The process of milling sugar begins after the sugar cane is harvested and is done as a way to to extract the juice from the cane. After harvest, sugar has to be milled as soon as possible to increase the freshness of the final products. In mills today, “sugar cane is weighed and processed before being transported to a shredder. The shredder breaks apart the cane and ruptures the juice cells.
Rolling mills are usually used in small and large scale sectors and made of cast iron or steel placed in frames, Smaller crushers are powered by animals, but could also be ran by a motor or small engine. Obviously, the outputs of the two of these vary; the animal drawn by a single animal yields about 50kg per hour, while a 5HP diesel yields about 300kg per hour. Also, the speed at which this mill makes a difference in the amount produced. The average operating speed is from 5 to 50 r.p.m.
Rollers are used to separate sugar juice from the fibrous material, called bagasse. The bagasse is recycled as a fuel for the mill boiler furnaces.” (SugarAustralia). The juice resulting from the milling process is then put through a purification process. After the juice becomes concentrated by being boiled down in an evaporator and turned into syrup, it is seeded with crystals in a process called crystallization. If the crystals aren’t uniform in size, more of the syrup is added throughout the boiling process. The syrup is further separated from the raw crystals by centrifuges. Molasses forms as a result of left over syrup left over in the centrifuge. Raw sugar that does separate from the syrup in dried and transferred to storage in the mills. (1)
The sugar milling process has not always been this easy and advanced. The earliest reference to milling sugar cane was made in the Ganges Valley in Southeastern Asia in the year 100 AD. The original mill that was used was the pestle and mortar by the Indians. Innovations have been made pretty frequently to streamline the practice of milling sugar cane but most were speculations based on artist’s drawings throughout the centuries. “An early eighteenth-century comment, “This comment (milling) presses out the juice and the English do no more to the canes; but the Spaniards have a press to squeeze out the remainder of the liquor after both the former grindings. Their works are small, but they are willing to make the most of them.” (SugarAustralia). In 1795, millers tried unsuccessfully to implement a hydraulic press in St. Vincent, but a better attempt came in 1851 by Sir Henry Bessemer. Although it wasn’t successful in milling, the technology was tweaked to be able to use it for bailing the bagasse, or fibrous material in the sugar cane. (2)
In 1858, Jeremiah Howard was given a patent for the design featuring a free moving top roller that had a hydraulic accumulator applying pressure, the design most commonly used in today’s sugar mills. The next few designs used this as a basis but changed a few elements to further improve the milling process. The next design in 1871, patented by Duncan Stewart, powers the lower rollers by hydraulic pressure from an accumulator supported by the brasses of the rollers. These designs are responsible for why mills work the way they do today.
Having identical extraction as the roll mill, the screw expellers are rarely used despite the fact they are more efficient than rolling mills.
Open pan boiling is also an option for processing sugar and is done so via furnaces. Basic furnaces are built in the ground with a block wall surrounding the top to produce a flat surface that will support a pan. A chimney has been included over time to carry the smoke away from the area the sugar is being processed. The shapes of the pans often decipher rather or not the sugar is heated evenly. Round pans have been found to be the most evenly heated shaped pan.
The original type of furnace is called the ‘standard bel’ furnace, which is used for burning bagasse in a chamber that is under a sequence or pans. When burning wet bagasse, the ‘shell’ furnace is used.
(1) “The Milling Process.” Sugar Australia. Sugar Australia, 2004. Web. 16 Apr. 2012.
(2) Deerr, Noël. The History of Sugar. London: Chapman and Hall, 1949. Print.