My preservation skills are what I like to call, a “work in progress.” Meaning, they’re limited.
I know how to can diced tomatoes using the water bath method, but that’s about the extent of my canning skills.
The ONLY catch though to freezing, is knowing how to freeze glass jars without them breaking.
You’d think that we should be able to stick a glass jar into the freezer and never have a problem. But for many people, it’s ALWAYS a problem.
- Broken jars in the freezer.
- Cracking jars as it thaws (and the inside oozes out everywhere).
- Exploding contents when the jar is opened.
If you’ve ever cleaned spaghetti sauce off your ceiling, you know exactly what I’m talking about.
As you can imagine, lots of freezing means lots of lessons learned. One of those lessons, is how to prevent glass jars from breaking in the freezer.
How to Prevent Glass from Breaking in the Freezer
1. Choose the right shape.
At room temperature, all glass jars are created equal. Not so much when you are freezing in glass jars.
Glass jars are usually one of two main shapes – straight sides or shoulders – and you want to freeze in jars with straight sides.
Glass jars with straight sides are just that – straight from the top all the way to the bottom. The opening at the top isn’t any smaller, or bigger, than the glass jar itself.
A perfect example are these wide-mouth mason jars (also pictured above). Notice how the glass jar doesn’t bow inward or outward? These are great for freezing (in either the pint or quart size).
Glass jars where the opening is smaller or larger has shoulders. The shoulder of the jar is the point where the jar bends inward or outward, between the opening and the widest part of the jar itself.
One example of glass jars with shoulders are these traditional mason jars. As you can see, the shoulder on those is slight. However, this 1/2 gallon glass jar has significant shoulders and this 1 gallon glass jar has shoulders so big, I would never consider freezing in it!
2. Choose the right type of glass.
Glass jars are usually made with one of two types of glass – tempered glass or non-tempered glass – and you want to freeze in jars made with tempered glass whenever possible.
Non-tempered glass contains microscopic air bubbles that expand and contract as the glass is heated and cooled, especially at extreme temperatures like during canning and freezing.
When those little air bubbles expand, they cause the glass to crack or even explode.
Tempered glass doesn’t have these little air bubbles because it has been strengthened during the manufacturing process. The end result is a glass jar that is 4-5 times stronger than non-tempered, or regular glass.
Do you have any glass storage containers that are safe to go from freezer to microwave to dishwasher over and over again? Kind of like these? Those are made with tempered glass.
Most mason jars manufactured now are tempered glass. However, most foods bought in glass jars (i.e. jellies, spaghetti sauces, pickles, etc.) are NOT tempered glass (pictured below, and notice that it also has shoulders).
It is entirely possible to freeze in non-tempered glass, and using the rest of the tips in this post to help you do this successfully. However, your chances of preventing glass from breaking in the freezer goes up when you choose tempered glass.
That’s one reason why 99% of my frozen items are in:
- 8 oz mason jars
- 16 oz wide-mouth mason jars, or
- 32 oz quart mason jars (and I keep the boxes they come in! See tip #10 below.)
I still keep and re-use old spaghetti sauce jars and pickle jars (and remove the labels using this method). But I use those to store leftovers in the fridge, or dry goods in the pantry – especially when I buy in bulk.
3. Leave room at the top for expansion.
The space at the top of the jar, from where your fill line is to the lid, is call headspace.
Food expands when you freeze it, and the only place it has to go is the space you leave at the top. In order to prevent glass jars from breaking in the freezer, you need to leave plenty of room at the top.
The general rule of thumb is 1″ of headspace, but it’s not a one-size-fits-all solution. The bigger your jar, the more you’re going to fill it. When where’s more content to freeze, there’s more to expand and you need to leave more room to allow that expansion to happen.
Generally speaking, skinny jars need more headspace and wide jars need less. Depending on the width of the jar, my own personal rule of thumb for leaving headspace in glass jars is:
- 8 – 14 oz jars, leave 1″ – 1 1/2″ headspace
- 14 – 24 oz jars, leave 1 1/2″ – 2″ headspace
- 24 oz jars or larger, leave 2″ or more headspace
If you have jars made especially for freezing, you’ll notice that most have a fill line. I’m not going to say the manufacturer is wrong, but personally, I’ll follow my own “rule” and leave more room, to err on the side of caution.
Leaving headspace does mean you won’t get quite as much contents in each glass jar, BUT it also means not having glass jar break in the freezer and possibly ruining whatever you frozen inside the jar.
Tip: If you choose to freeze jars with shoulders, treat the shoulder as the top and leave leave adequate headspace from there.
4. Store the jar on its side.
Building on #3, another way to leave plenty of space for expansion is to store the jar on its side. Essentially, laying the jar down.
When you store the jar standing up, it only has 2″ – 3″ of diameter worth of space for expansion. However, when you store the jar laying down, you now have the entire length of the jar AND 2″ – 3″ of depth as well.
