[littlelisa]

Why do jam recipes say to leave a few cm of 'headspace' in the jar? Wouldn't the air in the jar cause the jam to spoil? What's this for?

Thanks!!
Love Lisa

[Trillium]

Headspace in bottled items is for a vacuum space to form when you need to properly seal the bottle. When boiling the closed bottles to seal, the contents will bubble and if there's no headspace, they can bubble right out of the jar and ruin the seal. Items like tomato sauces and such need more headspace to avoid this problem; jams and jellies can have a smaller headspace.

If you're thinking of the minimal space in commercial bottles, it's because the contents were already boiled to a very high temperature and the lids will self seal with no further treatment. Home made goods rarely reach such high temperatures (no, boiling isn't hot enough for self seal - further boiling of the bottle aids proper sealing).

[catllar]

Wow, Trillium, that's brilliant - I learn something every day - never ceases to amaze me the stuff people know. Thanks for that.

[littlelisa]

Right, so if you're using screwtop jars, from what I understand the screwtop creates the seal and there's no need for boiling to seal. So less headspace required/advisable, right?
L

[Trillium]

the contents must be at their hottest in order to properly seal. If not, then boiling the closed bottles for up to 15 minutes will help make the proper seal and ensure safety.

When I do jams, for instance, I leave about 1/2" of headspace, close the jar with the 2 part ring and seal unit, put the closed jars into a covered pot of boiling water for 10 -15 minutes, remove and let cool. Usually as I take the jars out of the previously closed pot, the lids start to pop shut due to the temp change and the sealing. If not, you'll hear the lids popping as they cool.
When I do tomato sauce I leave at least 1" of headspace, often more, as the sauce will bubble a lot and sometimes bubble out of the bottle despite the ring holding it down.

[mumofstig]

Right, so if you're using screwtop jars, from what I understand the screwtop creates the seal and there's no need for boiling to seal. So less headspace required/advisable, right?
L

If you're making jams with ordinary screw top jars then you put the lids on as soon as the hot jam has gone in and screw down tight. AS they cool they will pull a vacuum and seal themselves. If you use lids with the pop seal in the middle you can see, and hear it go down with a pop and you don't need as much headspace.........I fill up to where the kneck of the jar narrows.......just don't leave any jam around the rim to spoil the seal.

[littlelisa]

Thank you!!! You folks are more genius than the whole of the rest of the Internet all put together (which failed to answer those questions!!!)
Cheers
L

[Kleftiwallah]

I have found an old pair of motorcycle gloves are far better than juggling with towels when clamping down the screw tops.     Cheers,    Tony.

[Trillium]

Interesting tip but useful only if you've got old motorcycle gloves hanging about, which very few of us do.

[Kleftiwallah]

Hang about outside A & E this time of year, there will be dozens of 'em ! ! !   Cheers,   Tony.

[catllar]

My motorcycle doesn't wear gloves!