That is an interaction of proteins and sugars. A free amino group of a protein reacts with a carbon-R group of a sugar, producing something called N-glycosylamine. This is unstable and is basically a downstream product. It breaks down into something called the melanoidins, which can be bitter to taste.
The Sour Plum also is known as a Cherry Plum is the main ingredient of a tangy Georgian sauce called Tkemali. Tkemali is the Georgian name for cherry plum, as well as sauce made of cherry plums. Tkemali is made from both red and green varieties. Flavor of the sauce varies between sweet and pungently tart.
