Crystallisation :
The mixture of syrup and crystals, technically called massecuite, is placed in the crystalliser, which is a horizontal agitating tank equipped with cooling coils. Here additional sucrose deposits on the crystals already formed and crystallisation is completed and the massecuite is centrifuged to remove the syrup.
The resultant crystals are high-grade raw sugar, and the syrup is re-treated to obtain one or tow further sets of crystals. The final liquid, after re-working, is called Black Strap Molasses, which is used for a number of purposes in the confectionery industry, one of which is in the manufacture of licorice.
The light brown raw sugar containing approximately 96 per cent sucrose is bagged and shipped to the refineries.
Separation of Crystals :
The granular mass, still highly fluid, flows into mixers fitted with beaters to prevent the cooling mass solidifying or forming large colonies of crystals. Thence it passes into centrifuges which separate the crystals from the syrup, which is washed away by high-pressure steam.
The washed crystals are transferred to inclined granulators fitted with narrow baffle plates which keep the sugar in constant motion in a current of hot air. The dried sugar passes out of the granulators on to canvas conveyors which carry it to the screening machines over magnetised drums for the extraction of metal. The sieved sugar is conveyed to the automatic packing and weighing machines.
All sugar rejected at each stage is redissolved and reprocessed, and the yield of refined sugar obtained from raw sugar of 96 ْ polarisation is approximately 93-94 per cent.
The extraction of sugar from sugar beet, which contains 13-17 per cent sugar, entails a considerable number of processes which differ in many respects from those used for cane treatment.
Control of Crystallisation :
The control of the tendency of sugar in supersaturated solutions to recrystallise depends on the presence of some invert sugar to coat the sugar crystals in their minute from with a film to prevent a cohesion.
This may be done by adding an acid substance such as cream of tartar ( potassium acid tartrate ), which will chemically change or invert part of the sugar to form non-crystals invert sugars and so prevent the reforming of sugar crystals in a batch of confectionery.
This method, however, is very much more difficult and less stable than that in which glucose syrup is used, since it is not easy to assess the changes which take place owing to the varying conditions of heating in the factory. Glucose syrup is very much more tractable, and therefore is more widely used.
Return to Raw Sugar