If you’ve ever tried freezing food, soups, or liquids in glass jars, you may have heard conflicting advice about which types of glass containers are safe for freezer storage. Some people have experienced shattering glass when pulling their jars out of the freezer, while others have successfully used glass jars for years without any issues. The secret to freezing safely in glass comes down to one critical design feature: the shape of the jar.
Many people assume all glass jars are freezer-safe, but that’s far from the truth. Specifically, glass shoulder jars (jars with a curved or sloped shoulder near the top) are not safe for freezing because of the way liquid expands as it freezes. On the other hand, straight-sided glass jars (those with no narrowing near the top) are much safer because they allow for even expansion of frozen contents.
Let’s break down the science behind why shouldered glass jars tend to crack or explode in the freezer, and why straight-sided jars are the better option for preserving your food safely.
📌 What Are Shoulder Jars?
A shoulder jar is a glass container that has a noticeable curved transition from the main body to the neck. The “shoulders” can be gently sloped or sharply curved, depending on the jar’s design. Some common examples include:
🔹 Mason jars with a rounded shoulder
🔹 Jam and jelly jars with tapered tops
🔹 Recycled pasta sauce or pickle jars
These jars are excellent for pantry storage, canning, and refrigeration, but they present a significant risk when placed in the freezer.
🧊 Why Shoulder Jars Crack in the Freezer
When you freeze any liquid, it undergoes a process of expansion. Water-based substances (like soups, broths, juices, or sauces) expand by approximately 9% when frozen. If a jar has curved shoulders, the expanding liquid pushes against the narrowing glass, increasing pressure in a small, confined area. Since glass is brittle and inflexible, this pressure often results in cracks, fractures, or even complete jar explosions inside your freezer.
Here’s a simple breakdown of the freezing process in a shouldered jar:
- The liquid starts to freeze from the outer edges first, moving inward.
- As it freezes, it expands, needing more space.
- Instead of expanding evenly upwards, the ice presses outward against the curved shoulders.
- The glass can’t flex or expand, so it cracks under pressure.
Even if the jar doesn’t break immediately, small fractures can weaken it, making it more prone to breaking when handled later.
🔹 Why Straight-Sided Jars Are Freezer-Safe
In contrast, straight-sided jars have a uniform shape from top to bottom, without any narrowing or curved areas. This design allows the expanding liquid to move upwards, rather than outward, reducing pressure on the glass walls.
Here’s why straight-sided jars are the best option for freezing:
✔ Even Expansion: Liquids freeze in a controlled manner, rising upwards instead of pushing against the sides.
✔ Less Pressure on the Glass: The lack of constriction points reduces the risk of cracks.
✔ Easy Removal of Frozen Contents: Wide openings make it simpler to scoop out frozen foods.
Common straight-sided freezer-safe jars include:
✔ Wide-mouth Mason jars (with no shoulders)
✔ Straight-sided glass storage jars
✔ Glass containers designed specifically for freezer storage
💡 How to Freeze Liquids in Glass Jars Safely
If you want to safely freeze soups, sauces, broths, or beverages in glass, follow these essential tips:
1️⃣ Always Use Straight-Sided Jars
✔ Choose jars that have no narrowing near the top.
✔ Wide-mouth Mason jars or dedicated freezer-safe glass containers work best.
2️⃣ Leave Enough Headspace
✔ Liquids expand as they freeze, so always leave at least 1 to 2 inches of empty space at the top of the jar.
✔ Never fill the jar to the brim—this will almost always lead to breakage.
3️⃣ Cool Liquids Before Freezing
✔ Never place hot liquids directly in the freezer—the rapid temperature change can cause thermal shock, leading to cracks.
✔ Let soups, broths, or sauces cool to room temperature before transferring them to the freezer.
4️⃣ Don’t Screw the Lid on Tightly
✔ When first placing the jar in the freezer, leave the lid slightly loose to allow for expansion.
✔ Once the liquid is fully frozen (after several hours), you can tighten the lid to prevent freezer burn.
5️⃣ Avoid Sudden Temperature Changes
✔ When taking a jar out of the freezer, let it thaw gradually in the fridge before reheating.
✔ Avoid placing a frozen jar directly into hot water or a microwave, as this can cause thermal shock and breakage.
🚫 What to Do If a Jar Breaks in the Freezer?
If you discover that a jar has cracked in your freezer:
🔺 Do not touch the broken glass with bare hands.
🔺 Carefully remove any large glass pieces with tongs or a thick cloth.
🔺 Dispose of the food, even if it looks uncontaminated—tiny glass shards may be invisible but dangerous.
🔺 Wipe down the freezer shelf with a damp cloth to remove any liquid spills.
🌱 The Sustainability Advantage of Freezing in Glass
Even though shoulder jars aren’t freezer-safe, freezing in glass is still a great eco-friendly alternative to plastic. Glass containers are:
🌍 Reusable – Reduce plastic waste by using glass instead of disposable freezer bags.
🌍 Non-Toxic – Free from BPA and other harmful chemicals found in plastic.
🌍 Better for Flavor Preservation – Glass doesn’t absorb odors or flavors like plastic does.
By choosing the right type of glass jars, you can safely enjoy all the benefits of sustainable food storage without the risk of breakage.
📝 Final Thoughts: Choose the Right Jar for the Job!
Not all glass jars are created equal when it comes to freezing. While shoulder jars are great for canning, dry storage, and refrigeration, they pose a serious risk of cracking in the freezer. To avoid frustration (and broken glass), always use straight-sided glass jars for freezing liquids.
Key Takeaways:
✔ Glass shoulder jars are NOT freezer-safe because they can’t handle liquid expansion.
✔ Straight-sided jars allow for even freezing and are much safer.
✔ Always leave headspace and cool liquids before freezing.
✔ Let frozen jars thaw slowly to prevent breakage.
By following these simple guidelines, you can freeze food safely in glass while reducing waste and keeping your kitchen organized.
🎯 Have you ever had a glass jar break in the freezer? Share your experiences in the comments! 😊
