When I make a jam, I put it while it's hot into a jar, then close the jar with its lid (a single-piece, screw-on metal lid) and put in cold water. After doing this, I can usually keep the jam outside the refrigerator for months. But, when I try to open the jar, it is very difficult and in many cases I have to use a plyer to fold the lid open.
Is there a way to seal a jam for long-term preservation, such that it can also be opened easily?
EDIT: I just bought a jam at the supermarket. It is canned just like my jams – in a glass jar with a single-piece screw-on metal lid. It can remain on the shelf for months without refrigeration. Opening it is very easy – I just twist the lid slightly and it opens. I don't need to use force or any special tool or technique. The lid remains intact and can be used again. HOW DO THEY DO THIS?
Best Answer
You just need to work out your forearms more. :)
If food gets on the rim of the jar it can greatly add to the friction after the lid is on. It actually makes the whole process unreliable- the food might prevent a seal from forming or it might harden and make the lid difficult to remove- either way make sure the rim of the jar is clean.
Canning jars create a vacuum when the heated air in the jar contracts. This vacuum is what holds the lid on tightly. You can alter the strength of the vacuum in two ways: 1- leave more air at the top of the jar (called headspace).
More air means that a smaller percentage of the air will be forced out and the seal will be weaker. Be careful that you don't make it too weak. If the seal fails the food will spoil.
Jam recipes meant for normal canning jars will include the recommended headspace. You should start there with your experiments.
2- Let the jam cool a little before applying the lid.
This will also result in less air being forced out and a weaker seal. This is something you can experiment with but it would be more finicky than #1. If you do try it, watch your jars closely for a while and put them in the fridge if the seal fails.
The canning process for jam in normal canning jars usually includes boiling the jars afterwards to ensure the jars seal properly. With those lids, sealing too strongly isn't a concern as the lids are disposable. Even with your jars, you want a very strong seal. If you are having trouble opening them perhaps you should try using a tool that will give you leverage but not damage the lid.
