Glass works if you leave room for expansion and cool the soup first. Jars break for two reasons: filled too full, or hot liquid meeting cold glass.
Liquid expands by roughly ten percent as it freezes, and glass does not. Leave space and the jar is fine. The second failure is thermal shock, which is why you never pour hot soup into a jar and put it straight into the freezer, and why you never run a frozen jar under hot water.
| Headspace | At least an inch, more for a wide jar |
|---|---|
| Jar shape | Straight sided with no shoulder, or it cracks at the neck |
| Cool first | To room temperature, then fridge, then freezer |
| Keeps for | Three months for best flavour |
| Thaw in | The fridge overnight, never in hot water |
How to do it without breakages
- Cool the soup completely. Hot liquid into glass, then into a freezer, breaks jars.
- Use straight-sided jars. A jar that narrows at the top traps the expanding ice and cracks at the shoulder.
- Leave a full inch of space at the top, or an inch and a half for anything wide.
- Leave the lid loose for the first few hours, then tighten it once the soup is solid.
- Thaw in the fridge overnight. If you are in a hurry, stand the jar in cool water and work up gradually.
What happens when you get it wrong
- A cracked jar. Almost always too full, or a shouldered jar.
- A shattered jar in the freezer. Hot soup in, straight into the cold.
- Broken glass in the soup. If a jar has cracked, throw the contents out. You cannot pick fragments out of liquid reliably.
- A lid that will not come off. Over-tightened before freezing, which is harmless but annoying.
Freeze soup flat in bags, which stacks better, thaws faster and cannot break.
Silicone soup moulds are good if you want portions without bags.
Frequently asked questions
Can you freeze soup in glass jars?
Yes, if you cool the soup first, use straight-sided jars and leave at least an inch of space at the top for expansion. Most broken jars are overfilled or shouldered.
How much headspace do you need?
An inch for a standard jar and more for anything wide. Liquid expands by about ten percent as it freezes and the glass will not give.
Why did my jar crack in the freezer?
Either it was too full, or the jar narrowed at the neck so the expanding ice had nowhere to go, or hot soup went into cold glass.
Can you use regular jam jars?
Only if they are straight sided. Jars with shoulders concentrate the pressure at the neck, which is exactly where they break.
How do you thaw frozen soup in a jar?
In the fridge overnight. Avoid hot water on frozen glass, which is thermal shock in the opposite direction and just as likely to break it.