Sour plums
Small, very tart cherry plums, either green and unripe or ripe in yellow and red, which Georgians cook down into sauce or use as the souring element in stews.
These are cherry plums, smaller and far more sour than ordinary plums. In spring they appear green and unripe in Turkish grocers, and in summer ripe or frozen in Eastern European food shops.
Substitute:
Unripe, still firm ordinary plums come closest. Ripe plums with lemon juice add acidity, but the fruit is sweeter and softer, and the sauce comes out less fresh and sharp.