I stir-dissolve some sugar in a cup of water and then keep it open for certain time (say a few hours), when I taste it I feel that it has became sour.
I have observed the same thing many times when I went to juice shop(s) and asked especially for sugarcane juice when I drank it, it tasted a bit sour (sometimes too sour to drink) and I just realized that the sugarcane (or other fruits) juice was extracted some hour(s) ago because I know that a fresh juice doesn't taste sour until kept (open) for a certain time.
From all this experience one thing I know is that the sugar solution or sugarcane (or other fruits) juice becomes more or less sour when kept for a certain time after extraction. It's most probably due to acid (which usually tastes sour) formation but I don't know what chemical changes turn a sugar solution or (sugarcane) juice sour (acidic) when kept for a certain time?
Any explanation will be greatly appreciated. Thanks.