I have noticed that when I eat a plum that at least near the skin layer my sweet receptors are first activated by the plum juice and flesh but the more I chew it(doesn't matter if it is flesh or skin) the more tart it gets and if I were to continue chewing it could probably get as sour as a lemon.
So why does the plum become tart when I chew on it but with the initial bite it is sweet?