When you read a good canning recipe, you’ll always find certain elements of the canning process outlined, including:
- processing time: just how long should the water boil, or the pressure cooker cook ‘-)
- for pressure canner recipes, pounds of pressure: either 5, 10, or 15 PSI on a weight-gauge pot? … or the wider variety of pressures you can read on a dial-gauge pot?
And, last but not least:
- HEADSPACE: a confusing, mythical number … usually expressed in linear inches (in the US, at least) … that has something to do with canning.
So what is this mystical thing called headspace? What’s its function in canning? And I know you can’t put more food in the jar than proper headspace allows, but can you put less food in a jar? Do different foods need different headspace, and why? And how do you determine just how much headspace you need for one of your homemade creations, especially when you’re mixing foods with different headspace requirements together?
These are all extremely valid questions
Most canning recipes say something like “fill jars, leaving one inch of headspace, and then process at blah, blah, blah ….” … so let’s start with a diagram that explains headspace visually:
As you can see, headspace does NOT mean “from the bottom of the ring” or “starting somewhere below the shoulders of the jar” or any other variation you may have heard. Headspace basically means the distance between the bottom of the flat, rubber-gasketed lid … and the top of the food in the jar … expressed by a linear measurement, usually stated (at least in the United States) in inches.
Therefore, if your recipe calls for one inch of headspace, you should fill your jar with food up to no more than one inch below where the rubber-gasketed jar lid will ultimately rest. In other words, your food should stop one inch below the top lip of the jar. Likewise, if your recipe calls for one-half inch of headspace, then you stop filling the jar when the food is one-half inch below the lip of the jar. And if your recipe calls for a quarter-inch of headspace … well, you get the idea
What does headspace do in canning? Now that you know what headspace is, it’s easier to explain what it does.
First, let’s talk about what happens when you heat food. Think about a pot of soup. When you put it on the stove and turn the heat on underneath it … what happens? It gets hot, yes … but the soup in the pot also expands. The overall volume of the soup gets somewhat larger when you heat it than it was when it was cold. The difference isn’t necessarily substantial, but it is measurable. All foods (and most things) expand somewhat when subjected to heat, some more than others.
When you’re talking about a canning jar, you have to take a few other things into consideration, too:
- a canning jar is a hermetically-sealed environment, capped by a (hopefully) one-way steam venting system … more commonly known as a two-part canning lid. A two-part canning lid has two functions. First, its job is to let steam and cooking gasses escape during the canning process … while keeping the inside liquids/solids inside … and the outside pressure canner liquid outside. Once that gas/steam vents, the lid’s job is to seal firmly against the lip of the glass jar … which is accomplishes thanks partly to the vacuum created when the jar vented steam under heat and pressure … and partly thanks to the glue-like qualities that heated rubber gasket brings to the game.
- the canning jar’s contents are usually some form of liquid … or solids/semi-solids suspended in some amount of liquid … or–in the case of raw-packed foods in the very beginning, before the canning process leaches fluid out of the foods themselves–moist solids. Occasionally we fill canning jars with dried ingredients, too, but they aren’t affected by the same significant expanding/shrinking problems that jars filled with wet food face.
- there is “headspace,” a pillow of air trapped above the liquid/food, which also affects the contents of the jar.
- canning jars are subject to the normal laws of physics, namely: solids tend to expand the least overall when subjected to high levels of heat; liquids tend to expand more than solids but less than gasses in the same situation; and, finally, gasses tend to expand more than either liquids or solids when you heat them.
- when you subject a filled canning jar to the kind of heat you find in a boiling water or pressure canner, the mixture inside the jar not only heats and expands … it also boils, sometimes substantially.
Knowing that, think back to your soup pot. What happens when it goes from just heating … into full-blown boiling? The volume expands drastically at that point: or–at least–it appears to, thanks to the bubbles boiling up in the liquid. Not only does the volume of the liquid expand even more when its heated to boiling (212 degrees fahrenheit), the release of gases from the food AND from the liquid once it boils aerates the liquid as well. This causes the liquid volume to expand even more, further raising the level of food/liquid in the pan … or in your canning jar.
