A reducing sugar is any sugar

Sugar

Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

 that either has an aldehyde

Aldehyde

An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

 group or is capable of forming one in solution through isomerisation

Isomerisation

In chemistry isomerisation is the process by which one molecule is transformed into another molecule which has exactly the same atoms, but the atoms are rearranged e.g. A-B-C → B-A-C . In some molecules and under some conditions, isomerisation occurs spontaneously...

. This functional group allows the sugar to act as a reducing agent

Reducing agent

A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...

, for example in the Tollens' test

Tollens' reagent

Tollens' reagent is a chemical reagent most commonly used to determine whether a known carbonyl-containing compound is an aldehyde or a ketone. It is usually ammoniacal silver nitrate, but can also be other mixtures, as long as aqueous diamminesilver complex is present...

 or Benedict's test

Benedict's reagent

Benedict's reagent is a chemical reagent named after an American chemist, Stanley Rossiter Benedict....

.

Chemistry

A sugar is classified as a reducing sugar only if it has an open-chain form with an aldehyde group. 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

Ketone

In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

 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

Fructose

Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...

 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. This type of isomerization is catalyzed by the base present in solutions which test for the presence of aldehydes. Monosaccharide

Monosaccharide

Monosaccharides are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose...

s which contain an aldehyde group are known as aldoses, and those with a ketone group are known as ketoses.

Examples

Reducing monosaccharides include glucose

Glucose

Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

, fructose

Fructose

Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...

, glyceraldehyde

Glyceraldehyde

Glyceraldehyde is a triose monosaccharide with chemical formula C3H6O3. It is the simplest of all common aldoses. It is a sweet, colorless, crystalline solid that is an intermediate compound in carbohydrate metabolism...

 and galactose

Galactose

Galactose , sometimes abbreviated Gal, is a type of sugar that is less sweet than glucose. It is a C-4 epimer of glucose....

.
Many disaccharide

Disaccharide

A disaccharide or biose is the carbohydrate formed when two monosaccharides undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharides, disaccharides form an aqueous solution when dissolved in water...

s, like lactose

Lactose

Lactose is a disaccharide sugar that is found most notably in milk and is formed from galactose and glucose. Lactose makes up around 2~8% of milk , although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from or , the Latin word for milk,...

 and maltose

Maltose

Maltose , or malt sugar, is a disaccharide formed from two units of glucose joined with an αbond, formed from a condensation reaction. The isomer "isomaltose" has two glucose molecules linked through an α bond. Maltose is the second member of an important biochemical series of glucose chains....

, also have a reducing form, as one of the two units may have an open-chain form with an aldehyde group. However, sucrose

Sucrose

Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...

 and trehalose

Trehalose

Trehalose, also known as mycose or tremalose, is a natural alpha-linked disaccharide formed by an α,α-1,1-glucoside bond between two α-glucose units. In 1832, H.A.L. Wiggers discovered trehalose in an ergot of rye, and in 1859 Marcellin Berthelot isolated it from trehala manna, a substance made...

, in which the anomeric carbons of the two units are linked together, are non-reducing disaccharides since neither of the rings is capable of opening.
In glucose polymer

Polymer

A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s such as starch

Starch

Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...

 and starch-derivatives

Derivative (chemistry)

In chemistry, a derivative is a compound that is derived from a similar compound by some chemical or physical process. In the past it was also used to mean a compound that can be imagined to arise from another compound, if one atom is replaced with another atom or group of atoms, but modern...

 like glucose syrup

Corn syrup

Corn syrup is a food syrup, which is made from the starch of maize and contains varying amounts of maltose and higher oligosaccharides, depending on the grade. Corn syrup is used in foods to soften texture, add volume, prevent crystallization of sugar, and enhance flavor...

, maltodextrin

Maltodextrin

Maltodextrin is a polysaccharide that is used as a food additive. It is produced from starch by partial hydrolysis and is usually found as a creamy-white hygroscopic spraydried powder. Maltodextrin is easily digestible, being absorbed as rapidly as glucose, and might be either moderately sweet or...

 and dextrin

Dextrin

Dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch or glycogen. Dextrins are mixtures of polymers of D-glucose units linked by α- or α- glycosidic bonds....

 the macromolecule

Macromolecule

A macromolecule is a very large molecule commonly created by some form of polymerization. In biochemistry, the term is applied to the four conventional biopolymers , as well as non-polymeric molecules with large molecular mass such as macrocycles...

 begins with a reducing sugar, a free aldehyde. More hydrolysed starch contains more reducing sugars. The percentage of reducing sugars present in these starch derivatives is called dextrose equivalent

Dextrose equivalent

Dextrose equivalent is a measure of the amount of reducing sugars present in a sugar product, relative to glucose, expressed as a percentage on a dry basis. For example, a maltodextrin with a DE of 10 would have 10% of the reducing power of dextrose , while sucrose, with a DE of 120, would have...

 (DE).

Detection

Benedict's reagent

Benedict's reagent

Benedict's reagent is a chemical reagent named after an American chemist, Stanley Rossiter Benedict....

 and Fehling's solution

Fehling's solution

Fehling's solution is a chemical test used to differentiate between water-soluble aldehyde and ketone functional groups, and as a test for monosaccharides. The test was developed by German chemist Hermann von Fehling in 1849.-Laboratory preparation:...

 are used to test for the presence of a reducing sugar. The reducing sugar reduces copper(II)

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 ions in these test solutions to copper(I), which then forms a brick red copper(I) oxide

Copper(I) oxide

Copper oxide or cuprous oxide is the inorganic compound with the formula Cu2O. It is one of the principal oxides of copper. This red-coloured solid is a component of some antifouling paints. The compound can appear either yellow or red, depending on the size of the particles, but both forms...

 precipitate. 3,5-Dinitrosalicylic acid

3,5-Dinitrosalicylic acid

3,5-Dinitrosalicylic acid is an aromatic compound that reacts with reducing sugars and other reducing molecules to form 3-amino-5-nitrosalicylic acid, which absorbs light strongly at 540 nm...

 is another test reagent that allows quantitative spectrophotometric measurement of the amount of reducing sugar 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 hydrolysed using dilute hydrochloric acid

Hydrochloric acid

Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric 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

Molisch's test

Molisch's Test is a sensitive chemical test for the presence of carbohydrates, based on the dehydration of the carbohydrate by sulfuric acid to produce an aldehyde, which condenses with two molecules of phenol Molisch's Test (named after Austrian botanist Hans Molisch) is a sensitive chemical test...

.