The reduction of cupric acetate by reducing monosaccharides and reducing disaccharides forms the basis of Barfoed's test reaction. Brick red precipitate is produced when cupric acetate is reduced to cuprous oxide. Typically, monosaccharides react in 1 to 5 minutes and form a crimson precipitate.
With the Barfoed reagent, monosaccharides including fructose, mannose, glucose, and others will turn red in 5 minutes.