Sucrose: Preparation of Sucrose, Properties of Sucrose and Test for Sucrose

Sucrose, with a molecular formula C₁₂H₂₂O₁₁, is a disaccharide sugar (containing two sugar unit) whose molecule is made up of one glucose molecule in a glycosidic linkage with one molecule of fructose. It is the common household sugar in the form of cube in packets or granulated sugar in sachets. Though sugar cane is the major raw material for the production of sucrose, beetroot from the sugar beet plant contains 15 to 20% its dry mass as sucrose. Small wonder beet sugar export accounts for 30 to 35% ot total world sugar output

Although sucrose is found abundantly in sugar cane and sugar beet, there is a wide range of plant sources of sucrose. They include Sorghum, carrots and in tropical fruits like the pineapple fruit.

The story of sucrose goes beyond the household whitish crystalline substance we use to sweeten our tea, coffee, chocolate and pap; or to bake our bread and cake. It is associated with unique chemical attributes that brings us to the arena of its preparation and properties.

The process of preparation of sucrose is directly associated with the production of ethanol. This is because the by-product liquid of sucrose extraction called molasses contains 60% of sugar and as such fermented to ethanol.

The preparation of the sucrose proper, involves the crushing of the sugar cane or beetroot, followed by using warm water (at about 80°C) to dissolve out the sucrose content. This result to the formation of a solution rich in sucrose and other sugars. The solution is then made to undergo purification where slaked lime and carbon(IV) oxide are used to treat the solution.

Once purified, the solution is concentrated by evaporation and left to cool. The cooling process is actually a crystallization process where brown sucrose crystals separate out leaving behind the molasses. The brown sugar is further purified with slaked lime and carbon(IV) oxide, and decolorized to pure white crystals using animal charcoal.

Glucose is regarded as a non-reducing sugar because it does not contain anomeric hydroxyl group. Though glucose contain both α and β pyranose isomers, and fructose the α and β furanose isomers; it is only one of the isomers that are linked to form the sucrose molecule. This is as evident in α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside structure of sucrose. Another important point (which confer the non-reducing status of sucrose) is that, the reducing end of both the fructose and glucose units are used to form the glycosidic bond instead of one donating its reducing end, and the other its non-reducing end. This is how it discourages bonding to other carbohydrate units, thus maintaining its disaccharide status.

Like other carbohydrates, fructose also undergoes dehydration on heating or warming in the presence of concentrated tetraoxosulphate(VI) acid to form carbon residue with the evolution of water.

C₁₂H₂₂O₁₁ → 12C + 11H₂O

It must be stated that sucrose melts at a lower temperature of about 210°C to melt into a yellowish brown substance known as caramel used in flavouring and confectionery.

Sucrose also undergoes hydrolysis with dilute tetraoxosulphate(VI) acid and hydrochloric acid to form the component monosaccharides: glucose and fructose. This is the basis of the inversion reaction in ethanol production from sucrose where the yeast enzyme, invertase, acts as catalyst for the reaction.

Sucrose + Water ↔ Glucose + Fructose

Sucrose is Inert to Fehling test, though it is positive to the test only after it has been hydrolyzed to its component monosaccharide units.

The conclusive test for sucrose is called the Seliwanoff's test. Seliwanoff reagent contains concentrated hydrochloric acid and resorcinol. The HCl hydrolyses the sucrose to release its fructose and glucose units. Then, the alkanone (ketone) group of the fructose reacts with the resorcinol to form a deep cherry red colour.

The procedure involve adding 5.00cm³ of freshly prepared Seliwanoff's reagent to 1.00cm³ of the sucrose solution. The mixture is then warmed in a water bath until it turns read - usually after ten minutes.