We’ve all seen jars that come out of the pressure canner: the ones that keep boiling for a while as they cool on your counter. Imagine how vigorously they were boiling before the pressure and heat dropped enough for you to open that canner. Think how high those bubbles were reaching up inside that jar right at the moment of maximum heat and pressure. Be seriously impressed, because you should be. It’s a feat of modern physics
Once you understand how high the contents of a canning jar can boil inside a pressure canner, you begin to realize more about why maintaining proper headspace is so vital. A canning lid consists of a rubber-gasketed seal, so it’s important to keep contaminants away from it during the canning process. Contaminants of any kind–solid or liquid–can cause a canning lid seal to fail, either during the canning process … or later. That’s why you sterilize the entire jar, and then clean the lip and lid so scrupulously before you put them together. That’s also why you need to maintain that minimum headspace recommended on various recipes. When the food, liquid, and air inside a canning jar is heated, it’s normal for the gas to escape … but you don’t want the solids and liquids expanding so much that they push chunks of food against the inside of the lid, too. That can cause a jar seal to fail right inside the canner.
Likewise, letting the liquid contents of a jar boil up against the inside of the lid is problematic as well. Why? Because the liquid in a canning jar often contains microscopic particles of food. Wedging a few of those between the rubber gasket and the glass lip of the jar doesn’t always cause the seal to fail right away, but it can cause your jar seals to fail months or even years down the road, once the small particles of food trapped between the lid and the jar begin to decompose, and weaken an otherwise solid seal.
So respect headspace. Period!
In addition, headspace recommendations are always minimum recommendations. In other words, if your recipe calls for a one-inch headspace, then you need to leave at least one inch of space between the top of the food and the lip of the jar. That’s not to say that you can’t leave more than one inch of headspace … you just need to leave at least that much.
The most common reason for leaving more than the required headspace in a jar is because that’s the last jar in a batch. Most people follow the tradition of immediately refrigerating and then eating their orphan (less than full) jar first … and I’m not discounting that practice. I often do it myself, especially when it comes to fruit spreads. Someone’s got to taste them to see how they came out, right?
However, don’t be afraid to put your partially-filled jars on your shelf, too … or afraid of going with somewhat-more-than-required when it comes to headspace, especially on items that you know you’re going to eat within the first year after canning them. I often leave anywhere between one and a half inches to three inches of headspace in jars that contain meat, because I think more about serving size on the eating end of canning, than I think … “Oh, I’ve got to fill this jar up to 100% capacity … or I’m not being 100% efficient.”
Yes, there are a couple of different things you need to take into consideration (beyond some inefficiency in my total jar and shelf capacity) when you decide to leave more headspace than a food normally requires. First, when you lower the volume of food in the jar, you lower the total amount of thermal mass in the jar: the amount of food that’s currently trying to absorb and deal with the heat you’re throwing at it.
To understand what that means, think back to our pot of soup. When you put one cup of food in one pot, and two cups of food in another pot … and then you cook the two pots over the same heat for the same duration … what happens? Yes, the pot with one cup of food in it will cook twice as much as the pot with two cups of food in it. The same thing happens in your canning jars when you put different amounts of food in the same size jars … and then can them together together in the same canner, using the time-frame established for full jars. The jar with less food in it will cook more inside the jar than the jar that holds more food. Therefore–in the case of orphans and pressure-canning–I try not to have *too* much of a difference between the level of food in my jars. I either even out the difference over several jars (without pushing into my required headspace on any one jar), or I just don’t can that last little bit. I may just decide to cook it for a snack while I’m canning the rest of the jars.