Storing the jar on its side does take up more room, but once the jar has been in the freezer for 24-48 hours, you can move it upright for longer-term storage.
5. Don’t tighten the lid right away.
When you tighten the lid to the jar, you’re basically telling everything inside that they’re not going anywhere. Not cool when you KNOW the stuff inside is going to expand!
By not tightening the lid to the jar all the way, you’re leaving a little escape route for any air that needs to get out. It’s not much, but it’s enough. And don’t worry, your jars won’t get freezer burned if you forget to tighten them in a couple days.
6. Loosen the lids when thawing.
For the same reasons as #4, except now you’re on the other side of the fence. Loosen the lids, or remove them entirely, as you thaw jars and you’ll help prevent the glass jar from breaking.
7. Cool filled jars slowly.
If there’s one sure fire way to break glass jars in the freezer, it’s thermal shock. Thermal shock happens when the shift in temperatures is extremely great, and essentially too much for the jar to take. A perfect example would be putting piping hot spaghetti sauce into a glass jar, and then putting that glass jar into the freezer.
The BEST way to avoid thermal shock, and prevent glass jars from breaking in the freezer, is to cool the jar as slow as you can. Here is how you can do that:
- Fill the jars with cool contents, OR fill the jar with hot contents and allowing it to cool to room temperature.
- Once the jar AND contents are at room temperature, place in the fridge at least overnight. 24 hours is best.
- Move the jar to the freezer door (not the back of the freezer, see #9 below) or the top of a chest freezer with the lid slightly loose.
- Once it’s completely frozen (24-48 hours), tighten the lid and move to the back of the freezer OR the bottom of the chest freezer, if desired.
8. Thaw frozen jars slowly.
Again, the same concept as #6, but in reverse. Because the odds of breaking a glass jar doesn’t stop at the freezer!
Here’s how to thaw frozen jars so you prevent the glass jar from breaking:
- If you have time, move the frozen jar to door of the freezer or the top of the chest freezer. If you don’t have time, move the frozen jar to the back of the fridge and loosen the lid.
- Allow the jar to thaw at least 75% before removing the jar to the counter to finish thawing to room temperature. Loosen the lid again, or remove the lid entirely. (There will be condensation on the outside of the jar, so I suggest placing the jar on a kitchen towel.)
- Once the contents is 90% thawed or greater, pour into another container to heat or warm.
Do NOT place the jar in hot water or the microwave to thaw. Even if the contents is mostly thawed, there’s still a chance the jar itself isn’t entirely at room temperature. It’s better to wash another dish than ruin the glass jar and possibly the food you stored inside it.
9. Avoid the cooling elements in the freezer.
The closer you place an item to the cooling element in a freezer, the colder the item gets, right? To prevent glass jars from breaking in the freezer, avoid storing glass jars on top of, or next to, or beside, the cooling element of the freezer.
- For an upright freezer, or a side-by-side fridge/freezer, this would be the very back.
- For a chest freezer, this would be the very bottom.
Your jar will still freeze and be plenty cold if you store in the door of an upright freezer, or near the top, or in a basket of a chest freezer. And the chances of your jar breaking go down!
10. Leave space between jars in the freezer.
I’ve already mentioned that the jars expand and contract when they’re in the freezer, as does the contents inside the jar. It might not seem like a big deal, but cramming the jars right next to each other in the freezer doesn’t leave any wiggle room for the jar to expand.
Plus as the jars freeze and once they’re frozen, the glass itself is more susceptible to breakage. Storing the jars close to each other can cause them to rattle every time you open the door. Plus storing them close can change the rate at which the jars freeze. Remember, you don’t want the jars to freeze too fast.
I like to use the box the mason jars come in to store my jars in the freezer. It leaves just enough room, and it also helps when I’m storing jars vertically in my chest freezer (we have an older model chest freezer similar to this one).
If you’re re-using jars or didn’t keep the box, you can place each jar in a sock and the sock will be the buffer.
11. Sometimes, despite your best efforts, glass jars just break anyway.
If you don’t want to risk breaking your glass jars, consider freezing in non-glass containers like plastic storage bags, re-using old yogurt and sour cream containers. When I freeze muffin batter or freeze multiple loaves of bread, I usually use plastic storage bags because that’s what works best for what I’ve made.
It’s possible to prevent glass jars from breaking in the freezer, but even if they do break, don’t let that deter you from saving money!
I teach in my eCourse, Grocery Budget Bootcamp, details on why you should buy extra of what’s on sale now and preserve, so you can save money later. Here are some other ideas for preserving:
- How to Dehydrate Fruit
- DIY Apple Chips (dehydrating apples)
- How to Dehydrate Herbs
- How to Dehydrate Carrots
- No-cook Freezer Jam
- DIY Fruit Butter
- Canning Diced Tomatoes
- How to Can Applesauce
- How to Blanch Greens
- DIY Dried Celery
- How to Freeze Fruit