Sucrose

Sucrose (common name: table sugar, also called saccharose) is a disaccharide (glucose + fructose) with the molecular formula C₁₂H₂₂O₁₁. Its systematic name is α-D-glucopyranosyl-(1→2)-β-D-fructofuranose. It is best known for its role in human nutrition and is formed by plants but not by other organisms.

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Physical and chemical properties

  Pure sucrose is most often prepared as a fine, white, odorless crystalline powder with a pleasing, sweet taste; the common table sugar. Large crystals are sometimes precipitated from water solutions of sucrose onto a string (or other nucleation surface) to form rock candy, a confection.

Like other carbohydrates, sucrose has a hydrogen to oxygen ratio of 2:1. It consists of two monosaccharides, α-glucose and fructose, joined by a glycosidic bond between carbon atom 1 of the glucose unit and carbon atom 2 of the fructose unit. What is notable about sucrose is that unlike most polysaccharides, the glycosidic bond is formed between the reducing ends of both glucose and fructose, and not between the reducing end of one and the nonreducing end of the other. The effect of this inhibits further bonding to other saccharide units. Since it contains no free anomeric carbon atom, it is classified as a nonreducing sugar. Sucrose melts and decomposes at 186 °C to form caramel, and when combusted produces carbon, carbon dioxide, and water. Water breaks down sucrose by hydrolysis, however the process is so gradual that it could sit in solution for years with negligible change. If the enzyme sucrase is added however, the reaction will proceed rapidly.

Reacting sucrose with sulfuric acid dehydrates the sucrose and forms elemental carbon, as demonstrated in the following equation:

C₁₂H₂₂O₁₁ + H₂SO₄ catalyst → 12 C + 11 H₂O

Commercial production and use

Main article: Sugar

Sucrose is the most common food sweetener, although it has been replaced in American industrial food production by other sweeteners such as fructose syrups or combinations of functional ingredients and high intensity sweeteners. This is due to the subsidization of corn in the United States, which has led to a vast surplus. Combined with sugar tariffs, this has driven the price of corn syrup far below that of sugar.

Sucrose is the most important sugar in plants, and can be found in the phloem sap. It is generally extracted from sugar cane or sugar beet and then purified and crystallized. Other (minor) commercial sources are sweet sorghum and sugar maples.

Sucrose is ubiquitous in food preparations due to both its sweetness and its functional properties; it is important to the structure of many foods including biscuits and cookies, candy canes, ice cream and sorbets, and also assists in the preservation of foods. As such it is common in many processed and so-called “junk foods.”

Sugar as a macronutrient

In mammals, sucrose is very readily digested in the stomach into its component sugars, by acidic hydrolysis. This step is performed by a glycoside hydrolase, which catalyzes the hydrolysis of sucrose to the monosaccharides glucose and fructose. Glucose and fructose are rapidly absorbed into the bloodstream in the small intestine. Undigested sucrose passing into the intestine is also broken down by sucrase or isomaltase glycoside hydrolases, which are located in the membrane of the microvilli lining the duodenum. These products are also transferred rapidly into the bloodstream.

Sucrose is digested by the enzyme invertase in bacteria and some animals.

Acidic hydrolysis can be used in laboratories to achieve the hydrolysis of sucrose into glucose and fructose.

In human nutrition

Sucrose is an easily assimilated macronutrient that provides a quick source of energy to the body, provoking a rapid rise in blood glucose upon ingestion. However, pure sucrose is not normally part of a human diet balanced for good nutrition, although it may be included sparingly to make certain foods more palatable.

Overconsumption of sucrose has been linked with some adverse health effects. The most common is dental caries or tooth decay, in which oral bacteria convert sugars (including sucrose) from food into acids that attack tooth enamel. Sucrose, as a pure carbohydrate, has an energy content of 3.94 kilocalories per gram (or 17 kilojoules per gram). When a large amount of foods that contain a high percentage of sucrose is consumed, beneficial nutrients can be displaced from the diet, which can contribute to an increased risk for chronic disease. It has been suggested that sucrose-containing drinks may be linked to the development of obesity and insulin resistance.^[1]

The rapidity with which sucrose raises blood glucose can cause problems for people suffering from defects in glucose metabolism, such as persons with hypoglycemia or diabetes mellitus. Sucrose can contribute to development of the metabolic syndrome.^[2] In an experiment with rats that were fed a diet one-third of which was sucrose, the sucrose first elevated blood levels of triglycerides, which induced visceral fat and ultimately resulted in insulin resistance.^[3] Another study found that rats fed sucrose-rich diets developed high triglycerides, hyperglycemia, and insulin resistance.^[4]

References

Notes

General references

• Yudkin, J.; Edelman, J., Hough, L. (1973). Sugar - Chemical, Biological and Nutritional Aspects of Sucrose. The Butterworth Group. ISBN 0-408-70172-2.