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Reducing sugar

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**Definition of Reducing Sugars**:
– A reducing sugar is capable of acting as a reducing agent.
– In an alkaline solution, a reducing sugar forms aldehyde or ketone.
– Monosaccharides are reducing sugars, along with some disaccharides and polysaccharides.
– Disaccharides can be reducing or nonreducing based on glycosidic bonds.
– Acetals in polysaccharides cannot easily become free aldehydes.
– A sugar is classified as reducing if it has an aldehyde or free hemiacetal group.
– Aldoses have an aldehyde group, while ketoses have a ketone group.
– Ketoses can tautomerize to form an aldehyde group in solution.
– Base catalyzes the isomerization of ketoses to aldoses.

**Structure and Characteristics of Reducing Sugars**:
– Disaccharides may be reducing or nonreducing.
– Disaccharides have one reducing end due to glycosidic bonds.
– Most polysaccharides have one reducing end.
– Nonreducing disaccharides have both anomeric carbons tied up.
– Disaccharides are held together by glycosidic bonds.

**Examples and Detection of Reducing Sugars**:
– Monosaccharides like galactose, glucose, and fructose are reducing sugars.
– Disaccharides like lactose and maltose have a reducing form.
Sucrose and trehalose are nonreducing disaccharides.
Glucose polymers like starch contain reducing sugars.
– Glycogen is a reducing sugar with one reducing end.
– Qualitative tests detect reducing sugars using copper(II) ions.
– Tollens reagent precipitates silver metal in the presence of aldehydes.
– 3,5-dinitrosalicylic acid allows quantitative detection of reducing sugars.
– Non-reducing sugars can be hydrolyzed to become reducing sugars.
– Molisch’s test responds positively to all carbohydrates.

**Medical Importance of Reducing Sugars**:
– Fehlings solution was used as a diagnostic test for diabetes.
– Measuring the reduction of Fehlings solution by glucose helps determine glucose concentration.
– Proper insulin dosage can be determined based on blood glucose levels.

**Food Chemistry and the Maillard Reaction**:
– The Maillard reaction involves reducing sugars reacting with amino acids.
– Maillard reaction products can be beneficial or toxic.
– The Maillard reaction decreases the nutritional value of food.

Reducing sugar (Wikipedia)

A reducing sugar is any sugar that is capable of acting as a reducing agent. 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. In such a reaction, the sugar becomes a carboxylic acid.

Reducing form of glucose (the aldehyde group is on the far right)

All monosaccharides are reducing sugars, along with some disaccharides, some oligosaccharides, and some 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 cannot 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, while the other is free and 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, cannot 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.

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