The percentage of reducing sugars present in these starch derivatives is called dextrose equivalent (DE). When starch has been partially hydrolyzed the chains have been split and hence it contains more reducing sugars per gram. In glucose polymers such as starch and starch- derivatives like glucose syrup, maltodextrin and dextrin the macromolecule begins with a reducing sugar, a free aldehyde. Įquilibrium between cyclic and open-chain form in one ring of maltose However, sucrose and trehalose, in which the anomeric carbon atoms of the two units are linked together, are nonreducing disaccharides since neither of the rings is capable of opening. Many disaccharides, like cellobiose, lactose, and maltose, also have a reducing form, as one of the two units may have an open-chain form with an aldehyde group. ![]() This includes common monosaccharides like galactose, glucose, glyceraldehyde, fructose, ribose, and xylose. Similarly, most polysaccharides have only one reducing end.Īll monosaccharides are reducing sugars because they either have an aldehyde group (if they are aldoses) or can tautomerize in solution to form an aldehyde group (if they are ketoses). With one anomeric carbon unable to convert to the open-chain form, only the free anomeric carbon is available to reduce another compound, and it is called the reducing end of the disaccharide.Ī nonreducing disaccharide is that which has both anomeric carbons tied up in the glycosidic bond. Even a reducing disaccharide will only have one reducing end, as disaccharides are held together by glycosidic bonds, which consist of at least one anomeric carbon. Reducing end ĭisaccharides consist of two monosaccharides and may be either reducing or nonreducing. This type of isomerization is catalyzed by the base present in solutions which test for the presence of reducing sugars. Therefore, ketones like fructose are considered reducing sugars but it is the isomer containing an aldehyde group which is reducing since ketones cannot be oxidized without decomposition of the sugar. Sugars with ketone groups in their open chain form are capable of isomerizing via a series of tautomeric shifts to produce an aldehyde group in solution. The aldehyde can be oxidized via a redox reaction in which another compound is reduced. Monosaccharides which contain an aldehyde group are known as aldoses, and those with a ketone group are known as ketoses. Ī sugar is classified as a reducing sugar only if it has an open-chain form with an aldehyde group or a free hemiacetal group. Terminology Oxidation-reduction Ī reducing sugar is one that reduces another compound and is itself oxidized that is, the carbonyl carbon of the sugar is oxidized to a carboxyl group. Also, the levels of reducing sugars in wine, juice, and sugarcane are indicative of the quality of these food products. Reducing sugars react with amino acids in the Maillard reaction, a series of reactions that occurs while cooking food at high temperatures and that is important in determining the flavor of food. ![]() However, acetals, including those found in polysaccharide linkages, cannot easily become free aldehydes. The cyclic hemiacetal forms of aldoses can open to reveal an aldehyde, and certain ketoses can undergo tautomerization to become aldoses. The aldehyde functional group allows the sugar to act as a reducing agent, for example, in the Tollens' test or Benedict's test. Reducing disaccharides like lactose and maltose have only one of their two anomeric carbons involved in the glycosidic bond, while the other is free and can convert to an open-chain form with an aldehyde group. ![]() Nonreducing disaccharides like sucrose and trehalose have glycosidic bonds between their anomeric carbons and thus cannot convert to an open-chain form with an aldehyde group they are stuck in the cyclic form. The common dietary monosaccharides galactose, glucose and fructose are all reducing sugars.ĭisaccharides are formed from two monosaccharides and can be classified as either reducing or nonreducing. Ketoses must first tautomerize to aldoses before they can act as reducing sugars. The monosaccharides can be divided into two groups: the aldoses, which have an aldehyde group, and the ketoses, which have a ketone group. In such a reaction, the sugar becomes a carboxylic acid.Īll monosaccharides are reducing sugars, along with some disaccharides, some oligosaccharides, and some polysaccharides. In an alkaline solution, a reducing sugar forms some aldehyde or ketone, which allows it to act as a reducing agent, for example in Benedict's reagent. Reducing form of glucose (the aldehyde group is on the far right)Ī reducing sugar is any sugar that is capable of acting as a reducing agent.
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