Oh..I haven't put it into the recipe..the recipe is something I copied from Finnish magazine..but usually citric acid is used as a form of preservative to keep colour/flavour from changing and keep bacterial growth down so the drink will store longer.
It is same as with elderflower cordial...where you would add citric acid as well. ;)
You get small amounts of citric acid from lemons but as it not as consentrated, you would need to used loads of fresh fruit to achieve same effect..and that would effect the flavour then...more like lemonade..