Minimum sugar in jam for preservation

[ameno]

Does anyone know what the minimum sugar content of a jam needs to be to have it keep on the shelf indefinitely when unopened, the way most jams will?

I ask because I just made a batch of winter squash jam, and it's really nice but I feel like it would taste better with less sugar. It seems to taste a lot sweeter than other jams I've made, despite using the same amount of sugar. I suspect this is because most fruit have an acidity to them which helps balance out the sweetness, but squash has no acidity at all.
I made the current batch with 900g of sugar for each 1kg of fruit.
Having sufficient sugar for setting is a non-issue as this isn't a normal set jam, anyway. If you cook down squash enough it naturally forms a paste which is thick enough to be spreadable, even though it never sets into a gel like fruit with pectin in will.

[Nicos]

Not sure tbh ameno.
Have to say that French jam ( confiture ) is made with 700 g sugar to 1kg fruit and I have several jars of homemade confiture still unopened from 2017 stored in my cellar.


Why don’t you water bath your produce then it should keep for several years - I use LeParfait jars as they have stronger glass but other makes are available.

[ameno]

Why don’t you water bath your produce then it should keep for several years - I use LeParfait jars as they have stronger glass but other makes are available.

I did consider that, but a) that's a pain in the arse, and simply jarring and lidding then putting them in the cupboard is much easier, and b) if the sugar is too low then they won't keep long at all once opened, even in the fridge. For example, I looked up a traditional Italian pumpkin jam recipe, which uses 350g of sugar for 1kg of fruit, and they say that once opened it will only keep 4-5 days, even in the fridge.

Going down to 700g may be worth trying, though.

[greenishfing]

Most of my ancient jam recipes are equal weights of fruit/sugar but my strawberry jam recipe is 75% sugar to fruit. It's Delia's original recipe and has never gone off on me.

[Nicos]

My 70% confiture has to be stored in the fridge once opened as we don’t eat much jam these days , but I seem to recall it keeps for several weeks outside of the fridge….gf’s comment supports that.

Though….does acidity also make a difference to storage life?

I’m not really a jam expert tbh…..

[ameno]

Though….does acidity also make a difference to storage life?

I can't imagine there is enough acid in the fruit to improve storage. You need quite a bit (preserving vinegar is 5% acid, for example).
The acid in the fruit just adds a tangy flavour, and also a small amount of acid is needed to make the pectin set properly.

[quanglewangle]

Unopened keeping really depends only on the jam, jar, and lid being sterile at moment of closure.

Jam is no problem since it will be hot enough. Lid can be popped into boiling water at last minute, shaken dry and quickly screwed on. We wash our jars, on their own, in the dishwasher on super-hygiene setting timed to finish at the moment they are needed - hot and sterile.

Needs gloves to be able to screw hot lids on hot jars!

Opened keeping? Don't know.