When Sugars are called Reducing Sugars?

The sugar type that exhibits its property as a reducing agent as it consists of free ketone and aldehyde functional groups is termed reducing sugars. These sugars have the capability to reduce the cupric ions into cuprous ions of the benedict’s solution. It includes sugars such as maltose, fructose, galactose, and so on. 

Biomolecules       

Every organism that grows, reproduces, and sustains its own life, is composed of molecules and atoms that are non-living. These atoms are interlinked by a covalent bond to form a molecule. Proteins, nucleic acid, carbohydrates, and lipids are a few of the complex macromolecules that make up the living species. Specifically, carbohydrates and protein are significant constituents of our food products. The interaction between these kinds of essential biomolecules constitutes the molecular basis of the organisms. Certain other simple molecules including minerals and vitamins also play their part in the composition of the organisms. Every biomolecule is made mainly of hydrogen and carbon atoms along with phosphorus, nitrogen, oxygen, and sulfur.       

Carbohydrates      

The polyhydroxy ketone or aldehydes or the compounds which are capable of creating such kinds of units on hydrolysis are called carbohydrates. These are ultimately manufactured by plants and create a major category of organic compounds that are produced naturally. It is collectively termed saccharides.       

Glucose, sucrose, cane sugar, starch are few examples of this macromolecule. Based on the process of hydrolysis, the numbers of constituting units of sugar molecules get varied and are categorized as monosaccharides, oligosaccharides, and polysaccharides.

Reducing Sugar 

All monosaccharides sugars and almost most of the disaccharides belong to the category of reducing sugars. Reducing sugar is determined by the state of oxygen present on anomeric carbon (carbon 1). If it is not linked to any other type of molecule, then such a type of sugar is called reducing sugar. This is specifically obtained by donating electrons. Tollen’s reagent which is recognized as a weak oxidizing agent can be used to oxidize these reducing sugars. 

Glucose is considered to be one of the most significant reducing sugars and is available in our body as blood sugar. Monosaccharides such as fructose, glucose, and galactose belong to the category of reducing sugars.   

For determining the reducing properties of various types of sugars, we use Benedict’s test. With the help of this test, we can reduce the reactive component and which results in the oxidation of anomeric carbon. So, any type of sugar which consists of hemiacetal is said to be reducing sugar. For sugar like sucrose to exhibit its property as reducing sugar, at first sucrose should undergo the process of hydrolysis followed by decomposition for forming fructose and glucose. There are various functions of reducing sugar like adding color, flavor and improving shelf life. 

Examples  

• Glucose: It is the major source of energy and it is present in our body as blood sugar. It is considered to be a reducing sugar due to the presence of the free aldehyde group in its structure. In plants, glucose is stored as starch and in human bodies, it is stored as glycogen.  
• Lactose: Glucose and galactose are combined to form lactose. Lactose is referred to as a reducing agent due to the presence of sugar in it.  
• Maltose: When two molecules of glucose are combined, it forms maltose. It is a reducing sugar that specifically exists in cereals. Alcohols are formed when maltose undergoes fermentation.  
• Fructose: In all sugars, fructose is recognized as the sweetest sugar. Several fruits have the presence of fructose in them. It is a reducing sugar as it consists of a free ketone group. Fructose when combined with glucose, results in the formation of the sugar molecule called sucrose, but sucrose is a non-reducing sugar.  

Difference between Reducing and Non-Reducing Sugars 

• In the case of reducing sugar, the free aldehyde or ketone group is present whereas, in non-reducing sugar, the free ketone or aldehyde group is absent.  
• Reducing sugar is present in hemiketal or hemiacetal form whereas non-reducing sugar is present in ketal or acetal form.  
• Reducing sugar exhibits the property of mutarotation whereas non-reducing sugar cannot exhibit them.  
• In reducing sugars, osazones formation takes place. But in non-reducing sugars, it does not show any such type of formation.  
• Most of the reducing sugars belong to the category of monosaccharides whereas most of the non-reducing sugars are considered to be polysaccharides. 

Tests 

There are about 2 very significant tests that are often useful in determining the presence of reducing sugar. These tests are namely the Fehling test and the Benedict test.  

Benedict Test 

In this test, the sample of food in which the existence of reducing sugar has to be determined is subjected to dissolve in water. Once the food sample gets dissolved in water, about a little amount of Benedict's reagent should be added to the solution after which the particular solution starts to cool down. After a period of 10 minutes, the change in the solution’s color begins to occur. If the color changed resulted in blue, it confirms the absence of reducing sugar. On the other hand, if it results in colors such as yellow, red,  orange, green, and then finally to dark red color or brown, then it shows the presence of reducing sugar in the particular food sample. 

The Benedict solution used in this test is composed of an anhydrous solution of sodium carbonate, sodium citrate, and copper II sulfate pentahydrate. At the time of reaction with the reducing sugar, the blue-colored copper sulfate present in the solution gets turned into copper sulfide which is in the color of red-brown. This specific test is significant only as a qualitative analysis of reducing sugars. 

Fehling’s Test 

In this case, the solution sample should be warmed for the homogenous mixing of food and water in the solution sample. After that, a Fehling solution is added to it. In order to confirm reducing sugar’s presence, the solution gets turned into a red precipitate color which resembles rust.  

The composition of Fehling’s solution is composed of mixing equal quantities of potassium sodium tartrate tetrahydrate and aqueous solutions of copper (II) sulfate pentahydrate. This is specifically significant in determining the monosaccharides, especially aldoses and ketoses. This test is also useful in diagnosing diabetes mellitus.  

Tollens’ Test 

Another reagent that is frequently used to identify the presence of functional groups of aromatic aldehydes and aldehydes with the presence of certain alpha-hydroxy ketones that usually gets tautomerized into aldehydes are termed the tollen’s reagents. The test performed with this reagent is named the tollen’s test. The reagent is composed of an alkaline solution of ammonical silver nitrate. It must be noted that the reduction of the aldehyde leads to the formation of primary alcohols whereas the reduction of ketones results in secondary alcohols. 

Biological Importance of Reducing Sugars

Generally, carbohydrates, specifically reducing sugar are considered to be the most abundant organic molecules that can be present in nature. They have a broad range of functions and roles in the biological world. They are responsible for providing a considerable fraction of regularly used dietary calories in most of the living beings surviving on the earth. Also, their ultimate role is to act as the storage form of energy in the body of living organisms. Carbohydrates also tend to serve as one of the components of the cell membrane and they function mainly in mediating several intermolecular communications in the living organisms’ bodies.  

Context and Applications 

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry.