What is the function of resorcinol in Seliwanoff’s test?

Resorcinol acts as a reagent that reacts with the carbonyl group of the ketose sugars under acidic conditions. The resorcinol forms a red or pink-colored complex with the ketone group in fructose. This reaction is enhanced by the presence of concentrated hydrochloric acid, which catalyzes the reaction, facilitating the identification of ketoses, especially fructose.