Sweet plums going tart as I chew 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?
Saliva breaks down sugars and starches. That is why candies dissolve so well, and it's why bread or crackers taste sweeter the longer you chew them. What saliva does not break down are acids, which are tart or sour. The longer you chew the fruit, the more sugars and starches are broken down and swallowed, leaving the tangier acids intact.