When I intentionally decide to create somewhat smaller servings inside my canning jars, thereby leaving more than one inch of headspace in all of the jars … I always process all the jars like they’re full, regardless. I never drop the contents down to less than about three-quarters of the original volume anyway, and for the few extra minutes of process time that volume will undergo, it’s just not worth risking canning cooties by trying to guess-ta-mate or recalculate my processing time to something lower. Processing time is more related to the killing of canning cooties than anything else … and that’s not simple math. It’s more about temperature and duration. Take a look at the differences between the times recommended for pints and quarts, and you’ll see instantly that it’s not a simple just divide the time in half, just like the volume-kinda thing. Calculating proper processing time is far more complex, so–rather than run the risk of either canning cooties or severely over-processed food–I tend to occasionally use less food/more headspace than is recommended … but not THAT much less/more. I try to limit it to 25% less food overall, but no more. If I want smaller servings than that, I just plan to make two meals out of every jar of food.
Secondly, the top surface of all canned foods will oxidize somewhat with time … i.e.: mix with some of the oxygen that’s trapped in the jar with the food, which will cause it to dry out slightly over weeks/months/years of storage. Therefore, leaving an even bigger headspace can cause your canned goods to be exposed to a bit more air than jars filled to exact specifications for headspace……
…..however, the overall volume of air trapped inside your canning jar is pretty tiny overall … plus, it’s been super-saturated with moisture.
To explain that, I need to explain another little bit of physics. You see, heating air and water together (to a sufficient temperature) causes the water to break down into air-born water particles (you know it as steam), which–in turn–causes the air around the source of the steam to become more heavily saturated with water vapor. You can easily see this in action with a tea kettle. When it boils, steam comes out of the end, and–with time–the whole kitchen will get rather steamy and damp-feeling. But if you put something cold into the path/in the neighborhood of the steam, it will cause the water vapor in the surrounding air to condense against the cool surface, and turn back into liquid form. With me so far?
Heating water and air together under pressure–like you do inside of a jar inside of a pressure canner–lets you take normal boiling temperatures that you can’t exceed on top of the stove with plain water (212 degrees), and jacks them up even higher: up to 228 degrees at 5psi … 240 degrees at 10psi … and 250+ degrees at 15psi.
Super-heating air under pressure like that causes it to be able to take in/hold even more water vapor than it could at room temperature, or even at normal boiling temperatures. That’s where super-saturation comes in. The hot steam swirling around in that jar for upwards of sixty to ninety minutes (or more, in some cases)–or even for as few as 10 minutes in a boiling water canner–shoves even more moisture into the air that’s trapped inside the jar: more than it normally contains … or that it even could contain under normal circumstances. This turns the air inside the sealed jar into a moisture-rich environment, much like what you feel when it’s about to rain … because that trapped air becomes super-saturated. From there–yes–it rains a little inside the jar as it cools. Some of the moisture returns to being liquid, which then bathes the top of your food as it settles … but not all of the moisture condenses. Most of it remains in the small amount of air that’s trapped inside your jar with your food, creating a damp environment will slow/prevent drying inside the jar for many weeks/months/years to come, as long as the jar seal remains unbreached …….
…… so, to bring this back around to our topic (finally…*grin*) the additional drying effect of leaving somewhat more than the required headspace in a jar will be minor, at best.
Do different foods need different headspace, and why? That’s also an excellent question. The answer is yes, and it depends on the chemical and structural makeup of the food.
Generally (but–please–always check your recipe or the UGA Canning site for special-handling instructions), headspace requirements usually go something like this:
- fruit spreads (jams, jellies, etc.) tend to require a quarter-inch of headspace; occasionally less if they’re particularly thick. Whole/halved/chopped fruit and fruit pie fillings–because they contain more unbound moisture–tend(s) to need more like one-half inch of space between the top of the fruit and the bottom of the gasketed lid, because they are more prone to the boiling in the jar problem.
