods including tomatoes and acidified figs to within ½-inch (1 cm) of the top of the jar. Pack low-acid foods to within 1 inch (2 cm) of the top of the jar.” ^[11] Fraser, Angela. Associate Professor/Food Safety Education Specialist. How Canning Preserves Food. Clemson University, Clemson, SC. Accessed March 2015 http://www.foodsafetysite.com/consumers/resources/canning.html.

Elizabeth Andress noted some research related to temperature in the headspace:

Magoon and Culpepper (1922) discovered that can vacuum is not always proportional to the average temperature of the contents, but is determined mainly by headspace temperature. A short exhaust, therefore, could result in a high vacuum if sealed immediately. A long exhaust would be ineffective if the headspace is allowed to cool.” ^[12] Andress, Elizabeth L and Gerald Kuhn. Critical Review of Home Preservation Literature and Current Research. IV. Equipment and its Management – History and Current Issues. Athens, GA: University of Georgia, Cooperative Extension Service. 1983. Accessed March 2015.

Penn State Extension says,

If jars are filled too full, food may boil out during processing and solids or seeds may catch under the sealing compound and prevent the jar from sealing…. you are filling jelly jars and need just a little more jelly to make another jar. If you take just a little from each of the other jars, you will have that extra jar—WHOA! The processing time may not be long enough to drive out all that extra air from the top of the jar resulting in a poorly formed vacuum seal. Besides, the extra air left inside the jar could cause the food to discolor. “Allow ½ inch headspace for tomatoes, pickles, and fruits. Vegetables need one inch headspace to allow for the expansion of the starch in the vegetable. Allow one inch headspace for red meats and 1¼ inch for poultry.

The headspace you start with may not be the same as when you finish. 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. Finally, siphoning may cause the loss of liquid in jars. Raw packing food and using heavy sugar syrups increase siphoning. Loss of liquid can be reduced by allowing the jars to sit in the canner 5 minutes after processing is completed before removing the jars from the canner. If liquid is lost during processing and the jar is sealed, do not open the jar to replace the liquid. Liquid loss is not a sign of spoilage.” ^[13] Penn State Extension. Why Allow Headspace? 14 September 2012. Accessed January 2015.

Headspace as volume or height?

As end users of home canning information, we are given headspace in terms of “height” from the top of the food contents to the underside of the lid. But, in the labs where these recipes are developed, do they actual measure the headspace in volume instead and just convert it back for us? Here, headspace is referred to in terms of volume by USDA researchers:

Conditions that reduce liquid loss to a negligible amount are (a) sufficient headspace to allow for the expansion of jar contents, in particular at least 35 ml. headspace for pint jars and 70 ml. headspace for quart jars…” ^[14]Andress, Elizabeth L and Gerald Kuhn. Critical Review of Home Preservation Literature and Current Research. IV. Equipment and its Management – History and Current Issues. Athens, GA: University of Georgia, Cooperative Extension Service. 1983. Accessed March 2015.

In 2021, we asked the National Center for Home Food Preservation whether they used volume or actual height when developing home canning recipes in the lab. They answered that they actually do just use height, in inches.

University of Georgia video

Headspace with Tattler lids

Many Tattler lids users advise a minor modification to headspace. See: Tattler Lid Headspace.

Headspace for freezing in mason jars

So Easy to Preserve

So Easy to Preserve sets out freezer headspace in a handy table:

Source:National Center site: Headspace to Allow Between Packed Food and Closure

[Note: rather than rounding ½ inch down to 1 cm and possibly having insufficient headspace, we rounded up to 2 cm, and ditto for other measurements.]

Freezing in Ball mason jars

For freezing in jars, Ball recommends ½ inch (2 cm). They don’t elaborate much, just say generally a half-inch, and in a site redo a few years ago, this advice disappeared completely, nor is their Ball Blue Book of much help. ^[15]Recommend Jar Usage. Accessed March 2015 at https://www.freshpreserving.com/sites/all/themes/freshpreserving/library/RecommendedJarUsage.pdf.

Freezing in Bernardin mason jars

Bernardin essentially recommends against using jars with shoulders for freezing. They are recommending only straight-sided jars. And, they say ½ to 1 inch (2 to 3 cm) headspace.

Note that proper headspace for freezing is required for safety reasons, particularly if you are using jars with shoulders. Without it, jars may crack, ruining the food inside them owing to the possibility of the food becoming contaminated with small shards of glass.

Further reading

Loss of liquid during home canning