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Production of beet sugar

White beet sugar is made from the beets in a single process, rather than the two steps involved with cane sugar.

Harvesting

The beets are harvested in the autumn and early winter by digging them out of the ground. They are usually transported to the factory by large trucks because the transport distances involved are greater than in the cane industry. This is a direct result of sugar beet being a rotational crop which requires nearly 4 times the land area of the equivalent cane crop which is grown in mono-culture. Because the beets have come from the ground they are much dirtier than sugar cane and have to be thoroughly washed and separated from any remaining beet leaves, stones and other trash material before processing.

Extraction

The processing starts by slicing the beets into thin chips. This process increases the surface area of the beet to make it easier to extract the sugar. The extraction takes place in a diffuser where the beet is kept in contact with hot water for about an hour. Diffusion is similar to the process by which the colour and flavour of tea comes out of the tea leaves in a teapot, but a typical sugar-beet-diffuser weighs several hundred tons when full of beet and extraction water. The diffuser is a large horizontal or vertical agitated tank in which the beets slices slowly work their way from one end to the other and the water is moved in the opposite direction. This is called counter-current flow and as the water goes it becomes a stronger and stronger sugar solution usually called juice. Of course it also collects a lot of other substances from the flesh of the sugar beet.

A typical raw juice from diffusion will contain perhaps 14% sugar and the residual pulp will contain 1 to 2% and a total of 8 to 12% solids.

Pressing

The exhausted beet slices from the diffuser are still very wet and the water in them still holds some useful sugar. They are therefore pressed in screw presses to squeeze as much juice as possible out of them. This juice is used as part of the water in the diffuser and the pressed beet, by now a pulp, is sent to drying plant where it is turned into pellets which form an important constituent of some animal feeds.

Carbonatation

The next stage of processing the sugar-liquor is aimed at removing the solids which make the liquor turbid. Coincidentally some of the colour is removed too. One of the two common processing techniques is known as carbonatation. Carbonatation is achieved by adding milk of lime [calcium hydroxide, Ca(OH)₂] to the liquor and bubbling carbon dioxide through the mixture. The gas reacts with the lime to form fine crystalline particles of calcium carbonate which occlude the solids. To obtain a stable flocculation, conditions of the reaction are carefully controlled. The clumps, as they form, collect a lot of the non-sugar substances so that by filtering out the chalk one also takes out these non-sugar substances. Once this is done, the sugar liquor is now ready for decolourisation

Unlike cane sugar production, the alternative process (phosphatation) is not used. Similarly, a separate decolourisation step is normally not used.

Boiling

For this last stage, the syrup is placed into a very large pan, typically holding 60 tons or more of sugar syrup. In the pan even more water is boiled off until conditions are right for sugar crystals to grow. Some sugar dust is added to the liquor to initiate crystal formation. Once the crystals have grown the resulting mixture of crystals and mother liquor is spun in centrifuges to separate the two, rather like washing is spin dried. The crystals are then given a final dry with hot air before being packed and/or stored ready for despatch.

As in the cane processing, the mother liquor still contains valuable sugar so the crystallisation is repeated several times. However non-sugars inhibit the crystallisation. This is particularly true of other sugars such as glucose and fructose which are the breakdown products of sucrose. Each subsequent step therefore becomes more difficult until one reaches a point where it is no longer viable to continue. This is usually after three steps.

Product

The final sugar is white and ready for use, whether in the kitchen or by an industrial user such as a soft drink manufacturer. As for raw sugar production, because one cannot get all the sugar out of the juice, there is a sweet by-product made: beet molasses. This is usually turned into cattle food or is sent to a fermentation plant such as a distillery where alcohol is made. It does not have the same quality smell and taste as cane molasses so cannot be used for rum production.

Sources :

• www.sucrose.com