How Sugar is produced from Sugarcane?

Most of we in our childhood might have wondered about the production of sugar from sugarcane. Moreover than production we wonder how fine crystals might have formed. Sugar (chemically sucrose) is produced mainly from sugarcane. Beside sugarcane, sugar beats and other chemical synthesis can be utilized for the manufacture.

Fig: Sugarcane

Sugarcane is traditionally refined into sugar in two stages. In the first stage, raw sugar is produced by the milling of freshly harvested sugarcane. In a sugar mill, sugarcane is washed, chopped, and shredded (tear into narrow pieces) by revolving knives. The shredded cane is mixed with water and crushed. The juices (containing 10-15 percent sucrose) are collected and mixed with lime to adjust pH to 7, prevent decay into glucose and fructose, and precipitate impurities. The lime and other suspended solids are settled out, and the clarified juice is concentrated in a multiple-effect evaporator to make a syrup with about 60 weight percent sucrose. 


What is multiple-effect evaporator?

A multiple-effect evaporator, invented by American Engineer Norbert Rillieux, is an apparatus for efficiently using the heat from steam to evaporate water. In a multiple-effect evaporator, water is boiled in a sequence of vessels, each held at a lower pressure than the last. Because the boiling temperature of water decreases as pressure decreases, the vapor boiled off in one vessel can be used to heat the next, and only the first vessel (at the highest pressure) requires an external source of heat.

The syrup is further concentrated under vacuum until it becomes supersaturated, and then seeded with crystalline sugar. Upon cooling, sugar crystallizes out of the syrup. Centrifuging then separates the sugar from the remaining liquid (molasses). Raw sugar has a yellow to brown color. Sometimes sugar is consumed locally at this stage, but usually undergoes further purification. Sulfur dioxide is bubbled through the cane juice subsequent to crystallization in a process, known as "sulfitation". This process inhibits color forming reactions and stabilizes the sugar juices to produce “mill white” or “plantation white” sugar.

The fibrous solids, called bagasse, remaining after the crushing of the shredded sugarcane, are burned for fuel, which helps a sugar mill to become self-sufficient in energy. Any excess bagasse can be used for animal feed, to produce paper, or burned to generate electricity for the local power grid.

Fig: Flow chart of refining of Sugar from Sugarcane (source: www.wikipedia.org)