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Milling sugar cane

One of the most prominent natural sweeteners in the world is sugar derived from sugar cane. But to get this popular product from paddock to plate takes months of growing, weeks of harvesting and milling and then processing before it can end up as the sweet, crystallised product we all know as raw sugar.

Once sugar cane is harvested, it begins its journey to sugar mills, where it will be processed and refined into a range of different sugar products. Sugar cane needs to be milled and go through the refinery process as soon as possible after harvesting. Many sugar producers have invested heavily in cane railways and rolling stock to optimise the process and maximise yield.

Where are sugar mills located?

Sugar mills are generally located close to sugar cane growing regions. In Australia they are mainly located in Queensland, as this is where a majority of Australian sugar cane is grown. Australian sugar mills span a large proportion of the eastern seaboard. There are mills as far north as Mossman in far north Queensland right down to Grafton in northern New South Wales. 

There are a total of 24 sugar mills in Queensland and New South Wales, which together produce between 4 - 4.5 million tonnes of raw sugar each year.  Sugar mills employ over 4500 people, requiring a range of different workers from mechanics, boiler-makers and cane train drivers to chemists, electricians and administrative staff. 

There are no sugar mills located in New Zealand as sugar cane is not grown there. New Zealand does however have a sugar refinery to process bulk raw sugar imported from Australian sugar mills.

How is sugar cane harvested and transported to the mill?

Sugar cane is harvested annually during June and December. There are two different harvesting methods – harvesting sugar cane green or burning sugar cane before harvest. Many growers will harvest their sugar cane green, using leftover cuttings as a mulch called ‘trash blankets’ to reduce weed growth, soil erosion and water evaporation. Green harvesting also enables farmers to harvest in times of rain or heavy winds when burning is not desirable or possible.

In some cases, older machinery restrictions prevent farmers from harvesting sugar cane green, so fields are burnt before harvest instead. Burning removes any dead plant matter, leaving stalks available for harvest by older machinery. In some colder areas, green harvesting is also unfavourable as cane losses negate any yield advantages, and trash blankets can result in poor germination and slower growth. 

When harvesting green, harvesters move through sugar cane fields and mechanically separate sugar cane leaves and tops from the sugar-bearing stalk. The leaves and tops of the sugar cane are either left on the soil as a trash blanket or collected and utilised to produce a range of by-products like electricity and biofuels, cattle feed, disintegrant and even tree-free paper products or some medicines.  

Once sugar cane is mechanically harvested, it’s put into large containers called ‘bins’ which have a capacity of between 4 and 14 tonnes. These bins are hauled by cane trains to sugar mills to be milled and processed into raw sugar within. In Queensland, over 4,000kms of track and over 52,000 bins are used to transport cane from field to mill. Sugar mills are generally located in the same regions as the growing area so the sugar cane can be processed swiftly within 16 hours of harvesting. 

What happens to sugar cane at a mill?

Once the harvested sugar cane is transported to the mill, it’s time to turn it into raw sugar, ready for export, refining or storage. The sugar cane is weighed and then dumped onto a conveyor belt, taking it to the crushing plant. The sugar cane is shredded by large rollers and then processed again through a series of additional crushing mills, which will remove the majority of the cane juice.   

The juice is then put through a clarification process where impurities are removed. Following this is the evaporation process which removes any excess water. What remains is a sweet syrup which is boiled under a vacuum and seeded with smaller sugar crystals that enables more crystals to form. This mixture is then put through a centrifuge to separate the sugar crystals from the dark syrup and then dried to give the final raw sugar product. 

The raw sugar produced in sugar mills is not yet food grade. At this stage it is either transported to refineries for processing into the sugar products we know and eat (like icing sugar, molasses, white sugar and brown sugar), or it is exported to overseas refineries for similar processing. 

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

What are some of the by-products produced by sugar cane milling?

Raw sugar is the most extensively produced product from sugar cane, however there are wide and varied additional uses for the plant, ranging from fuel sources and clothing through to pharmaceuticals and paper production. 

In an effort to maximise the usage of the sugar cane crop, every part of the plant is used throughout the production process. Nothing is wasted in an effort to maximise this crop's versatility and usefulness throughout the entire production process. Below are just some of the products that are derived from the sugar cane plant. 

Bagasse
Bagasse is a coarse, fibrous material that’s left once the sugar cane plant has been crushed to extract the sweet fluid inside its stalks. Many sugar mills in Australia use this by-product as a renewable energy source to power the mills and associated production facilities. Bagasse is also used in the production of plastics, poultry litter, Xylitol and activated carbon. 

Molasses
Molasses is a thick, viscous liquid made by boiling down cane sugar syrup and extracting the sugar crystals. Depending on how long the sugar cane syrup is boiled, and the amount of sugar crystals extracted from it, molasses can be turned into distinct product types. These include treacle (also called golden syrup), dark treacle, Blackstrap molasses, fertiliser and animal feed. Molasses is also one of the main ingredients in brown sugar. 

Filter Cake
Filter cake is the residue that’s leftover from the sugar cane juice filtration process. It’s a good source of phosphorous, organic matter and moisture. Filter cake is often used as a fertiliser or a structural soil improver to help counteract some of the soil degradation that occurs during the growing and harvesting of the sugar cane plant.


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