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Reducing Sugar. Any sugar (q.v.) that contains a reactive aldehyde group (HC=O), as does dextrose (q.v.), or a reactive keto group (C-CO-C), as does fructose (q.v.). Such a sugar is capable of reacting with the soluble blue cupric ion of Fehling's solution (q.v.) to produce an insoluble reddish cuprous oxide, being itself oxidized to an acid (q.v.) in the process. At the elevated temperatures of baking, reducing sugars enter into the browning reaction (q.v.), also referred to as the Maillard reaction, by combining with amino acids (q.v.) to form melanoidins (q.v.) that contribute importantly to crust (q.v.) color and bread flavor (q.v.).