Reducing sugar - wikipedia Reducing sugar Reducing form of glucose ( the aldehyde group is on the far right ) A reducing sugar is any sugar that is capable of acting as a reducing agent because it has a free aldehyde group or a free ketone group. All monosaccharides are reducing sugars, along with some disaccharides, oligosaccharides, and polysaccharides. The monosaccharides can be divided into two groups : the aldoses, which have an aldehyde group, and the ketoses, which have a ketone group. Ketoses must first tautomerize to aldoses before they can act as reducing sugars. The common dietary monosaccharides galactose, glucose and fructose are all reducing sugars. Disaccharides are formed from two monosaccharides and can be classified as either reducing or nonreducing. Nonreducing disaccharides like sucrose and trehalose have glycosidic bonds between their anomeric carbons and thus can not convert to an open - chain form with an aldehyde group ; they are stuck in the cyclic form. Reducing disaccharides like lactose and maltose have only one of their two anomeric carbons involved in the glycosidic bond, meaning that they can convert to an open - chain form with an aldehyde group. The aldehyde functional group allows the sugar to act as a reducing agent, for example in the Tollens ' test or Benedict 's test. The cyclic hemiacetal forms of aldoses can open to reveal an aldehyde and certain ketoses can undergo tautomerization to become aldoses. However, acetals, including those found in polysaccharide linkages, can not easily become free aldehydes. 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. Also, the levels of reducing sugars in wine, juice, and sugarcane are indicative of the quality of these food products. Contents 1 Terminology 1.1 Oxidation - reduction 1.2 Aldoses and ketoses 1.3 Reducing end 2 Examples 3 Characterization 4 Importance in medicine 5 Importance in food chemistry 5.1 Maillard reaction 5.2 Food quality 6 References Terminology Oxidation - reduction A 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. Aldoses and ketoses A sugar is classified as a reducing sugar only if it has an open - chain form with an aldehyde group or a free hemiacetal group. Monosaccharides which contain an aldehyde group are known as aldoses, and those with a ketone group are known as ketoses. The aldehyde can be oxidized via a redox reaction in which another compound is reduced. Thus, a reducing sugar is one that reduces certain chemicals. 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. Therefore, ketone - bearing sugars like fructose are considered reducing sugars but it is the isomer containing an aldehyde group which is reducing since ketones can not be oxidized without decomposition of the sugar. This type of isomerization is catalyzed by the base present in solutions which test for the presence of aldehydes. Aldoses or aldehyde - bearing sugars are reducing also because during oxidation of aldoses, there are certain oxidizing agents that are reduced. Reducing end The reducing end of a disaccharide is the monosaccharide with a free anomeric carbon that is not involved in a glycosidic bond and is thus capable of converting to the open - chain form. Recall that disaccharides consist of two monosaccharides and may be either reducing or nonreducing. However, 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. With one anomeric carbon unable to convert to the open - chain form, only the free anomeric carbon is available to reduce another compound. A nonreducing disaccharide has both anomeric carbons tied up in the glycosidic bond. Similarly, reducing polysaccharides have only one reducing end, assuming that there is no branching. Examples All monosaccharides are reducing sugars because all monosaccharides have an aldehyde group ( if they are aldoses ) or can tautomerize in solution to form an aldehyde group ( if they are ketoses ). This includes common monosaccharides like galactose, glucose, glyceraldehyde, fructose, ribose, and xylose. 