Sugar refining

Article index

		Beet sugar manufacture
		Cane sugar manufacture

Sugar (sucrose) is produced from two principal sources, sugar beet and sugar cane. Sugar beet tends to be grown in Europe and sugar cane in hotter climates such as the West Indies. Sucrose is also available in a form called “liquid sugar”; this has been refined but with the crystallisation step omitted. Sugar crystals may also be ground to make powdered or confectioner’s sugars; anti-caking agents such as maizestarch are added to keep these sugars free flowing.

Beet sugar manufacture

Sugar beet processing started in the beginning of 19th century. (The Zuckerfabrik Waghausel/Baden of Suedzucker A.G., founded in 1837, is the oldest sugar factory in the world that is still in operation!) The crop is delivered to the factory and the beets are washed and sorted to remove extraneous matter such as stones and dirt. Nowadays the delivery is organised by a traffic control system (just in time) so that almost no beet is stored on the farm. The beets are sliced into thin strips known as “cossettes”. Sugar is extracted from the cossettes in a “diffuser” which uses water introduced countercurrently at around 70°C.

The extracted sugar solution is heated with milk of lime and carbon dioxide. This produces a precipitate which carries out with it impurities such as gums, waxes and resins.

After clarification, the solution is evaporated in multiple effect evaporators to produce a syrup of around 50-65% solids known as “thick juice”. The juice is filtered, transferred to vacuum pans and seeded with very fine sugar crystals to initiate crystallisation of the sugar in the juice.

The sugar is centrifugally separated from the juice, washed and then dried to yield the final product. The syrup is re-cycled into the vacuum pans for further crystallisation of the sugar. The spent syrup is termed molasses.

The beet pulp generated from the extraction process is dewatered. Molasses may be added before the mixture is dried and pelletised for animal feed.

Cane sugar manufacture

Raw cane sugar is usually separated, purified and crystallised in the country of origin. The cane sugar, which is in a crudely refined state, undergoes final processing when transferred to the country of use.

The raw sugar is typically a minimum of 96% sucrose. The impure crystals, with adhering molasses, are “affined” in a saturated sugar solution to soften the surface molasses film which can then be removed by centrifugation. The affined sugar is dissolved in reclaimed liquors (“light waters” from the refining process). Carbonation (treatment with milk of lime and carbon dioxide) follows. This removes suspended impurities such as waxes, gums and starches. The sugar syrup is filtered and decolourised using ion exchange resins and activated carbon to produce “fine” liquor, which may be sold as a finished product or passed on for crystallisation in vacuum pans. The fine liquor is concentrated by evaporation. When the liquor is slightly supersaturated, the pan is “seeded” with fine icing sugar. The mixture is centrifugally separated to extract crystalline sugar, which is dried, conditioned for packaging or bulk loaded. Each pan boiling yields around 50% of the available sugar. The liquor separated during centrifugation (“jets”) is re-boiled for further extraction. Three boilings yield white sugar. A fourth boiling yields “off white” industrial sugar. Jet four together with liquor from affination goes to a recovery house for 3 further boilings to produce brown sugars which go back to the start of the process and are treated as raw sugar. When it is no longer economically feasible to extract further sugar, the spent liquor is known as “molasses”. Various intermediary products from jets one to four and the corresponding syrups from recovery and boiling are sold as the starting materials for syrups such as golden syrup and treacle. Molasses are used for animal feed, fermentation and a number of non-food uses.

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