Sugar processing happens in two stages. A cane mill crushes the cane, cleans and boils the juice, and spins out raw sugar, a brown crystal still coated in molasses. A refinery then washes, dissolves, clarifies, decolourises and recrystallises the raw sugar into white sugar. Beet sugar is made white in a single factory, without the raw sugar stage.
What is the sugar production process at a cane mill?
Cane starts to lose sugar once it is cut, so mills stand in the cane fields and run only during the harvest. The steps are the same everywhere.
Milling
Rotating knives and shredders break the cane open. The prepared cane then passes through a tandem of four to six roller mills, which squeeze out the juice. Hot water is added at the last mill to wash out more sugar. Some factories use a diffuser, in which juice is percolated through the shredded cane 12 to 15 times. The fibre left over is bagasse. It is burned in the mill's boilers to raise steam, which drives the turbines and heats the evaporators and pans, so a cane mill supplies most of its own energy.
Clarification
Raw juice is acidic, with a pH of about 4.0 to 4.5, and carries soil, fibre, waxes and proteins. Milk of lime (calcium hydroxide) is added to raise the pH to about 7 or 8. This stops the sucrose breaking down and makes the impurities clump together. The juice is heated, and the solids settle in a clarifier. Clear juice is drawn off the top. The mud is filtered to recover the juice in it.
Evaporation
Clear juice is mostly water. It is concentrated in a multiple-effect evaporator, a row of vessels in which the vapour from one heats the next under falling pressure. The result is a syrup of 60 to 65° Brix, the scale for dissolved solids by weight.
Crystallisation
The syrup is boiled further in a vacuum pan. The vacuum lets it boil at a low temperature, which protects the sugar. When the syrup is supersaturated, fine seed crystals are added, and more syrup is fed in as the crystals grow. The pan ends up full of a thick mix of crystals and syrup called massecuite.
Centrifuging
The massecuite is spun in perforated baskets at 1,000 to 1,800 revolutions per minute. The syrup is thrown out through the holes and the crystals stay behind. The syrup is boiled and spun again, usually three times in all, giving A, B and C sugars of falling purity. The syrup left after the last boiling is final molasses, also called blackstrap. It still contains about one third sucrose that cannot be crystallised economically, and it is sold for cattle feed, alcohol and yeast. See molasses.
The crystal that leaves the centrifuges is raw sugar. It is stored and shipped in bulk.
How is raw sugar refined to white sugar?
Refineries are usually at the ports of importing countries and run all year on raw sugar from several origins.
- Affination. The raw sugar is mixed with a warm, saturated syrup to soften the film of molasses on the crystals, then spun in centrifuges and washed with hot water. Most of the colour is in that film, so this step removes a large part of it.
- Melting. The washed crystals are dissolved in hot water to give melt liquor.
- Clarification. One of two methods is used, both starting with milk of lime. In carbonatation, carbon dioxide is bubbled through the limed liquor and forms a precipitate of calcium carbonate that traps impurities. In phosphatation, phosphoric acid is added and the liquor is limed to a pH of 7.0 to 8.0; a floc of calcium phosphate rises and is skimmed off. In both cases the liquor is then filtered.
- Decolourisation. The clear liquor passes through columns of bone char or granular activated carbon, or through ion-exchange resin, which adsorb the remaining colour.
- Crystallisation. The decolourised liquor is evaporated and boiled in vacuum pans with seed crystals, as at the mill, and spun in centrifuges. One batch of liquor typically gives several strikes of white sugar.
- Drying. The crystals are tumbled in a stream of warm air in rotating drums, which takes the moisture from about 1% to 0.02% or less. The sugar is then screened and packed.
Mill white sugar
A mill can make a white sugar directly, without a refinery. The juice is treated with sulphur dioxide and lime (sulphitation), or with lime and carbon dioxide, before it is boiled. The product is called plantation white or mill white sugar. It is less white and less pure than refined sugar and its colour tends to rise in storage. See mill white sugar.
Beet sugar: the sugar factory process in one stage
Sugar beet is grown in temperate climates and typically holds between 12% and 21% sugar. The factory process is shorter than cane's.
The beets are washed and sliced into thin strips called cossettes. Hot water draws the sugar out of the cossettes in a diffuser, leaving raw juice and beet pulp, which is dried for animal feed. The juice is purified with lime and carbon dioxide and filtered to give thin juice. It is evaporated to thick juice of roughly 60% sucrose and crystallised in vacuum pans. Because the purification is thorough, the first crystal is already white. There is no raw beet sugar in trade. Refined beet sugar and refined cane sugar are both sucrose and meet the same white sugar specification.
What does ICUMSA colour measure?
ICUMSA is the International Commission for Uniform Methods of Sugar Analysis, a standards body founded in 1897 that publishes the laboratory methods used in sugar contracts. Colour is one of them. A sugar solution is filtered and its absorbance of light is measured at a wavelength of 420 nanometres in a spectrophotometer. The result is expressed in ICUMSA units (IU). The lower the number, the whiter the sugar.
The number is a measure of colour only. Purity is measured separately, by polarisation, in degrees Z. A specification needs both, with moisture and ash.
The main trade grades
| Grade | What it is | Reference limits |
|---|---|---|
| Raw cane sugar | Mill output for refining | ICE Sugar No. 11 futures: raw centrifugal cane sugar, basis 96 degrees average polarisation |
| VHP (very high polarisation) | High-purity raw sugar, the main export type from Brazil | Set by contract; colour commonly quoted as ICUMSA 600 to 1,200 |
| ICUMSA 100 to 150 | Mill white or crystal sugar | Codex plantation or mill white sugar: polarisation 99.5 °Z min, colour 150 IU max |
| ICUMSA 45 | Refined white sugar | ICE White Sugar futures: polarisation 99.8 degrees min, moisture 0.06% max, colour 45 IU max |
The Codex Alimentarius standard for sugars also defines a general white sugar grade with polarisation of not less than 99.7 °Z and colour of not more than 60 IU. VHP has no international standard, so the polarisation and colour limits must be written into the contract. Product pages: raw cane sugar VHP and ICUMSA 45 sugar. Our ICUMSA 45 buyer's guide covers the refined grade in detail.
How sugar is traded
Raw sugar ships in bulk, FOB the origin port, to refineries. White sugar ships mostly in 50 kg bags, in containers or break-bulk. Contracts state the grade, the ICUMSA colour and polarisation limits, the crop year, the Incoterm and the shipment period.
CBIL brokers sugar between a named mill, refinery or seller and a named buyer. Colour, polarisation, moisture and weight are certified by an independent inspector such as SGS or Intertek at loading.
Request a sugar quote with the grade, quantity, packing and destination port.