Sugar, an edible, water-soluble crystalline carbohydrate, is essential to energy production in all living beings. Although most commonly associated with sucrose found in sweet-tasting foods, sugar is also found in most fruit (fructose) and in almost every snack, processed food or flavoured beverage as high fructose corn syrup. On a chemical level, a sugar is deemed "reducing" if it reduces an oxidising agent within a chemical reaction, while those which don't reduce oxidisation are called "non-reducing."
Sugars with chemical structures that include one free anomeric carbon atom (such as glucose, maltose, lactose and fructose) use this free end to reduce oxygen during a chemical reaction by donating electrons to the other (joining) molecule.
Other sugars (such as sucrose) have closed chemical structures, where open atoms are used to bind the structure together and, therefore, do not have the free electrons to donate to the joining molecule. Because of this, oxidisation is not reduced during the reaction.
Types of Reducing Sugars
All monosaccharides (simple sugars that can't be broken down into smaller molecules) are reducing sugars. Two of three types of disaccharides (sugars with two chemical rings), maltose and lactose, have the open chemical structure needed to act as reducing agents. The simple structure of monosacchrides allows them to be broken down twice as quickly as disaccharides, while disaccharides must be broken into their smaller parts first.
Types of Non-Reducing Sugar
The third type of disaccharides, sucrose, and polysaccharides (sugars with multiple chemical rings) are non-reducing sugars. Polysaccharides--starches--have closed structures, which use free atoms to bond together their multiple rings, and take a much longer time to be broken down.