Why Straight-Sided Mason Jars Are The Safest Choice For Freezing

Freezers are a miracle when it comes to kitchen appliances. They are one of the best ways to preserve a food's nutrients and flavors so you can enjoy it later. But in order to retain the integrity of food, it's important to choose the right vessel to freeze it in. Plastic containers, freezer bags, and even glass jars give you plenty of options, and everyone has their favorite choice for the job. But if your container of choice is a glass one, particularly a Mason jar, only one type is a smart option when it comes to freezing food safely, and it comes down to the shape of the jar itself.

Mason jars, which are also known as canning jars, were invented by John Landis Mason in the 1850s as a way to improve the home canning process. They come in a wide variety of sizes and can be used for many different purposes, from organizing items in your utensil drawer to storing food in the freezer. But the main thing that differentiates Mason jars from one another — and determines if they are freezer-safe — is their shape, which is either straight or shouldered. Straight-sided jars are as straightforward as they sound, with the side of the jar following a straight line from rim to bottom, and these are the ones you'll want to use for freezer storage. Shoulder jars, the ones you want to avoid using in the freezer, have a neck that drops down from the opening, then extends out in a rounded shape before continuing down to the base.

Whether you are filling Mason jars to freeze leftovers or trying an unconventional hack for making ice cream, you'll want to be aware of how the freezing process works. Food expands as it freezes, so no matter what shape of jar you use, if you fill it too full, expansion can cause the glass to break as the moisture in the food runs out of room. For this reason, you'll want to make sure there is plenty of breathing room — or what is known as "headspace" — of at least ½ an inch to account for this expansion. With shouldered jars, however, even with headspace, there is even more limited room for expansion as the structure of the jar is essentially constricting in on the same space the food is using to expand, which can ultimately lead to cracking, breaking, or even shattering of the glass.

Another cause of glass breakage is thermal shock, in which the glass is affected by rapidly changing temperatures. Freezer-safe jars are made with more sturdy tempered glass; while these jars can tolerate high or low temperatures in canning and freezing situations, they can still shatter if exposed to extreme temperature changes (like moving from the freezer and running it under hot water to defrost). With that being said, any glass jar that is used in the freezer has at least some risk of breaking — it's the unfortunate nature of glass, after all — so it's important to follow best practices for freezing food such as allowing it to cool sufficiently in the fridge before sticking it in the freezer.