Plastic storage containers and freezer bags are popular with cooks as handy ways to freeze food for use at a later time. With concerns about plastic potentially leaching harmful chemicals into the food, as well as its impact on the environment, many cooks are interested in using glass containers – from canning jars to Pyrex dishes – but are concerned about breakage. Glass is excellent for freezing food and, as long as you follow the precautions for safe freezing, glass should freeze well without breaking and last for many years.
Store Glass in the Freezer Safely
If you follow the guidelines for freezing food in glass you can avoid breakage. It's important to leave what's called "headspace" in glass containers that are going into the freezer. This is because frozen food tends to expand, and that pressure can break the glass. For safety's sake, leave 1 inch to 2 inches above the food level to allow room for expansion.
Be sure the food is cooled thoroughly before putting glass containers into the freezer. Since recent storage guidelines recommend cooling food in the refrigerator immediately, rather than cooling it on the counter, you can refrigerate glass containers until the food is cool, then place them in the freezer.
Sudden temperature changes can cause breakage, so thaw the frozen glass container in the refrigerator. Trying to speed up thawing time by microwaving or placing the glass container in water can cause it to break. Using glass containers safely requires a bit of advance planning, but you can get into the habit of moving a container from the freezer to refrigerator the night before you need to use the food.
Choose Tempered Glass Containers
Look for containers that are made from tempered glass, which means the glass has been heated at high temperatures and cooled rapidly so it is much tougher than regular glass. It's intended to withstand many uses that regular glass is not. If it does break, tempered glass shatters into small pieces that don't have pointed edges so you're not likely to be cut from sharp glass.
Tempered glass is made from either borosilicate or soda and lime silicate – both work well. The glassware may have this information stamped on it, or it may say "tempered glass" or "microwave, oven and freezer safe," which indicates it's made from tempered glass.
Freeze Food in Glass Jars
For the most reliable freezing, look for jars made from tempered glass, such as those used for canning fruits and vegetables. They may not actually say "tempered" on them, but if they are canning jars, you can assume they are tempered. Canning jars, like those made by Ball or Kerr, are a popular choice.
On the other hand, many people freeze food in any glass jar they can get their hands on and rarely have a problem with breakage. The secret is to leave 1-inch to 2-inches of headspace above the food level in the jar, but there's a catch: If your jar has shoulders, meaning it curves at the top to a narrower opening, the headspace must be below the shoulders of the jar to prevent breakage.
Leave space between jars to help them freeze faster and prevent them from hitting another jar and breaking.
Freeze Food in Glass Storage Dishes
Square or rectangular glass storage dishes make stacking easier and are less precarious than using glass jars. Many cooks use these containers just until the food has frozen solid, then pop it out and wrap or bag it, freeing up the dish for other purposes. But you can also leave the glass dish in the freezer with the food in it until you're ready to thaw the food.
Whether you choose square or rectangular dishes, or jars with or without shoulders, glass containers are great for freezing food. But after all your diligence, just don't clink the container against a hard surface once it's out of the freezer. Then again, as long as it's tempered glass, it can take a few clinks and bumps.