What is sucrose?
Sucrose is one of the oldest sweetening agents and the most used caloric sweetener, both for home and commercial use.
Also known as common table sugar, brown sugar, liquid sugar, sugar, table sugar, refined sugar, or white sugar, it consists of one unit of glucose plus one of fructose linked together by α-(1→2) glycosidic bond.
Together with starch and lactose, it is one of the three most common carbohydrates taken up with diet.
It is synthesized by plants through photosynthesis using as main components water, carbon dioxide (CO2) and solar energy, energy that is stored in the chemical bonds of the molecule. So, sucrose is virtually present in all plants, but its extraction is economically viable only from:
- Sugarcane (Saccharum officinarum); about 75% of world production comes from sugarcane, 146 million tons on 2004.
- Sugar beets (Beta vulgaris ssp. vulgaris var. altissima), especially in Europe; its sucrose content, thanks to continuous selections in the last two centuries, passed from 4.5% to 16%-18%.
The extraction, except for the initial steps, is very similar from both sources.
The history of sucrose
Sucrose apparently was discovered in India in VI century A.D. as a result of boiling cane juice (in ancient times honey was the most popular sweetener); Arabs introduced it in Europe.
In 1747, Andreas Sigismund Marggraf, a german chemist, discovers sucrose in sugarbeets and in 1802 Franz Achard, one of his students, set the first industrial process for its extraction. Then, the same process will be fitted to sugarcane.
Extraction from sugarcane
Sugarcane, once cut, must be rapidly processed, because of it deteriorates very quickly.
The first step of processing, after harvesting, is milling and crushing for juice extraction, thanks to water as well; the crushed cane exiting the last mill, called bagasse, is dried and used as a fuel source. The next step is juice filtering. To avoid that sucrose present in the filtrate is cleaved into fructose and glucose by organic acids present, in a process called “inversion” (see invert sugar), calcium hydroxide (Ca(OH)2) also called slaked lime or milk of lime is added.
The “neutralized” juice is heated up to 95°C (200°F) and clarified by the action of calcium hydroxide, that is, the precipitation (incomplete) of impurities and residues (glucose, fructose, fiber, pectins, inorganic ashes, amino acids, proteins and others) occurs, forming a “mud”, then separated by gravity or centrifugation.
By heating, a partial water evaporation is obtained, water that reaches a concentration of about 35% of the juice. Ca2+ ions present, derived from Ca(OH)2, are removed by CO2 that is bubbled through the liquor; the reaction between CO2 and Ca2+ ions produces a calcium carbonate precipitate on the basket.
The following step is the crystallization by which, through a series of centrifugation and evaporation stages, sugar is separated from the molasses (used as an animal feed additive, to produce ethanol, citric acid, compressed yeast, and rum): the product is raw cane sugar ready for sale.
White refined sugar from raw cane sugar
White refined sugar is obtained from raw one as a result of separation of molasses residues, that have an entirely negligible nutritional value.
After dissolving raw sugar in hot water Ca(OH)2 is added again to precipitate remnant molasses residues and so to obtain a further clarification.
At this point, the solution including yellowish residues is passed through on activated carbon that absorbs the residues in solution. Finally, white sugar is obtained through a series of crystallizations and centrifugations.
Extraction from sugar beet
The extraction of sucrose from sugar beets occurs in a way similar to that seen for sugar cane.
After being harvested and washed they are cut into slices and put in water at temperature between 60-70°C (140°-158°F), treatment that causes the breakage of cellular membranes and the release of sucrose together with other substances. In this solution, called raw juice, together with sucrose in a concentration between 10%-15%, there are also many impurities, both inorganic, as salts, and organic, as glutamic acid, proteins (including enzymes responsible of oxidative processes), phenols, acids, saponins, betaines, pectins that confer a color ranging from brownish to black and that obviously must be removed. The eventual heat-resisting microorganisms are eliminated adding a disinfectant as e.g. sulfur dioxide (SO2).
As for sugar cane, also in this case Ca(OH)2 is used to precipitate parts of impurities and, then, CO2 will be bubbled through the liquor to precipitate calcium carbonate that carries with it some impurities. The next step is the addition of SO2 to the solution to prevent successive reactions of darkening and degradation. The final steps of evaporation and crystallization correspond to those seen for cane sugar and yield to the production of a sugar with a purity above 99.7%.
Sucrose: enzymatic cleavage
The reaction that cleaves sucrose into its component monosaccharides fructose and glucose (α-(1→2)-glycosidic reaction) is catalyzed by a multifunctional enzyme, localized on the brush border membranes of enterocites. The enzyme, called sucrase-isomaltase, has two active sites: one catalyzes the cleavage of sucrose while the other the release of new straight chains from limit dextrins (α-(1→6)-glycosidic reaction on which amylase acts (for this reason it is also called debranching enzyme). Finally, in both active sites it is able to catalyze the hydrolysis of maltose (maltase activity, (α-(1→4)-glycosidic reaction).
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