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From Wikipedia

Reducing sugar

A reducing sugar is any sugar that, in a solution, has an aldehyde or a ketone group. This allows the sugar to act as a reducing agent, for example in the Maillard reaction and Benedict's reaction.


A sugar is only a reducing sugar if it has an open chain with an aldehyde or a ketone group. Monosaccharides which contain an aldehyde group are known as aldoses, and those with a ketone group are known as ketoses.

Many sugars occur in a chain as well as in a ring structure and in solution it is possible to have an equilibrium between these two forms.

The aldehyde can be oxidized via a redox reaction. The chemical that causes this oxidation becomes reduced. Thus, a reducing sugar is one that reduces certain chemicals. Even though a ketone cannot be oxidized directly, a keto sugar may be converted to an aldehyde via a series of tautomeric shifts to migrate the carbonyl to the end of the chain. Therefore, keto sugars are also reducing.


Reducing monosaccharides include glucose, fructose, glyceraldehyde and galactose. Many disaccharides, like lactose and maltose also do have a reducing form, as one of the two units may have an open-chain with an aldehyde group. However, sucrose and trehalose, in which the anomeric carbons of the two units are linked together, are non-reducing disaccharides.

In glucose polymers as starch and starch-derivatives like glucose syrup, maltodextrin and dextrin the macromolecule begins with a reducing sugar, a free aldehyde. More hydrolysed starch contains more reducing sugars. The percentage reducing sugars present in these starch derivatives is called dextrose equivalent (DE).


Benedict's reagent and Fehling's solution are used to test for the presence of a reducing sugar. The reducing sugar reduces copper(II) ions in these test solutions to copper(I), which then forms a brick red copper(I) oxide precipitate. 3,5-Dinitrosalicylic acid 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. 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.

From Yahoo Answers

Question:For example, sucrose is a non-reducing sugar & maltose is a reducing sugar. Please explain.

Answers:Reducing Sugars : Sugars that contain aldehyde groups that are oxidised to carboxylic acids are classified as reducing sugars. Common test reagents are : Benedicts reagent (CuSO4 / citrate) Fehlings reagent (CuSO4 / tartrate) They are classified as reducing sugars since they reduce the Cu2+ to Cu+ which forms as a red precipitate, copper (I) oxide. Remember that aldehydes (and hence aldoses) are readily oxidised (review ?) In order for oxidation to occur, the cyclic form must first ring-open to give the reactive aldehyde. So any sugar that contains a hemi-acetal will be a reducing sugar. But glycosides which are acetals are not reducing sugars. A reducing sugar is any sugar that, in basic solution, forms some aldehyde or ketone. This allows the sugar to act as a reducing agent, for example in the Maillard reaction and Benedict's reaction. Reducing sugars include glucose, glyceraldehyde, lactose, arabinose and maltose. All monosaccharides which contain ketone groups are known as ketoses, and those which contain aldehyde groups are known as aldoses. Significantly, sucrose is not a reducing sugar. It is in fact known as a non-reducing sugar. Benedict's reagent is used to determine if a reducing sugar is present. If it is a reducing sugar, the mixture will turn green/orange/red. Fehling's solution can also be used for the same purpose, as both contain copper (II) ions, which are reduced to a brick red precipitate of copper (I) oxide when the solution is heated. A reducing sugar occurs when its anomeric carbon is free. Since sugars occur in a chain as well as a ring structure, it is possible to have an equilibrium between these two forms. When the hemi-acetal or ketal hydroxylgroup is free, it is not locked, not linked to another (sugar)molecule, the aldehyde (or keto-) form (i.e. the chain-form) is available for reducing copper (II) ions. When a sugar is oxidized its carbonyl group (i.e. aldehyde or ketone group) is converted to a carboxyl group. Non REDUCING SUGARS : Fructose, glucose and galactose are all hexoses. However, whereas glucose and galactose are aldoses (reducing sugars), fructose is a ketose (a non-reducing sugar). It also has a five-atom ring rather than a six-atom ring. Fructose reacts with glucose to make the dissacharide sucrose. the chemical difference between reducing and non-reducing sugars Reducing sugars (monosaccharides, e.g. glucose and some disaccharides, e.g. lactose and maltose) act as reductants when heated with a weakly alkaline solution of copper (II) sulfate (e.g. Benedict s solution) to form an orange-brown precipitate of copper (I) oxide. The Cu2+ is reduced to Cu+, which reacts with OH- to give Cu2O. 2Cu2+(aq) + 2e- + H2O(l) Cu2O(s) + 2H+(aq) Aldehyde (-CHO) group in a reducing sugar can be the source of electrons that reduces copper (II) to copper (I) R-CHO (aq) + H2O(l) R-COOH(aq) + 2H+(aq) + 2e- Benedict test of glucose and sucrose, University of Manitoba, Winnipeg, Manitoba, Canada. Fehling s solution (strongly alkaline copper sulfate) and Tollen s reagent (silver nitrate) can also be used. Negative (1) and positive(2) Fehling test , Sciences de la vie et de la terre, Didier,(Sciences of life and the earth) Paris, France. The reaction only occurs if the carbohydrate exists in equilibrium between a ring and open-chain form. The open-chain form contains CHO and CO-CH2OH functional groups, which are easily oxidised to carboxylic acids R-COOH and R-CHOH-COOH respectively. Since the carbohydrates are oxidised the reagent must have been reduced, therefore the carbohydrates are reducing agents. Thus the sugars that cause a reaction are called reducing sugars.

