How do they harvest sugar from the sugar cane plant? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.
How is cane harvested for sugar?
1. Cane is crushed through a series of rollers. Water is passed through the rollers in countercurrent fashion (i.e., virgin sugarcane is put in contact with highly concentrated sugar juice, while virgin water is put in contact with cane that has been washed multiple times) to maximize extraction.
2. The sugar juice is flocculated and treated with lime to remove particulates and precipitate salts.
3. Sugar juice is boiled and reduced in a series of multiple-effect evaporators to reduce water content and push up sucrose concentration to about 40–50%. This resulting mixture is known as thick juice.
4. The sugar juice is sent into a series of vacuum pans (three being typical), where the sugar juice is evaporated under vacuum after the addition of nucleation crystals. Each vacuum pan is referred to as a “strike.” In each strike, at the end of a cycle the resulting solution with suspended crystals, called a massecuite, is centrifuged to remove the sugar, which is further washed to remove residual solution. The remaining solution, called a molasses, is sent to the next strike. The first sugar is the highest quality and is used almost exclusively for white refined sugar. The second sugar is of lower quality and is usually blended into the first sugar, further bleached, sold as “raw sugar” or used in brown sugar. The third sugar is suitable usually only for nucleation or for brown sugar. The last molasses is referred to as blackstrap molasses.
5. (Optionally) Molasses is treated with sulfur dioxide for preservation. Unsulfured molasses is suitable for human consumption, but the majority is sulfured and fed to cattle.
6. (Optionally) Collected sugar is bleached and either granulated or formed into loaves.
There are various other side processes that are also essential to the proper functionality of a sugar mill. For example, bagasse (the woody cake left after cane is crushed and sugar extracted) is burned for power production and disposal. Precipitated salts are continuously removed from limed cane juice and usually returned to the fields. Finally, if the mill also produces ethanol, it is often advantageous to dilute either the second molasses or blackstrap molasses and ferment it. In the former case, it allows you to not have to deal with low-quality sugar, while in the latter case it allows you to continue fermenting year-round instead of only during the cane season because preserved molasses is not perishable.
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