Why is there so much citric acid in citrus fruits? And how did it evolve i.e. what did it come from? Is it a by-product of the ripening process? Why have citrus fruit evolved a particularly high amount of citric acid compared to other fruits? Was it due to particular selection pressure for acidic fruits or was there a genetic bottle neck that caused this such as human cultivation?
I've been trying to search for this but google scholar isn't throwing up anything so I'd really appreciate some help.