Freezing mason jars is a wonderfully sustainable alternative for food storage, and a great way to easily swap out plastic from your everyday use. But there are a few things you need to know before putting mason jars in the freezer or microwave.
Using glass jars to store leftovers is a simple way to reduce your food waste. And since you can store single meal portions of soups and stews, they’re also perfect for multiple day lunch prep.
I’m always looking for ways to live more sustainably, and for anyone else trying to make eco-friendly changes, I always recommend starting small. You don’t have to overhaul your entire life on day one.
Freezing leftovers in mason jars is the perfect way to ease into low-waste living.
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Can mason jars be frozen?
Before we dive in, we should probably answer the burning question on everyone’s mind: can you freeze mason jars?
In general, you can freeze the contents of a mason jar for 3-5 months.
FYI: In the US, you might hear mason jars referred to as ‘ball jars’ since Ball was the first company to make canning and mason jars. Kerr is another big brand in mason jars, along with Golden Harvest.
Of course, to do this without exploding your jars is a bit more complicated.
So the real question is: how can you put mason jars in the freezer safely?
The answer comes down to two things: liquid expansion and temperature control.
Although the act of freezing mason jars is seemingly straightforward–you put the contents in and then put it in the fridge–many people forget to account for liquid expansion.
Unlike most liquids, water expands when it freezes. If you fill your mason jar completely before freezing it, there’s no room for the water inside the stored food to expand, and you’ll end up with a mess of shattered glass.
The other culprit of exploding jars is rapid temperature changes. While mason jars can withstand freezing and boiling temperatures, they cannot go from one extreme to another in a short time. Putting a jar of hot soup directly into the freezer is a recipe for disaster.
While we’re here, you might also be wondering can you freeze canning jars? They’re the same thing as mason jars, so you’ll be fine so long as you follow the below recommendations.
I’ve got my technique down now, but I had a few mishaps when I first started storing leftovers in glass jars. Here’s exactly what you need to know about putting mason jars in the freezer.
How to freeze mason jars safely
#1. Use the right kind of mason jar
There are two kinds of mason jar shapes: with ‘shoulders’ and without. Ideally I recommend using wide mouthed, straight sided mason jars because shoulders prevent air flow and are more likely to crack.
But if you find yourself in a pinch freezing mason jars with shoulders, please ensure you do not fill them any higher than at least one inch below the shoulder.
#2. Make sure they’re freezer safe
This sounds like a no brainer, but not all mason jars are freezer safe, although the vast majority are. This faux paux can lead to heartache if you don’t read the label carefully enough!
#3. Avoid extreme temperatures
The number one mistake people make when freezing mason jars is to pour the leftovers straight in, even if it’s still boiling hot. Don’t do this! Glass cracks under thermal pressure. What this means is that anything hot needs to cool down to room temperature before it can be stored safely in a mason jar.
After the contents reach room temperature, pour into the mason jar, secure the lid and let it cool down in the fridge for a few hours. You can skip this step, but I have a much higher success rate when I don’t!
#4. Don’t overfill the jar
Most freezer-safe mason jars have a freeze line near the top. Sometimes it’s just an extra line beneath the cap, some, like these Ball jars state that it is the freeze line.
However, with liquid I normally still give an extra .5 – 1 extra inch of space from this line just to be on the safe side. With frozen produce or preserves this isn’t an issue.
#5. Avoid rusty lids
Even in the freezer, metal mason jar lids and rings will start to rust. Luckily, you can avoid this by swapping the metal for reusable plastic lids.
While I’m all about avoiding plastic where I can, it’s better to use long-lasting plastic lids than to keep tossing out rusty metal ones.
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What happens if the mason jar breaks?
Luckily if a mason jar breaks in the freezer, it typically cracks in a few places, but retains its overall shape. Nevertheless, to avoid any shards, remove it as carefully as possible wearing gloves. Next place it in a bucket to let the contents thaw completely.
The glass must be thrown out. You cannot recycle broken glass as it is likely to endanger the collectors who sort through it by hand. That having been said, I also recommend wrapping up any big shards so that garbage collectors don’t cut themselves on any pieces sticking out of the bag.
Make sure to dispose of the jar’s contents as well. I do not recommend composting this as there might be small pieces of glass inside, which are not compostable.
Can you microwave mason jars?
Some mason jars are microwave-safe, but not all. It depends on a few factors, namely the age of the jar. Many newer manufactured canning jars will have a microwave-safe symbol on the bottom.
Never microwave a mason jar right from the freezer! This will absolutely shatter the glass. Instead, defrost the jar and its contents completely before putting it in the microwave.
And it should go without saying, but make sure to remove the metal ring and lid before you microwave mason jars.
So, can you freeze mason jars? Absolutely! Just be safe and smart about it! A little bit of change in the kitchen a long way towards a more sustainable you and a happier, healthier planet.