The Refining Process

Appears in
Oxford Companion to Sugar and Sweets

By Darra Goldstein

Published 2015

  • About
About half of the non-sucrose components, including color bodies, are contained in the thin molasses film around the raw sugar crystals. The affination process involves mingling the raw sugar with a syrup, and centrifuging off the liquid in high-speed batch centrifuges. This is an important step when processing lower-purity raw sugars, but it requires more equipment and energy. Therefore, this step is usually omitted when higher-purity raw sugars are processed.

The sugar is dissolved in water to give a solution of about 66 grams of sugar per 100 grams of liquor. The next step in the process involves clarification and filtration of this liquor. In most refineries, this is achieved by one of two processes:

  • Carbonatation, where lime is added to the liquor and a gas containing carbon dioxide (usually boiler flue gas) is sparged into the liquor in vessels called saturators to create a chalky precipitate of calcium carbonate, which occludes impurities as it is formed, facilitating the subsequent filtration process.

  • Phosphatation, where lime is added as well as phosphoric acid, forming a precipitate of calcium phosphate. A flocculant (a high molecular mass polyelectrolyte) is added that causes the precipitated material to agglomerate into clumps called flocs, capturing some of the non-sucrose components in the process. This flocculated material is removed by dissolved air flotation, and the scum removed is treated to recover sugar before being discarded.