The Processing and Chemistry of Sugarcane


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Sugarcane requires physical and chemical processing. Image by Xenia.

Sugarcane (genus Saccharum), like the bamboo which it slightly resembles, is a type of grass! Unlike bamboo, however, stalks of sugarcane are solid. The canes also require processing before we can use the sugary contents as a sweetener.

Processing this crop is primarily a physical process, but there’s quite a lot of chemistry behind the changes that sugar cane juice undergoes before it makes it to our sugar bowl.

Sugar Cane: Extracting and Cleaning the Juice

Sugarcane processors chop the stalks and press the sections using using large rollers to release the juice. They then clean the juice with calcium hydroxide a.k.a. slaked lime – before evaporating it to concentrate the sweetness.

We derive slaked lime, Ca(OH)2, from quick lime, calcium oxide or CaO, by adding water. That reaction sequence is:

CaO + H2O → Ca(OH)2

At this point, they must make the acidic juice alkaline.

Heating and Breaking Down Sugar Cane Juice

The processors heat the juice to 75 degrees C (167 degrees F), which, combined with the slaked lime, decomposes the enzymes that would break down the sugar.

Invertase hydrolyzes sucrose into glucose and fructose. Note the the chemical structures of sucrose, glucose and fructose, as well as the schema for the reaction path produced by the presence of invertase. Image compilation by Vincent Summers, all rights reserved.

The predominant enzyme is invertase, a large molecule that splits the disaccharide, sucrose or table sugar, into two monosaccharides, glucose and fructose.

Although these two are also sugars, they are not desired for use as table sugar, so preventing that reaction is necessary to keep the sugar in the proper form.

Another positive effect produced by the  slaked lime is that it clarifies the juice.

Sugar Cane Juice: Evaporation and Boiling

The clarified juice is now of suitable purity for further processing, but is dilute. Before processors boil and cool the juice to produce crystallization, they must evaporate much of the water out.

Boiling greatly increases the concentration of sugar – they can dissolve much sugar in boiling water, but as the solution cools, sugar crystallizes out. At this stage, the sugar is brown due to the presence of traces of the mother liquor (molasses). At this point, the brown sugar crystals are shipped to refineries.

Sugar Processing By-Products

After processing removes most of the sucrose from the enriched liquid or syrup, what remains behind is molasses. Molasses contains some mineral content not found in the refined, crystallized sugar, including iron, calcium, potassium and magnesium – especially the form of molasses with the most sugar removed from it; “blackstrap” molasses. Molasses is used in the production of alcoholic spirits, especially rum.

The solids that remain after the juice has been pressed from the stalks – called bagasse – are often used as fuel at the processing site. Bagasse is also used in the manufacture of certain paper products and as an ingredient in building materials.

Chemistry of Sugar, Brown Sugar, and other Sweeteners

It’s convenient to have sweeteners like erythritol and chemically-created sugar-substitutes such as sucralose, but the chemical makeup of simple table sugar is as fascinating as the result is sweet.

Need something to put in your coffee? Chemistry’s got you covered.

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