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Refining sugar cane

After sugar cane is processed in a mill, it is transported to refineries in the form of raw sugar. The final product from a sugar mill isn’t the raw sugar we’re all familiar with on supermarket shelves, it is a non-food-grade raw sugar that requires further refining before it’s ready to be sold and consumed.

The raw sugar produced at the mills has gone through a refining process which removes sugar cane pulp and fibres, soil, etc., however it still requires further refining to remove impurities like molasses, plant matter, glucose, fructose, calcium, potassium salts, gums, amino acids and colours from the sugarcane.  

Sugar refineries make a range of different sugar products like white and raw table sugar, brown sugar, liquid sugar, golden syrup and treacle. Each product goes through a slightly different process before it’s packaged and transported to be stocked on supermarket shelves. 

What is the process of refining sugar?

Before raw sugar can be made into the sugar products we’re familiar with, it will go through a series of steps to remove impurities, water content and other organic matter. This process produces a clear, sweet liquid that is almost pure sucrose. The liquid then goes through a crystallisation stage to produce sugar crystals. 

The size of the sugar crystals will determine what type of product it is turned into (raw sugar, soft brown sugar, coffee crystals, caster sugar, etc.).

The major stages in the sugar refining process are

  • Affination
  • Purification
  • Decolourisation
  • Crystallisation

Affination

Affination is the first process the milled raw sugar will go through. During affination milled raw sugar is mixed with hot concentrated syrup to help soften the hard molasses coating on the raw sugar crystals. This molasses coating, which contains many initial impurities, is dissolved into the syrup and then separated from the sugar crystals using a centrifugal machine. The syrup still has recoverable sugars and is typically processed in another area of the refinery to make molasses. 

Once the raw sugar is washed or ‘affined’ it is then dissolved into hot water to form a sugar liquor. The liquor goes through a screening process to remove any fibrous materials or organic matter that may still remain after the affination process. 

Purification

The process for purifying the sugar liquor is called carbonatation. During carbonatation calcium hydroxide (also called milk of lime or limewater) and carbon dioxide gas is added to the sugar liquor. This creates a chemical reaction that causes organic impurities such as gums, amino acids and colour components to bond to a calcium carbonate precipitate. This precipitate is then removed from the liquid sugar mix. 

This precipitate, or leftover solid, (also called Mill mud, press mud, filter mud or carbonatation mud) is combined with other by-products to make soil conditioners, which are an important source of nutrients for sugar cane fields. 

Decolourisation

Once raw sugar goes through the affination and purification process it comes out as a honey coloured liquor known as ‘raw liquor’. To remove the remaining colour the liquor is passed over either bone char, granular carbon or ion exchange resin which absorbs the remaining colour. The liquor will be very clear at this stage.

Crystallisation 

Before being crystallised the liquor is passed through evaporators to remove excess water and also passes through a UV light to sterilise the solution. The final crystallisation stage converts the sucrose into a more usable format. This is an important part in the final refining process as it allows pure sucrose to crystallise out of the liquor, leaving most of the remaining impurities in the associated syrup. 

The process starts by pressurising the liquor in ‘vacuum pans’ to help reduce boiling temperature and avoid further formation of colour compounds. Seed crystals are then added to help with the spontaneous formation of fine sucrose crystals. Once seeded the final product goes through a centrifuge to separate sugar crystals from the remaining syrup. 

Once crystallised, the sugar is dried and graded by crystal size prior to packaging and storage. The remaining syrup is used as a starting material for other sugar products such as golden syrup, treacle and soft brown sugar.

For a diagram of refining process, see our booklet Where Does Sugar Come From?


REFERENCES

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