Let me kick things off with a personal failure.
One winter, I decided to be practical, save myself future dinner stress. So, I ladled an entire batch of homemade chicken soup into mason jars. Let it cool, sealed the lids, felt so proud of myself for thinking ahead, and stuck the jars into the freezer like I had cracked the meal prep code. The next morning, I opened the door and… disaster. Glass everywhere, frozen soup stuck to the shelves like some sort of glacier. That sinking “oh no” moment in the pit of my stomach? Yeah, I still remember it perfectly.
If you’ve ever done this, let me assure you: it’s not the soup’s fault. It’s the jar.
The one criteria that dictates it all: shoulders
Here’s what I wish I had had someone explain to me earlier in life. Not all mason jars are created equal. You can have two mason jars next to each other, both say “Ball” or “Kerr,” and look basically indestructible. But, one is going to survive in the freezer and the other is going to explode. The explanation? Straight sides versus shoulders.
Straight side jars, like jars that are sometimes used for jams, are the winners. No gradual narrowing at the top, single width from bottom to rim. When you freeze liquid that is in a straight-sided jar, it has the ability to expand upward as it freezes. Safe, contained, and no drama.
But, the classic mason jar where the sides gradually come in to a shoulder (say that 3 times fast)? Complete trap. That narrowing point is where pressure accumulates as the liquid expands, and boom… glass breaks. It doesn’t matter how “sturdy” it seems; physics wins.
Why freezer safe is not a simple designation
Quick refresher on science: when water freezes, it expands about 9 percent, meaning the soup, stock, or whatever you have in there are all trying to take up more space. If your container isn’t able to absorb that expansion, it breaks. And when the container is glass… well, you get the horror story I told before.
That’s why the best way to store anything in jars is to use straight-sided, freezer-safe jars, and this is important, leave plenty of headspace at the top.
Headspace: the invisible safety reserve
Even if you are using the “correct” jar, there is a way to ruin everything by filling it too full. I learned the hard way (RIP dinner). As a rule, leave 1 to 1.5 inches of space. More if you’re freezing something chunky (like soup with beans or pasta). And when freezing, don’t crank the lid down super tight. Just sit the lid on loosely until the food is frozen solid. That means that the expanding liquid is not pushing against a locked lid.
Tight lid + zero headspace = cracked jar + wasted food. Every time.
How to find actual freezer-safe jars
Both Ball and Kerr manufacture jars that literally say “Freezer Safe” on them. Straight sides, even stamped on the box. That’s your best bet. After that, if you’re shopping, look for:
- No shoulder, same width, top to bottom
- Has wording, packaging, or embossing that says “Freezer Safe”
- Tempered glass
So even if not all straight-sided jars say “Freezer safe,” if they are tempered with no shoulders, you’re usually fine. Just use common sense and don’t press your luck.
About re-using store jars (and why it’s a gamble)
I’ve tried freezing in all types of old jars, pasta sauce jars, salsa jars, you name it. Sometimes they survive, sometimes they crack right away. Most store jars are not tempered to freeze. It’s a gamble and honestly one that I would not encourage unless you like cleaning up frozen tomato splatters on your freezer walls.
A few habits that are helping me along now
After too many cracked jars, I’m going to share what really works for me:
- Cool food all the way before freezing. Hot liquid, in cold glass? Thermal shock.
- Freeze jars upright, and separated at first. Once solid, you can reorganize them however you like.
- Label your jars with the date and contents because in 3 months, tomato sauce and chili look the same.
- Weird but it works: freeze with the lid open for a couple hours, then screw the lid on after it’s solidified. That way the excess pressure won’t push against the seal.
It’s not that you can never freeze in glass. It’s about being aware of what jars will actually take the job. Freezer-safe jars are reusable, eco-friendly, and great for all types of things, such as homemade pesto and bone broth. But shoulders? Shoulders are trouble.
So next time you’re digging into a jar of leftover food, look at the shape of the jar. If you see straight sides—freezer safe! If you see the shoulder curve—time to be a drop-away! And trust me, future you doesn’t want to spend an evening chiseling frozen soup from a freezer shelf.
