Reducing Sugars- Definition, Characteristics, Examples, Uses


Sugars are carbohydrates that provide energy and serve as building blocks for other molecules in living organisms. Sugars can be classified into different types based on their structure and chemical properties. One such type is reducing sugars, which are sugars that can act as reducing agents in chemical reactions.

A reducing agent is a substance that can donate electrons to another substance, while being oxidized itself. In other words, a reducing agent can reduce the oxidation state of another substance by giving up some of its own electrons. A reducing sugar is a sugar that has this ability to reduce other substances by donating electrons.

How does a sugar become a reducing agent? The key factor is the presence of an aldehyde (-CHO) or a ketone (-CO) group in its molecular structure. An aldehyde group is a carbon atom double-bonded to an oxygen atom and single-bonded to a hydrogen atom. A ketone group is a carbon atom double-bonded to an oxygen atom and single-bonded to two other carbon atoms. These groups can form aldehyde or ketone compounds in water or alkaline solution, which allow them to react with weak oxidizing agents .

Not all sugars are reducing sugars. Some sugars have their aldehyde or ketone groups involved in glycosidic bonds, which are covalent bonds that link two sugar units together. These bonds prevent the sugars from forming aldehyde or ketone compounds in solution, and thus prevent them from acting as reducing agents. These sugars are called non-reducing sugars.

Reducing sugars can be found in all categories of carbohydrates: monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Monosaccharides are the simplest sugars, consisting of one sugar unit. Disaccharides are composed of two sugar units linked by a glycosidic bond. Oligosaccharides are made of a few (3-10) sugar units linked by glycosidic bonds. Polysaccharides are long chains of many sugar units linked by glycosidic bonds.

All monosaccharides are reducing sugars, because they have free aldehyde or ketone groups. Examples of monosaccharides are glucose, fructose, galactose, ribose, etc. Some disaccharides are reducing sugars, while some are non-reducing sugars. For example, lactose and maltose are reducing disaccharides, because they have one free aldehyde group in one of their sugar units. Sucrose and trehalose are non-reducing disaccharides, because they have both their aldehyde groups involved in glycosidic bonds. Some oligosaccharides and polysaccharides are reducing sugars, while some are non-reducing sugars. For example, starch and glycogen are reducing polysaccharides, because they have one free aldehyde group at one end of their chains. Cellulose and chitin are non-reducing polysaccharides, because they have no free aldehyde groups.

Reducing sugars have many important roles and applications in biology, chemistry, food science, and medicine. They participate in various biochemical reactions, such as glycolysis, fermentation, and the Maillard reaction . They also serve as indicators of the quality and freshness of food products, such as wine, juice, and sugarcane. They can be detected by various tests, such as Benedict`s test and Fehling`s test.

In this article, we will explore the characteristics, examples, tests, and uses of reducing sugars in more detail. We will also compare them with non-reducing sugars and learn how to distinguish them from each other.