The most common reason why mason jars break in the freezer is because the contents expanded beyond what the jar could accommodate. This can happen due to overfilling the jar, freezing contents that are too warm, or screwing the lid on tight before the contents are completely frozen.
The reasons most jars break in the freezer are putting hot contents straight into the freezer, overfilling the jar, using a narrow jar – or all of the above.
To prevent glass food containers from breaking in the freezer, it is recommended to use containers that are labeled as freezer-safe, leave enough space for expansion, and let the food cool down to room temperature before placing it in the freezer.
Tip #4: Leave Space Between Jars
Leave some space between the jars in the freezer. Cramming them right up next to each other does not leave room for the jars to expand and they can also rattle against each other. I simply place them in the freezer leaving a bit of space between them.
Ideally the bottles should be frozen slanted at an angle of 45° to enlarge the surface area and allow for the expansion of the freezing liquid. Alternatively, use bottles with a wider diameter to reduce pressure on the glass walls. Rapid cooling can create thermal shock, which can cause the glass to break.
It's important to note that not all jars are freezer-safe. Jars with tapered sides or straight sides are freezer safe; shouldered jars are not.
Glass may break when subjected to temperatures below freezing. This may occur because the contents freeze and their expansion cause the glass to crack (if the cap does not come off).
Regular mouth jars are not recommended for freezing because of the shoulders. As we know, when liquid freezes it expands. Therefore it is critical that enough headspace is available to allow for this expansion. However, even leaving plenty of headspace the regular mouth jars can crack.
I have used this method to quickly defrost glass jars of broth from frozen (and without using a microwave!) You simply place your jar in a saucepan of cold water on the stove top and put it on a low setting. It doesn't take long until you can start scraping out the defrosted broth to heat it up in another saucepan.
A lot of people are worried to freeze food in glass jars because they think the glass will break when frozen. Glass breaks because of stress, it is rigid and solid, so when pressure is applied it tends to crack. Although it's rigid, glass actually shrinks a little when it's cooled and expands a little when heated.
Most foods will freeze well in glass including fruit, vegetables, soups and sauces, baby food, leftovers, biscuits, and vegetable peels and scraps (great for making stock).
While you can freeze larger quantities in glass mason jars, and many broth lovers do, you run the risk of breakage as liquids expand during freezing which can crack your jars resulting in wasted.
Only if it says freezer-safe (usually tempered glass). Otherwise, it can crack and that's no fun. Also make sure to remember that liquids expand, and glass has no wiggle room, so if you're freezing in a freezer-safe glass container, you still want to leave a little room at the top.
Basically, the glass is being asked to accommodate a too-rapid temperature change. As your jar is lowered into boiling water, the glass attempts to expand slightly. When this expansion happens unevenly, stress builds up, and the brittle glass simply can't take it.
Mason jars can tolerate heat up to approximately 392°F (200°C) under gradual heating conditions but are prone to thermal shock with sudden temperature changes. Newer mason jars made from tempered glass are generally microwave-safe, but precautions should be taken to avoid sudden temperature changes.
The BEST way to avoid thermal shock, and prevent glass jars from breaking in the freezer, is to cool the jar as slowly as you can. Here is how you can do that: Fill the jars with cool contents, OR fill the jar with hot contents and allow it to cool to room temperature.
Mistake #3: Overfilling the mason jar
This is the biggest mistake that I see people make! The jar will shatter if it's overfilled. That's because the contents will expand as they freeze and if the jar is overfilled, there's nowhere else for it to go. So how full should you fill the jar before freezing?
It is important to only freeze in mason jars that do not have shoulders (jars that are straight sided or “tapered”) are the easiest because the contents can expand upward in the jar during freezing, especially if you're freezing liquids (jams, jelly, soup, broth, etc).
You can reuse glass canning jars, but don't be tempted to reuse canning lids, she advises. The gasket compound in used lids may fail to seal on jars, resulting in unsafe food. When jars are processed, the gasket on new lids softens and flows slightly to cover the jar-sealing surface.
It's the curve on shoulders. Straight and tapered can handle the freeze but the curve on shoulders weakens the glass.
Use one-piece plastic lids when freezing your jars (not the metal rings and lids that came with them). Not only does this help you open jars more easily, but you can wash them multiple times without worrying about rusting.
If glass isn't installed properly with a sturdy, supportive framework and sealed to avoid moisture and airflow, it's prone to breaking on its own. Excessive weight from the glass itself or pressure from wind and weather can also stress the glass beyond its breaking point.
If you don't choose the correct glass containers or you try to speed through the freezing and thawing processes, the glass can crack. You have thermal shock to thank for that. It's what happens when the temperature of a substance changes too quickly, and it can result in cracking or even shattering.