The carbon atom at which the initial deprotonation takes place is a stereocenter. If, for example, D-glucose (an Aldose) rearranges to  D-fructose, the ketose, the stereochemical configuration is lost in the enol form.  In the chemical reaction the enol can be protonated from two faces, resulting in the backformation of glucose or the formation of the epimer D-mannose. The final product is a mix of D-glucose, D-fructose and D-mannose.