Question:What are the properties of amylose (alpha-amylose) that make it a reducing sugar? WHAT DOES IT REDUCE? HOW? I understand the presence of alpha(1-4) glycosidic bond and that it is a linear (unbranched) polymer of glycogen. BUT, how does it function as a reducing sugar? I don't understand and I don't get the reducing vs non-reducing ends? THANKS SOO MUCH FOR YOUR TIME AND EFFORT :) Naomi

Answers:A reducing sugar is one that will form an aldehyde or a ketone in solution. Most sugars are reducing sugars: glucose, fructose, galactose, etc. Sucrose is NOT a reducing sugar. Reducing sugars can act as reducing agents in reactions. That makes them also easy to test for by testing reducing power with Benedict's reagent:sodium carbonate, sodium citrate and copper (II) sulfate. The reducing sugar will change blue copper (II) ions to red copper (I) ions which will precipitate. The color change indicates the presence of a reducing agent. This is of course when it is a monosaccharide. It will not act as a reducing agent while it is polymerized as in glycogen. A reducing end (ketone or aldehyde) is reactive and will be oxidized (donates electrons) by other molecules.

Question:This is for A level Science homework. I've searched around on the internet but cannot seem to find a good and clear answer. I need to know... 1. How is reducing sugar different to non-reducing sugar? 2. Why will non-reducing sugar not work with Benedict's Reagent? (for testing, it has to be heated with NaOH or KOH beforehand) Any help would be greatly appreciated! :)

Answers:A reducing sugar contains a functional group which can donate electrons to reduce whatever accepts the electrons. Remember reduction is electron gain. In the Benedict's test electrons from the reducing sugar are donated to the Cu++ (copper sulphate - CuSO4) in the Benedict's solution which are reduced to Cu+ = red copper oxide which is the colour you see if the test is positive for a reducing sugar. Example is glucose which has a CHO (aldehyde group) which donates electrons (Glucose is an aldose). Non - reducing sugars do not have an available reducing group to react and donate electrons. Eg Sucrose is a disaccharide of glucose linked to fructose by a glycosidic bond and has no reducing group free. After ACID hydrolysis and subsequent neutralisation of the acid ( Benedict's only works in alkaline conditions) the reducing groups are now available on the glucose and so it will now react witrh Benedict's.

Question:Why would there be a higher concentration of reducing sugars in spring compared to that of winter? Im so confused.

Answers:The starch which is the polysaccharide storage material will be hydrolysed by amylases to produce reducing sugars (glucose) which are needed for respiration to produce energy for the new spring growth.

From Youtube

Reducing Sugar :Dextrose is a reducing sugar which can change silver ion into silver metal.

Reducing and Non-reducing Sugars :This webcast takes a look at notation for disaccharides and the difference between reducing and non-reducing sugars.