The Importance of Headspace in Canning

What is Headspace in Canning
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 I cannot tell you how happy it makes me to see more and more people learning to can food at home. I have noticed, though, that many newbies proudly showing off their work online are making some mistakes. Totally normal! None of us knows everything about canning when we first start out. But one mistake I see a lot is ignoring headspace.

Headspace is the amount of unfilled space between the food in the jar and the rim (top) of the jar. Using the correct headspace not only ensures a good seal on your home canned foods, but it also prevents some potentially dangerous situations.

Why Correct Headspace Matters

Using too little headspace means the jarred food can't properly burp or expand during the canning process. This means the food is forced out of the jar, leaving particles behind on the jar rim. This, in turn, means the lid can't properly seal. From what I've seen, the most common occurrence of using too little headspace is in pie filling. Usually 1-inch headspace is called for - and while that might seem like a huge amount of "unused" jar, the truth is pie filling expands a lot once it's processed.

A much more common problem (and one I battled with when I was new to canning), is leaving too much headspace in the jar. This keeps air in the jar, making the food inside darken over time. Worse, it can potentially lead to food spoilage, even when the jars seal.

Whatever recipe you use should always indicate what headspace is required. (If it doesn't, it's a sure bet it's not a tested safe, "approved" recipe and therefore you shouldn't be using it.) Generally speaking, though, jams, jellies, and juices require 1/4-inch headspace, fruit requires 1/2-inch, and things that go into a pressure canner (low acid foods) need 1-inch headspace.

Bubbling is Vital

In home canning, "bubbling" refers to taking a plastic device (like a plastic spoon handle) or wooden skewer and running it between the jar and the food to remove air bubbles from the jar. (Never use metal for bubbling, since it could damage jars, causing them to break.)

If you don't bubble jars, you may end up with false headspace - that is, you may think you have the correct amount of headspace, but once air works out of the jars during processing, the contents will fall, leaving far more headspace than is ideal.

Courtesy Tom Head
By the by, if you discover air bubbles in your jars after processing, don't worry. You will never get all the air out of the jars. The idea is simply to remove as many air bubbles as possible.

What About After Processing?

Sometimes when you remove jars from the canner, you may find the headspace has changed. Usually, the headspace now looks bigger, but sometimes it may seem to have disappeared. This is nothing to worry about. If you start out with the correct headspace when the jars go into the canner - and the lids seal - the food is safe to eat and store on a shelf.

Sometimes you'll end up with increased headspace because hot food may shrink during the canning process. Other times the increased headspace is caused by "siphoning," or loss of liquid in the jars due to improper technique. Siphoning is generally caused by raw packing (not pre-heating) food that is canned in a heavy syrup; not allowing the jars to cool in the canner a full 5 minutes after processing; or simply running the canner at too high a boil. The end result may be food that discolors at the top, but this is not a safety issue.

If you neglected to bubble your jars and the headspace drops, you could end up with a poor seal on your jars - which could potentially release during storage and lead to spoiled food.

How to Measure Headspace Accurately

If you're just starting out, it's helpful to actually measure your jars' headspace, rather than eyeball things. Back when I was learning to can, I used an old fashioned ruler for this job, but nowadays, there's an even better tool, called (creatively) a headspace measuring tool. This is better than a standard ruler because  it latches onto the jar rim, making for more accurate measuring. (Most headspace measuring tools double as bubblers, too.) To use this type of ruler, hold it upright, with the tip inserted inside the jar. The food should just touch the tip of the correct measurement on the ruler. If you've over-filled your jar, spoon out some of the food. If you've under-filled the jar, add a little food and re-bubble.

Here's Ball's version of a headspace measuring tool.

And here's a less expensive version.

Once the jar is bubbled and the headspace is correct, wipe down the rim of the jar with a damp cloth. Better yet, wet the cloth with a little white vinegar; this helps remove sticky and oily substances better than just water. This step helps ensure a good seal.

Another option, though possibly not as accurate, is to use a canning funnel with headspace marks on it. (Here is a partially stainless steel version.)

When you become a more experienced canner, it's acceptable to eyeball headspace. To aid in this, use the jar's threads. Turn the jar so you can see all three threads. The first thread (the one closest to the jar rim) is where the 1/4 inch headspace mark is. The middle thread indicates 1/2 inch. Just below the last thread is the 1 inch mark.

By paying attention to headspace, you'll improve your canning tremendously, ensuring all the food you can is safe to eat. A little attention to this detail now, and you'll have home canned food that will last many years to come.
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