- tomato products require different amounts of headspace as well, also dependent on how thick the final tomato product is. For example, a jar of chopped tomatoes–canned in tomato juice–contains a fair amount of water, so they’re going to boil fairly high in the jar once they reach proper canning temperatures. Therefore, they require a one-half inch headspace to give room for the liquid to boil. Compare that to things like tomato paste and homemade ketchup, which have a much lower water content. They only require one-quarter inch (or even one-eighth inch) headspace … since they don’t boil as viciously/actively in the canning jar as the more watery tomato products do.
- veggies need between one-half inch and one inch of headspace, depending on the type of veggie you’re canning, if it’s raw-packed or cooked first, how porous it is, and what liquid you’re using to pack them in. Veggies packed in water tend to need more headspace than veggies packed in some form of thicker liquid, thanks to the boiling in the jar problem.
- meats, stews, and soups tend to all need at least one-inch of headspace, because most of them have problems with wanting to crawl right out of a canning jar in a pressure canner, thanks to their high water content … as well as the fat content of the meat, which can cause popping/fat-projectiles in the jar … and also make a jar lid fail.
- a few foods even require more headspace. Again, please check University of Georgia’s National Center for Food Preservation’s website for the headspace recommendations on most popular fruits, veggies and meats.
And, finally, how do you determine just how much headspace you need for one of your homemade creations, especially when you’re mixing foods with different headspace requirements together?
This is always the dilemma with canning home-cooked foods. Sure, many of us use many of the same ingredients they discuss canning individually on the UGA site. We start with meat and veggies and other normal sorts of ingredients, but then most of us combine them in new and creative ways, based on family/personal food histories and personal taste preferences. We create/use recipes the UGA has never even seen, much less tested. And–let’s be honest–the UGA and other university-based, Department of Agriculture-funded canning research centers have tested the basics of canning … but they’re woefully underfunded, and they get more underfunded every year. They’re not likely to EVER have vast resources available for you that out line hundreds or even thousands of variations on the theme of carrots … enough to cover every possible carrot recipe on the planet … much less everything else we humans put in our mouths.
So what do you do when your secret family recipe for carrot fricassee doesn’t look anything like the recipes they give you as examples on the UGA site? How do you determine proper headspace for your jars?
When I’m faced with this decision (and I can so many non-traditional things … that I constantly find myself in this position), I take what I know about my recipe, and compare it to the list I made above … the one that talks about how much headspace the various types of foods need. In most cases–especially when you have your basic chunks of food, suspended in a liquidy sauce-kinda dish … you’ll note that 1″ of headspace is probably your best, safest bet. That’s the recommendation for most meats, soups, and stews … plus, it’s a sort of safe minimum in the world of canning headspace.
Remember, headspace is a minimum number … not a maximum number … so if you always canned everything with a minimum of one inch of headspace (because very few things need more), made sure to put enough water in your pressure canner (so that it didn’t boil dry), and then regulated your heat properly throughout the canning process … you should be just fine seal-wise, no matter what you’re canning. You just wouldn’t necessarily be using all of your jars to absolute 100% efficiency. Similarly, when I’m trying to decide on a processing pressure and time for one of my non-traditional recipes … I tend to go with the processing instructions on the UGA site for “soup” … which also recommends a one-inch headspace. That’s the best all-purpose canning instructions you can follow for homemade foods that combine lots of different ingredients, especially that classic meat and/or veggies in broth or sauce formula we all love so much. That’s soup, right?
One final note: I know a lot of you can foods at home as a way to save money. You have growing families and/or limited resources, so the thought of ruining a few jars of something you’re experimenting with can feel like a very undesirable place to be in. However, when you’re stepping away from those tried-and-true UGA recipes … you have to accept that you’re taking a chance with your food. You will run the occasional risk of making something that you decide–in retrospect–that you never want to eat, ever again … but the joy of taking the rules … and combining them with your own experience and imagination, hoping to create a new joy on your canning shelves … is a very rich and rewarding pursuit nonetheless. I recommend it heartily, even if you have to do it on a shoestring. The rewards are far more valuable than a couple of dollars worth of ingredients