How To Manufacture Sucrose from Sugar Cane

Sucrose is ordinary table sugar. It occurs chiefly in sugar cane and sugar beets. In smaller amount it is present in maple sap, honey, and several fruits.

Manufacture of Sucrose ( Table Sugar )

In Pakistan and other tropical countries, the main source of sucrose is sugar cane. The modern method for the manufacture of ' Direct Consumption ' sugar from cane consists of the following steps.

• Juice Extraction 

The crushed cane is passed through a roller mill to squeeze out juice. The partially exhausted ' cane mat ' emerging from the mill is passed on to a tank, called Diffuser, by a chain conveyor . Here the maximum extraction of sucrose is done by washing with hot water and dilute juice on counter-current principle. This technique gets sugar extraction up to 98 per cent. The cellulosic material discharged from the diffuser is called Bagasse and is used as fuel under boilers.

• Juice Purification 

The raw juice contains 14-25 per cent sucrose and much impurity such as organic acids, inorganic salts, proteins, and colouring matter. It is purified by the operations listed below:

            1)Defecation: The juice is heated with high pressure steam and treated with 2-3 percent lime in a steel tank. This operation called defecation throws out organic acids as insoluble calcium salts , coagulated protein and colouring matter. The precipitate is removed by filtration.

           2)Carbonation: Through the filtered juice is then passed carbon dioxide. This operation known as carbonation, removes the excess, of lime as calcium carbonate which entraps colouring matter, colloidals, and some inorganic salts. The ' mud ' that settles is separated by filtration.

           3)Decolorisation: In Pakistan, the clarified juice is decolorised by treating with sulphur dioxide. This operation called sulphitation while it bleaches the brown colour of the juice, completes the neutralisation of lime. The insoluble calcium sulphite is removed by filtration.

The brown colour juice is also decolorised by percolation through beds of adsorbents, for instance, bone char, granular carbon, and ion exchange resins.

• Concentration and Crystallisation

The clear solution is then concentrated by boiling under reduced pressure in Multiple Effect Evaporation. In these, the steam produced in the first evaporator is used to boil the juice in the second maintained at a lower pressure.

The concentrated juice is finally passed to the Vacuum Pan where further evaporation reduces the water content to 6-8 per cent. Here partial separation of crystals takes place, The mixture of syrup and crystals, known as Massecuit, is then discharged into a large tank, the Crystallising Tank, fitted with cooling pipes. The crystals grow and form a thick crop.

• Separation of Crystals by Centrifugation, and Drying

The massecuite is then sent to centrifuges whereby sugar crystals are separated from the syrup. The crystals are here sprinkled with a little water to wash any syrup sticking to the surface. The wet sugar is dried by passing down a rotating drum with steam of hot air flowing counter-current to it. The residual mother liquor, from which the crystals have been removed, is called Molasses.