Why add lemon juice to jam?

My neighbour has very generously allowed me to grab heaps of plums from her tree and I have sugar but now see I have to add lemon juice to the recipe, and although my tree is laden with lemons not a one of them is ripe!Can anyone help me please! Many thanks

you can also use citric acid
there are few reasons to add lemon juice/citric acid etc .
it helps jam to set, it affects the colour (making it brighter), cuts the sweetness so the jam tastes more like fruit than sugar (if you are using sugar) Some fruits are naturally high in acid but others are low so we need to compensate for this by adding acid at the start before we begin cooking.
You will get more juice out of the lemon if the lemon is warm. Ten seconds in a microwave or popping lemons in a bowl of hot water for couple of minutes will enable you to extract all the juice. If you do not have fresh lemons or citric acid, you can use lemon juice in the bottle two tablespoons of lemon juice is equivalent to the juice of one lemon.
Or substitute for lemon juice by dissolving one level teaspoon of tartaric or citric acid powder in half a teacup of water for each lemon.
If you have none of the above you can use equivalent of one lemon - 140ml of gooseberry or redcurrant juice. It will affect flavour BUT DO NOT LEAVE OUT THE ACID.
These fruits contain enough acid:
cooking apples, early blackberries, blackcurrants, cranberries, damsons, gooseberries, loganberries, plums, raspberries, redcurrants, morello cherries,

Thanks 245sam - read somewhere that there's enough pectin in the plum stones to help it set - would this be right!

Interesting thought.
Made plum jam the other day and for the first time - I cut out the stones, took longer to set.

Thought that was for apricots, not plums! Made plum jam last week, stones removed, set as per normal. I didn't add water tho, maybe too much water increases setting time!

its apricots - you remove the stones and crack them open, wrap into the muslim /cheese cloth and cook with the apricots, it enhances the taste.in europe all stones are dried and then cracked open - 'fake' almonds
Plums do have a lot of pectin naturally so setting is not a problem. But for a more distictive taste crack open 6 of the stones and remove the kernels and put them in with the fruit - leave as they are or do as above with the apricots.
Over ripe plums will have less pectin but still enough

its apricots - you remove the stones and crack them open, wrap into the muslim /cheese cloth and cook with the apricots, it enhances the taste.in europe all stones are dried and then cracked open - 'fake' almonds
Plums do have a lot of pectin naturally so setting is not a problem. But for a more distictive taste crack open 6 of the stones and remove the kernels and put them in with the fruit - leave as they are or do as above with the apricots.
Over ripe plums will have less pectin but still enough.
I do not add water to any of my jams/ or as they should be called here fruit spreads - most of them are sugar free just fruit, lemon juice and pectin.
some will have up to 15 - 20 % of sugar but that is for combination of tastes only (strawberry with vanilla pods etc needs a bit of sugar to help the vanilla flavour)

If your plums are firm you don't need to use lemons- plums generally give a good set. I always have some tartaric acid and/or citric acid in the cupboard to add if I have a jam that is not setting. They are in the same aisle at the supermarket as spices etc.

katalin2 - I've got some citric acid . gotta love TM recipes and all you helpful folks

I didn't add lemon, but I did leave the stones in and had great fun picking them out and my jam set beautifully. I was a first timer too, love my jam :)

Did it work! You should get a really firm set with citric acid. You could try making it without, and just add at the end if it is not setting- that's what I tend to do.