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. However, sucrose and trehalose, in which the anomeric carbons of the two units are linked together, are nonreducing disaccharides since neither of the rings is capable of opening. Equilibrium between cyclic and open - chain form in one ring of maltose 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. More hydrolyzed starch contains more reducing sugars. The percentage of reducing sugars present in these starch derivatives is called dextrose equivalent ( DE ). Glycogen is a highly branched polymer of glucose that serves as the main form of carbohydrate storage in animals. It is a reducing sugar with only one reducing end ; no matter how large the glycogen molecule is or how many branches it has, each branch ends in a nonreducing sugar residue. When glycogen is broken down to be used as an energy source, glucose units are removed one at a time from the nonreducing ends by enzymes. Characterization Several qualitative tests are used to detect the presence of reducing sugars. Two of them use solutions of copper ( II ) ions : Benedict 's reagent ( Cu in aqueous sodium citrate ) and Fehling 's solution ( Cu in aqueous sodium tartrate ). The reducing sugar reduces the copper ( II ) ions in these test solutions to copper ( I ), which then forms a brick red copper ( I ) oxide precipitate. Reducing sugars can also be detected with the addition of Tollen 's reagent, which consist of silver ions ( Ag ) in aqueous ammonia. When Tollen 's reagent is added to an aldehyde, it precipitates silver metal, often forming a silver mirror on clean glassware. 3, 5 - Dinitrosalicylic acid is another test reagent, one that allows quantitative detection. It reacts with a reducing sugar to form 3 - amino - 5 - nitrosalicylic acid, which can be measured by spectrophotometry to determine the amount of reducing sugar that was present. Sugars having acetal or ketal linkages are not reducing sugars, as they do not have free aldehyde chains. They therefore do not react with any of the reducing - sugar test solutions. However, a non-reducing sugar can be hydrolyzed using dilute hydrochloric acid. After hydrolysis and neutralization of the acid, the product may be a reducing sugar that gives normal reactions with the test solutions. All carbohydrates respond positively to Molisch 's reagent but the test has a faster rate when it comes to monosaccharides. Importance in medicine Fehling 's solution was used for many years as a diagnostic test for diabetes, a disease in which blood glucose levels are dangerously elevated by a failure to produce enough insulin ( Type 1 diabetes ) or by an inability to respond to insulin ( Type 2 diabetes ). By measuring the amount of oxidizing agent ( in this case, Fehling 's solution ) reduced by glucose, it is possible to determine the concentration of glucose in the blood or in the urine. This then enables the right amount of insulin to be injected to bring blood glucose levels back into the normal range. Importance in food Chemistry Maillard reaction Main article : Maillard reaction The carbonyl groups of reducing sugars react with the amino groups of amino acids in the Maillard reaction, a complex series of reactions that occurs when cooking food. Maillard reaction products ( MRPs ) are diverse ; some are beneficial to human health, while others are toxic. However, the overall effect of the Maillard reaction is to decrease the nutritional value of food. One example of a toxic product of the Mailard reaction is acrylamide, a neurotoxin and possible carcinogen that is formed from free asparagine and reducing sugars when cooking starchy foods at high temperatures ( above 120 ° C ). Food quality The level of reducing sugars in wine, juice, and sugarcane are indicative of the quality of these food products, and monitoring the levels of reducing sugars during food production has improved market quality. The conventional method for doing so is the Lane - Eynon method, which involves titrating the reducing sugar with copper ( II ) in Fehling 's solution in the presence of methylene blue, a common redox indicator. However, it is inaccurate, expensive, and sensitive to impurities. References Jump up ^ Pratt, Charlotte W. ; Cornely, Kathleen ( 2013 ). Essential Biochemistry ( Third ed. ). Wiley. p. 626. ISBN 978 - 1118083505. ^ Jump up to : Nelson, David L. ; Cox, Michael M. ( 2008 ). Lehnniger : Principles of Biochemistry ( Fifth ed. ). W.H. Freeman and Company. p. 241. ISBN 978 - 0716771081. ^ Jump up to : Campbell, Mary K. ; Farrell, Shawn O. ( 2012 ). Biochemistry. Cengage Learning. p. 459. ISBN 978 - 0840068583. Jump up ^ Nelson, David L. ; Cox, Michael M. ( 2008 ). Lehnniger : Principles of Biochemistry ( Fifth ed. ). W.H. Freeman and Company. p. 243. ISBN 978 - 0716771081. ^ Jump up to : Davidson, Eugene A. ( 2015 ). `` Carbohydrate ''. Encyclopædia Britannica. access - date = requires url = ( help ) Jump up ^ Klein, David. ( 2012 ). Organic Chemistry ( First ed. ). John Wiley & Sons. p. 1162 - 1165. ISBN 978 - 0471756149. ^ Jump up to : Klein, David. ( 2012 ). Organic Chemistry ( First ed. ). John Wiley & Sons. p. 1159. ISBN 978 - 0471756149. Jump up ^ Leung, David W.M. ; Thorpe, Trevor A. ( April 1984 ). `` Interference by edta and calcium ions of the 3, 5 - dinitrosalicylate reducing sugar assay ''. Phytochemistry. Pergamon Press. 23 ( 12 ) : 2949 -- 2950. ISSN 0031 - 9422. doi : 10.1016 / 0031 - 9422 ( 84 ) 83048 - 4. Jump up ^ Dills, William L. Jr. ( November 1993 ). `` Protein fructosylation : fructose and the Maillard reaction ''. The American Journal of Clinical Nutrition. American Society for Nutrition. 58 ( 5 ) : 779S -- 87. ISSN 0002 - 9165. Jump up ^ Jiang, Zhanmei ; Wang, Lizhe ; Wu, Wei ; Wang, Yu ( June 2013 ). `` Biological activities and physicochemical properties of Maillard reaction products in sugar -- bovine casein peptide model systems ''. Food Chemistry. Elsevier. 141 ( 4 ) : 3837 -- 3845. ISSN 0308 - 8146. doi : 10.1016 / j. foodchem. 2013.06. 041. Jump up ^ Pedreschi, Franco ; Mariotti, María Salomé ; Granby, Kit ( August 2013 ). `` Current issues in dietary acrylamide : formation, mitigation and risk assessment ''. Journal of the Science of Food and Agriculture. Society of Chemical Industry. 94 ( 1 ) : 9 -- 20. ISSN 0022 - 5142. doi : 10.1002 / jsfa. 6349. Jump up ^ Leotério, Dilmo M.S. ; Silva, Paulo ; Souza, Gustavo ; Alves, Aline de A. ; Belian, Mônica ; Galembeck, André ; Lavorante, André F. ( November 2015 ). `` Copper -- 4, 4 ′ - dipyridyl coordination compound as solid reagent for spectrophotometric determination of reducing sugar employing a multicommutation approach ''. Food Control. European Federation of Food Science and Technology ; International Union of Food Science and Technology. 57 : 225 -- 231. ISSN 0956 - 7135. doi : 10.1016 / j. foodcont. 2015.04. 017. Types of carbohydrates General Aldose Ketose Furanose Pyranose Geometry Anomer Cyclohexane conformation Mutarotation Monosaccharides Dioses Aldodiose Glycolaldehyde Trioses Aldotriose Glyceraldehyde Ketotriose Dihydroxyacetone Tetroses Aldotetroses Erythrose Threose Ketotetrose Erythrulose Pentoses Aldopentoses Arabinose Lyxose Ribose Xylose Ketopentoses Ribulose Xylulose Deoxy sugars Deoxyribose Hexoses Aldohexoses Allose Altrose Galactose Glucose Gulose Idose Mannose Talose Ketohexoses Fructose Psicose Sorbose Tagatose Deoxy sugars Fucose Fuculose Rhamnose Heptoses Ketoheptoses Mannoheptulose Sedoheptulose Above 7 Octoses Nonoses Neuraminic acid Multiple Disaccharides Cellobiose Isomaltose Lactose Lactulose Maltose Sucrose Trehalose Turanose Trisaccharides Maltotriose Melezitose Raffinose Tetrasaccharides Stachyose Other oligosaccharides Acarbose Fructooligosaccharide ( FOS ) Galactooligosaccharide ( GOS ) Isomaltooligosaccharide ( IMO ) Maltodextrin Mannan - oligosaccharides ( MOS ) Polysaccharides Beta - glucan Oat beta - glucan Lentinan Sizofiran Zymosan Cellulose Chitin Chitosan Dextrin / Dextran Fructose / Fructan Inulin Galactose / Galactan Glucose / Glucan Glycogen Hemicellulose Levan beta 2 → 6 Lignin Mannan Pectin Starch Amylopectin Amylose Xanthan gum Retrieved from `` https://en.wikipedia.org/w/index.php?title=Reducing_sugar&oldid=751386712 '' Categories : Carbohydrate chemistry Biomolecules Carbohydrates Hidden categories : Pages using citations with accessdate and no URL Talk Contents About Wikipedia Bosanski Català Čeština Deutsch Español Français Bahasa Indonesia Italiano עברית Basa Jawa 日本 語 Polski Português Suomi தமிழ் Türkçe 中文 Edit links This page was last edited on 25 November 2016, at 09 : 29. 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Question: why all monosaccharides are reducing sugar but not all disaccharides
Answer: 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