Sugar cane is milled to extract the juice. Sugar beets are sliced, soaked in hot water, which may contain sulfur dioxide, chlorine or ammonium bisulfite as a disinfectant, and pressed to extract the juice. Damaged beets may require the addition of an enzyme such as dextranase to aide in sugar extraction.
The cane or beet juice is then strained and clarified with heat and lime (calcium oxide or calcium hydroxide); small quantities of soluble phosphate may also be added. In sugar beet processing, the juice may be mixed with sulfur dioxide to prevent color formation and adjust pH. A heavy precipitate forms and is separated from the limed juice by gravity or centrifugation. The precipitate can be reprocessed to extract any remaining sugar.
In the beet sugar refining process, the mixture is passed through resin-filled columns to separate the sugar from the non-sugars. Using a chromatographic separation process, it is possible to recover up to 90% of the sugar in beet molasses.
Evaporation is the next step, which occurs in an evaporator station and vacuum pans. The syrup is then clarified again by adding lime, phosphoric acid, and a polymer flocculent, then aerated and filtered.
To crystallize the sugar, some mills seed the vacuum pans with isopropyl alcohol and ground sugar.
Once the sugar crystals are dried and cooled, the sugar is again refined by washing and clarification.
Two clarification methods are commonly used: pressure filtration and chemical treatment; chemical clarification is the preferred method. Two chemical methods are commonly used: phosphatation and carbonation; both processes require the addition of lime.
The next step is decolorization. The two most common color adsorbants are granular activated carbon and bone char, manufactured from degrease cattle bones. The decolorized sugar liquor moves through heaters, multiple effect evaporators, vacuum pans, and then ultimately is seeded to form crystals. Next, the crystals are washed in a centrifuge and then dried, screened, conditioned, and stored until packaging.
The refined cane or beet sugar may be further processed into invert sugar by dissolving it in water to make liquid sucrose, and then adding hydrochloric acid and sodium hydroxide, or enzymes to hydrolyze the bond between glucose and fructose.(2)