The following post is from Katie of Kitchen Stewardship:
I shared what turned out to be a pretty popular post a few years back about how to freeze in glass jars, and I have felt kind of guilty about it the past year. It felt really helpful at the time: tips on where to get repurposed jars for free, how to eye up what you have to store, the importance of headroom.
But readers reported problems: They were still breaking glass jars in the freezer (and wasting food and shedding many tears, I tell you) at an alarming rate!
I realized that my post only skimmed the surface of what someone needed to know to successfully freeze food in glass jars, and the problems people shared in the comments over the years left me with a few questions, so I decided to set up my own science experiment at home.
Here are the questions I sought to explore:
1. Do newer freezers and/or uprights tend to break jars more readily than older and/or chest freezers?
2. Does the jar’s position in the freezer make a difference? For example, being in the door, sitting right on “quick freeze” coils, or nesting on top of other already-frozen foods.
3. Do different shapes of jars impact the breakage rate?
4. How much headroom do you really need?
5. Does freezing with the lid off solve the problem?
6. Does laying the jar down help?
Warning: Some glass jars were harmed in the writing of this post. I’m sorry. Please don’t report me to PETJ.
Freezer Experiments: Let’s Break Some Glass Jars!
I filled a bunch of jars that I could afford to lose with water, exactly 400 mL for the smaller tomato jars and 500 mL in the larger spaghetti sauce jars. I did some A/B testing as follows:
- One jar laying down directly on the shelf with quick-freeze coils in my upright freezer and the matching jar (the spaghetti sauce jars) laying down in my chest freezer, near the top.
- One jar with the lid off, standing up directly on the coils.
- Three jars with the lids on: one standing up on something else in the upright freezer (i.e. a buffer zone from direct contact with the coils), one standing up in the door and one laying down in the door.
No breaks at all! So that taught me nothing.
Time for some serious fun. I added 1/3 cup more water to each jar, which ended up farrrrrr more full than I’d ever risk for freezing soup or other leftovers in glass jars.
- To my great and pleasant surprise, neither of the spaghetti sauce jars broke at all! They were both laying down, one in the chest and one on the upright’s shelf.
- Both the jars in the door of the upright, one laying down and one standing up, froze completely with no breaks as well.
- And then we get to finally learn something – the jar standing up with its lid on, in the upright, on top of something else that was already frozen shattered completely. Since the jar and the amount of liquid in it matched the ones in the door exactly, I feel pretty confident in saying that freezing more slowly has a positive impact on avoiding broken jars of food.
- The jar that was standing directly on the coils with its lid off was doing great for most of its time during the freezing process, but right at the end it completely exploded!
Check this out:
I love science.
This jar was very full, really TOO full for freezing, but I still think it shows that keeping the lid off, while potentially being helpful for some to avoid broken jars, is not a perfect solution.
- The larger spaghetti sauce jars were placed upright, one in the upright freezer on top of some food (protecting it from direct contact with the coils), and the other was placed upright in the chest freezer.
- The smaller jars, filled SO much more full than I’d ever risk in the freezer, were placed upright in the chest freezer and laying down directly on the coils in the upright freezer.
My goal was to continue to test the idea that freezing more slowly (chest freezer) would be gentler on the jars than freezing more quickly (in the upright, which is turned really cold). The two jars standing upright in the chest freezer matched the one that did not break standing up in the door of the upright, as well as the ones that cracked badly standing up in the main part of the upright freezer, both on the coils and cushioned by other food.
I also wanted to see if the “laying down” theory would hold water…er, ice…even in the most volatile placement in the freezer. Remember that we already successfully shattered jars with the same amount of liquid standing up in the upright freezer, even with the lid off. Now I was laying one down (which was far too full in my opinion).
- The spaghetti sauce jar in the upright freezer completely burst within 12 hours, confirming our earlier result with the smaller jar in the same situation.
- The other spaghetti sauce jar and a smaller jar were both standing upright at the top of the chest freezer. When I checked on them after discovered the mess that the other spaghetti sauce jar made when it exploded, both still had bubbles in the center. I was careful not to wiggle/shake them too much because I didn’t want to interfere with the freezing, but it was good data that my suspicion about rate of freezing was correct – it definitely took longer to freeze at the top of the chest freezer (far from the source of cold).
- The note I wrote about both of the chest freezer jars once fully frozen said: “No breaks – DEFinitely froze slower.” However, what a surprise to find that as the jars were thawing on my counter, a chunk fell right out of the spaghetti sauce jar! It had cracked quite seriously, and I didn’t even notice it at first. So…you still can’t push your luck too much, even with a slow freeze. I wonder if it’s fair to say that smaller jars will offer more wiggle room on headspace.
- The smaller jar that was laying down on the upright coils, the one that was way too full to freeze – was completely fine!
Now I finally feel like I have really solid advice for freezing food in glass jars, and I don’t have to feel guilty about sharing only halfway decent tips anymore!
The Final Answer: The Secret to Freezing Food in Glass Jars without Breakage
The science behind the glass jars breaking in the freezer, of course, is that liquids expand when cold and push outward against the jar, which ultimately cracks under pressure (much like I do some days at about 5:20 p.m…).
Some breakage can happen as well because of rapid changes in temperature. We can use that knowledge of science plus my experiments to create some rules for freezing food in glass jars.
1. Of course, always leave an inch or so of headspace, more in bigger jars and less necessary in smaller jars.
2. Cool the food in jars completely before freezing. I always put everything in the refrigerator overnight before freezing just to be safe.
3. Choose your jars wisely – it turns out that jars with a “shoulder” are more prone to break (like narrow-mouthed Mason jars) because the top layer freezes first, and then the solid food begins to be pushed upward toward the mouth of the jar in a shape that will not fit past the “shoulder” area. If you have a jar with a shoulder, treat the shoulder as the top when figuring head space (or lay it down).
4. Very cold and very efficient freezers are more prone to break jars because they freeze faster. Whenever possible, freeze your glass jars either in the door of an upright freezer or the top of a chest freezer.
5. Some find that leaving the lid off your jar can help avoid breakage, but if you overfill the jars, they’re still going to break.
Even with all those strategies, and even when allowing for many inches of headspace, some readers still had trouble with glass jars breaking. One theory is that, particularly with quick-freezing appliances, that top layer of liquid/food freezes solid like a pond, and then the remaining liquid below exerts too much outward pressure on the sides of the jar, causing breaks.
The solution, and my official I’m-a-foodie-science-geek recommendation?
Freeze glass jars of food laying down.
The increased surface area, reduction of pressure on any “shoulder” area and the fact that laying down even expands the headroom in a way create a triple line of defense against even the fastest freezing, uber coldest freezer.
Ah, now my work to protect your homemade stocks, soups and other goodies from freezer-crack-itis is accomplished, and I can return to my regular duties of scaring the wits out of you with research on why everything in the world is going to hurt your family.
How many glass jars would you guess you have broken in the freezer?
|Katie Kimball has been “green” since 5th grade when she read 50 Things Kids Can Do to Save the Earth. She remains slightly disappointed that she didn’t actually save the whole thing back then, but now that she has 3 kiddos counting on her, she keeps plugging away hopefully. Katie blogs at Kitchen Stewardship about real food and natural living and is the author of Healthy Snacks to Go and other eBooks, available for Kindle.|