What is Reducing Sugar?

Reducing sugars are those that can oxidize others compounds by donating electrons, whereas they themselves get reduced are called as Reducing Sugar. A reducing sugar will contain an aldehyde group called -CHO- or a ketone -CO- group. These reducing sugars have free carbon at the end, thus called by the name reducing ends.

These sugar are placed into the carbohydrate group; thus, monosaccharides, disaccharide, oligosaccharide and polysaccharide are all the reducing sugars.

Aldoses and ketoses are two types of sugar present in the monosaccharide group. Aldoses possess the aldehyde group, whereas the ketoses have the ketone group.

Tautomerization is the process where a compounds isomers are converted to form tautomer. This is called as tautomerization. Ketoses tautomerizes aldoses. Ketose example are fructose and aldose examples are glucose and galactose.

Most of food contain sugar which is in the form of glucose. Lactose, sucrose, maltose are all disaccharide, polysaccharide examples are starch, chitin, cellulose and glycogen. However, sucrose is a disaccharide but not a reducing sugar, as it has glycosidic bonds which prevents it from getting converted to aldehyde, but lactose and maltose are reducing sugars as they can undergo the conversion.

Reducing Sugar vs non-Reducing Sugar

Sugars without an aldehyde or ketone group are called as non- reducing sugar. Sugars that donate electrons to other molecule and itself get reduced, while oxidizing the other is called reducing sugar, and they themselves never get oxidized. Example of a non-reducing sugar is sucrose as it has glycosidic bonds which prevents it from getting converted to aldehyde, thus falling into the non-reducing group.

Characteristics of Reducing Sugar

According to the reducing sugar definition, a sugar is said to be reducing if it donates electron and oxidizes the other. Thus, calcium also loses an electron in an oxidation- reduction reaction in a chemical or biological interaction. The characteristics of reducing sugar are:

a) Free aldehyde and ketone are present in the reducing sugars.

b) A hemiacetal structure is formed in reducing sugar, where carbon get linked to few oxygen molecule forming alcohol or ether.

c) Reducing sugars might produce aldehyde group compounds, in aqueous solution.

d) Reducing sugar usually don’t get oxidized, however they can be with the help of mild oxidizers such as metal salts.

e) Osazones is formed by the reducing sugars, which produce mutarotation.

f) The process in which reducing sugar and amines interact with each other resulting the food to change its color i.e., browning of food. This is called as Maillard reaction. This reaction can be seen if the food stays at room temperature or is heated for a longer duration. This browning is also observed in breads, chocolates, coffee, cakes and various processed and baked foods, which is visible on the surface or the sides of the food.

Oxidation-Reduction Process

Reduction can be defined as the gain of electrons, whereas loss of electrons in a reaction is called as oxidation. Oxidation and reduction are called as redox reaction and its example are browning of fruits, metals corrosion, photosynthesis, respiration are few examples. There is oxygen, hydrogen and electron transfer in a redox reaction. The examples are below:

Electron Transfer: Zn + Cu⁺² → Zn⁺² + Cu

Oxygen atom transfer: C + 2H₂O → CO₂ + 2H₂

Hydrogen atom transfer: N₂H₄ + O₂ → N₂ + 2H₂O

Reducing Sugar Test

There are two test which are performed and are:

i. Benedict's Test

The samples to be tested are mixed with water and the Benedicts reagent is added. This process is exothermic, so the solution is allowed to cool down and after a period of 10 minutes, color change is seen. The color change to red, yellow, orange or to dark red or brown, it indicates reducing sugars are present. If the color changes to blue then reducing sugar is absent.

The benedict solution, comprises of copper sulphate pentahydrate and sodium citrate anhydrous. If the reducing sugar is present it will react with copper sulphate to form copper sulfide which is brown in color. However, it is a qualitative test.

ii. Fehling Test

The sample to be tested is mixed with water and the solution is heated and then mixed with the sample. The presence of reducing sugar can be indicated by a red color precipitate. Fehling’s solution is made up of copper sulphate and potassium sodium tartrate tetrahydrate. This test is done in laboratory to check whether the person has diabetes mellitus which will be indicated by presence of reducing sugar in the urine. Only monosaccharide can be identified by Fehling test.

Another test to determine the presence of aldehyde and ketone is tollens test. It comprises of silver nitrate, where primary alcohol is formed during reduction of aldehyde and secondary alcohol is formed by reduction of alcohol.

Reducing Sugar Examples

A monosaccharide reducing sugar is glucose, which is known as blood sugar in the humans. It has various roles, such as provides energy and helps in functioning of brain.

Another monosaccharide reducing sugar is fructose, which is the sweetest sugar. Other reducing sugars are maltose, glyceraldehyde and arabinose. Another reducing sugars are galactose and lactose, where lactose is seen in dairy products.

Biological Importance of Reducing Sugar

Reducing sugars, such as carbohydrates are found throughout the world in various forms. These have various functions such as storage of energy, providing glucose to the brain and various other functions.

They are also found in the cell membrane and carries out various interaction in living body. They also determine the taste and color of food in Maillard reaction.

Reducing Sugar Uses

They are of great importance in various fields such as in the medicine field, they determine with the help of Fehling’s test whether the person is diabetic or not.

It also helps to determine the sugar concentration in blood or urine of a human body, thus helping in determining if the blood glucose levels are higher, then what amount of insulin is to be taken.

In products such as wine, juices, sugarcane and others it helps to determine the amount of reducing sugar present.