What flavouring is used in the Belgian beer Kriek?!
Cheap versions will use Cherry syrup.
Proper ones, for instance Cantillon, put whole cherries in the base Lambic beer and leave them for several years to dissolve (at the end of this time only the pit will be left - and in very acidic beers even that dissolves)
Sour cherries (Morello variety) Kriek is a fermented cherry beer.