Crystal formation

1. KS4 Crystal formation • Learning objective: • To understand the process of forming sugar crystals

2. Raw materials Raw sugar juice is a diluted form of sucrose (C12H22O11). Sucrose is formed by a reaction called condensation where glucose and fructose molecules are combined. C6H12O6 + C6H12O6 → C12H22O11 Sugar crystals Sugar pearls Caster sugar

3. Raw materials Raw materials used by sugar factories to create sugar crystals include: • Sugar beet • Calcium carbonate (calcium hydroxide and carbon dioxide) • Coal • Water • Natural gas

4. Processing of sugar beet Sugar beet is washed to remove dirt and stones. The beet is then sliced and soaked in warm water where the sugar diffuses out into the water creating a raw sugar juice. When the sugar is extracted from the beet, in both water and sucrose, hydrogen bonding is the main source of intermolecular attractions. However, there will also be instantaneous dipole-dipole attractions (van der Waals). The sugar juice is purified by adding lime and carbon dioxide. This creates calcium carbonate which precipitates, trapping impurities. The resultant thin juice is then filtered. The next stage of evaporation, increases the concentration of the juice to form a thick juice. The solids content is increased from 16% to 65%.

5. Crystallisation Crystallisation of the sugar is difficult to control. The thick juice is placed in pans which boil the juice under reduced pressure (vacuum) to lower the boiling point. This reduces the risk of caramelisation (a competing reaction). A solution which is about to crystallise is known as super saturated. The thick juice is seeded with tiny sugar crystals which provide the nucleus for larger crystals to grow at a lower super saturation. This reduces the problem of uncontrolled crystallisation.

6. Isolating the sugar crystals Once the crystals have reached the desired size, the process is stopped. The resultant solution is a mixture of crystal sugar and syrup, known as massecuite. This is spun in centrifuges to separate the sugar from the mother liquor. At 65% concentration, the thick juice is difficult to filter. Centrifuging the thick solution is more efficient in separating the sugar crystals from the syrup. Once the sugar crystals are washed, dried and cooled, they are conveyed to storage silos.

7. Making the most of it Once the sugar crystals have been removed the remaining juice is returned to the process to be spun again. The thick juice can be stored in large silos and be returned to the factory outside of the beet processing campaign to allow sugar to be produced throughout the year. This period of time is called the juice refining period and lasts about 10-12 weeks. Time is also set aside for the factory to be cleaned and maintenance work carried out.

8. Competing reactions A competing reaction is an unwanted reaction that can reduce the yield of sugar from the sugar beet. These can begin with storage of the beet at the farm and include: ● Enzyme degradation of the beet ● Fungal spores rotting the beet ● Caramelisation ● Bacterial activity creating invert sugar It is important for these to be minimised to increase the amount of sugar crystals produced at the sugar factory.