Understanding the Importance Headspace in Canning
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Quite a bit of the time, I find that new canners are getting hung up on headspace in the beginning of their journey with home food preservation. Let’s talk about it!
To define, headspace is the unfilled space between your food and the top rim & lid of the jar. It is the space the food expands to during the high temperatures of the canning process. Depending on what you’re canning, the specified amount of headspace will be different. Why? Because of the temperature. A short water bath canning time will be exposed to lower heat than a longer boiling water bath canner processing time and a pressure canner will expose food to a much higher heat above boiling because of the use of pressure. The higher the heat, the bigger the expansion of the food.
For starters, a general rule of thumb for headspace is:
- high acid foods: jams & jellies are 1/4″ headspace (short water bath canning times = 10 mins)
- high acid foods: fruits, sauces & salsas, pickles, are 1/2″ headspace (longer water bath canning times)
- low acid foods: meats, veggies are 1-inch headspace (pressure canning -higher temperature)
- pie fillings, thickened with clear jel are 1 + 1/4-1/2″ headspace (water bath canning but the thickener “grows” during the canning process)
Once you commit this to memory, you’ll never have to look up how much headspace again. It’s rather easy!
Proper headspace is helpful in reducing lid seal failure. By adhering to the guidelines, you’re giving yourself the best chance of success with the seal of the lid.
The purpose of headspace isn’t to prevent botulism or food poisoning; it is to have the best chance at your jars sealing. If you’re canning and not leaving enough headspace, you run a high risk of your jars not sealing due to the food expanding during canning and pressing on the underside of the lid the entire time and siphoning out liquid and fats and getting between the lid and the rim of the jar. Bummer to do all that work just for your lids not to seal! The higher the processing temperature, like that when pressure canning, the bigger the expansion. So anything pressure canned is going to expand exponentially more than a jam that’s water bath canned for 10 minutes. Same as a jam compared to a pie filling… the pie filling is processed for more time meaning it’s exposed to more heat.. meaning it will expand more.
If you’ve canned for a while, you probably already know this.
I frequently forget to add extra headspace to my pie filling recipes because I’m a goofball and end up having the filling oozing out of the jars… my fault. But there’s definitely a reason you leave that extra bit of space. When you overfill and have food forced out between the lid’s sealing compound and the rim of the jar, you’re running quite the risk of your jar not sealing if food is left behind that disrupts the lid sealing to the jar rim. I’ve gotten lucky and had lids seal anyway, and I’ve also had large amounts of a batch not seal due to my absent mindedness. A hard lesson to learn!
I’ve also learned that too little headspace is far more of an issue than leaving too much. If you’re at 1 1/4″ headspace for a pressure canner full of roast beef, it will be totally fine and botulism isn’t gonna be all “heyyooooooooo wrong headspace lemme just go nuts.” That’s not a thing and I’m not sure how that connection was ever made. The worst thing that’s going to happen with too much headspace is that all the air isn’t going to be completely driven out of the jar which will affect the seal (rare and really only if you leave a massive amount of headspace which would be purposeful and not just a margin of error… so you shouldn’t be canning anyway) and the top of the food will possibly discolor when it does seal.
If you take only one thing from this blog post, it should be this. A little too much headspace is better than not enough to ensure the best chance of a good seal on the jars. Making yourself insane over perfection with headspace isn’t necessary and just err on the side of being close enough and you will be fine! While the correct amount of headspace is helpful, perfection is not required to be successful with canning.
Jar Landmarks For Headspace Measurement
When I’m canning, I don’t have one of those pointless head space measuring tools that my kids are bound to steal and lose somewhere in the dark abyss of my couch cushions. I learned to go by landmarks on the canning jars. 1 1/2″ is the shoulders, 1″ is the first ring on the jar above the shoulders, 1/2″ is the second, and 1/4″ is the last. Check out the graphic below.
Don’t get all bunched up in it having to be perfect! Close is good enough! That’s how I learned and how I will continue to do so. My Mom didn’t have a headspace tool when she taught me!
My Headspace Is Lower After Canning
Another big question I get is about headspace being lower after canning. Loss of liquid from the jars is called siphoning. It usually has to do with the expansion and rapid cooling of jars, causing the liquid to sputter out from under the lid because of too quick of heating or too quick of cooling. It’s pretty simple to mitigate – see my post on siphoning here.
IF you raw packed and didn’t add liquid, in the instance of canning raw pack foods, meat or rebel canning dry packed vegetables, it’s because when heated, the food releases it’s liquid and shrinks. TOTALLY FINE and normal. If the jar is sealed, it is FINE.
Another cause could be not removing the air bubbles prior to processing. By de-bubbling, you are removing the air bubbles around and between the food pieces and allowing liquid to take its place. If you have no pretty big bubbles, once the jars are heated with enough processing time, sometimes those bubbles escape and cause your correct headspace you started with to be a bit lower than you thought. Also, not a big deal but just remember to poke the bubbles out if you can.
TL;DR: Use proper-ish headspace when canning so your you can guarantee your jars seals.
Improper headspace in the form of overfilled jars with not enough headspace left will not allow room for the expansion of food, meaning that food will be pressing on the underside of the lid, sputtering out between the lid and jar rim, and possibly interrupting the lid seal to the jar.
Too much headspace can result in all the oxygen not being pushed out of the jar, resulting in oxidized food that discolors and looks gross and sometimes a seal failure later on due to a false or poorly formed vacuum seal.
Don’t overcomplicate canning.
An ounce of common sense goes miles.
Headspace is important but that so important you should be frozen in fear about it.
I really appreciate that you go over the kitchen realities of this canning question – what you need to sweat and what can be fudged a bit. AI has a hard time getting that right because its knowledge is aggregated :)
One question that’s been bugging me… how come the size of the jar isn’t a factor? Should a 8 oz and 1/2 gallon jar of jam both have 1/4″ of headspace?
yes they should, because times are adjusted according to the size of the jar meaning the same amount of time that the center of the jar is heated leaving enough room for the product to swell :)