Why is the skin of a plum sour?

It's malic acid (the site below is a great resource for this stuff!).

Why is the skin of a plum sour?
because it is.
Reply:"I think it contains quinine"



nope it doesn't
Reply:quinine is definitely not because it is bitter....

i think it is the high concentration of ascorbic acid that is why it is sour... see sources of vitamin C

http://en.wikipedia.org/wiki/Vitamin_C



here about plum: http://en.wikipedia.org/wiki/Plum



best answer?
Reply:Hmm, due to the acid?
Reply:I think it contains quinine.