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?

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    In my experience, it's only tart at the skin and sweet on the inside. – Jay Apr 15 '16 at 17:11

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.

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    The sugars dissolve, sure, but they don't really break down in your mouth. I don't think you're swallowing the sugars and leaving the acid in your mouth. – Cascabel Apr 16 '16 at 4:15
  • Saliva breaks down the bonds between the simple sugars that are the building blocks of complex sugars and starches. It is the job of saliva to reduce sugars to their simplest form so that we can process them. That is why a starch doesn't taste sweet. Complex substances such as proteins, fats, and yes, acids, require different means of breakdown, usually involving stronger chemical agents such as the hydrochloric acid in your stomach, peptides and other enzymes. Once the sweet of sugar is processed out, the food in our mouth takes on the flavor of the remaining components, including sour acid. – Shalryn Apr 17 '16 at 14:01
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    Again all true except... the simple sugars still taste sweet, right? – Cascabel Apr 17 '16 at 15:13

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