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A recent article which outlined tips for successfully freezing milk and other dairy, especially when the milk is fresh from the farm or non-homogenized (cream top), triggered questions about what type of material is best for freezer containers.

Several very insightful comments from that article related to this concern got me rethinking my choice of plastic for freezer containers, and it seemed an article on the subject was warranted to explore the issue more thoroughly.

All things considered, which truly is the best material to use for freezer containers – plastic or glass? And, while glass may be the obvious green choice, it is really practical to use safely especially with children in the house, and if so, how?

Plastic Freezer Containers

The good news is that if you’ve been using plastic as your freezer containers of choice, the practice is probably not leeching a ton of chemicals into your food contrary to what some would lead you to believe. Cold plastic does not leech chemicals at a rapid rate like heated plastic does. So, while you should never put hot food or liquids into plastic containers, storing cold or frozen food in plastic is usually safe as long as you aren’t scratching the plastic with metal utensils, using cheap, single use plastic or old containers that are visibly breaking down. In addition, acidic liquids (like apple cider vinegar) and foods (tomato sauce) should never be stored or frozen in plastic as the acidity alone tends to leech chemicals out of the plastic.

Here are some other tips from Environmental Working Group (EWG) for safe use of plastic in the kitchen for food storage and as freezer containers (1):

  • Don’t microwave food or drinks in plastic containers — even if they claim to be “microwave safe.” Heat can break down plastics and release chemical additives into your food and drink. In addition, microwaves heat unevenly, creating hot spots where the plastic is more likely to get too hot, break down, and release chemicals.
  • Use plastic containers for cool or frozen liquids and food only — never hot.
  • Don’t reuse single-use plastics. They can break down and release hormone disrupting chemicals when used repeatedly.
  • Avoid old, scratched plastic containers. Exposures to the chemicals in plastic may be greater when the surface is worn down.
  • Wash plastic containers by hand with mild dishsoap and only using warm (never hot) water.

If you follow these guidelines for using plastic for freezing, it is a relief to know you are most likely not exposing yourself and your family to chemicals. However, plastic is clearly not as green a choice environmentally speaking as glass for freezer containers.

If you would prefer to up your game with regard to freezer containers and transition to glass, which is clearly the more planet-friendly choice, you will be happy to learn (as I was) that it is possible to use glass safely!

Several folks have shared with me recently how to use glass freezer containers safely and make cracks and breakage a thing of the past.

Using Glass Freezer Containers

The key to safe usage of glass for freezer containers is choosing the right glass jars. If you’ve had a lot of breakage when attempting to freeze food in glass, take heart. You’ve probably just been using the wrong type of jars.

Two things in particular impact the chance of breakage when using a particular glass jar as a freezer container:

Old glass jars will break much more readily than new ones. Make sure that the glass containers you select for freezing are new or relatively so and put a plan in place to replace them every so often if you use them as freezer containers on a regular basis.

Glass Jar Shape is Crucial to Freezing Success

While the age of the container is important, the shape of the glass container is critical. Be sure that the glass jars you choose as freezer containers are straight sided if at all possible. This is because if there is any narrowing of the container toward the top, the container will probably crack during the freezing process.  If you remember from high school chemistry, as liquids containing water freeze, they expand upwards, hence the tendency to crack glass containers if that expansion process is restricted in any way by the container’s shape.

If there is no narrowing of the container during this upward expansion, problems and cracking/breakage are unlikely. Don’t worry if the jars you want to use curve at the bottom. The narrowing and curving toward the top of the jar is what matters most.

It is common sense, of course, not to fill a glass container that will be put in the freezer filled to the top with liquid. Even if a straight sided jar is used in that case, it will still crack when the expanding liquid encounters a screwed on lid. If you wish to be extra safe to ensure no cracking of a glass jar, leave off the lid entirely until the liquid has completely frozen inside the jar, then gently screw on the lid but never too tightly. Waiting until the food/liquid inside the jar is frozen before putting on the lid will also extend the life of the jar for freezing purposes.

If you have curved jars that narrow toward the top, no need to replace them if you wanted to use them as freezer containers. Simply leave enough room for expansion so that the frozen liquid won’t ever reach the narrowed portion of the jar.

Safest Way to Use Glass as a Freezer Container

If freezing in glass still has you nervous even after reviewing the guidelines above, it would be best to stick with commercial canning jars that are labeled as freezer safe.

Freezer safe jars (like these) are not only straight sided, but also have a handy fill line marked right on the jar so you know exactly how much food or liquid to put in to prevent any breakage issues.

Buying canning jars labeled as freezer safe with a maximum fill line etched right on the glass takes all the guesswork out of the process. These containers permit the use of glass – the greenest and healthiest material for freezer containers – to be used without worry.

BPA Free Lids for Your Glass Jars

I have one more tip to pass along when choosing and using glass for freezer containers. Be sure to use jars that come with BPA free lids.

Metal, screw-on lids that have white plastic on the underside of the lid are not preferable to metal lids that are plain metal on the underside. Alternatively, you can choose BPA free plastic lids (like these) for your glass jars, but in my opinion, the plain metal screw on lids are better because they don’t use any plastic at all.

Do you have any other tips for safe, green freezer containers? Please share with all of us in the comments section.

Sarah, The Healthy Home Economist