Sugar

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For other uses, see Sugar (disambiguation).
For common table sugar, see Sucrose.
"White sugar" redirects here. For the Joanne Shaw Taylor album, see White Sugar (album).
Magnification of grains of refined sucrose, the most common free sugar
Sugar, granulated
Nutritional value per 100 g (3.5 oz)
Energy 1,619 kJ (387 kcal)
Carbohydrates 99.98 g
- Sugars 99.91 g
- Dietary fiber 0 g
Fat 0 g
Protein 0 g
Water 0.03 g
Riboflavin (vit. B2) 0.019 mg (2%)
Calcium 1 mg (0%)
Iron 0.01 mg (0%)
Potassium 2 mg (0%)
Percentages are relative to
US recommendations for adults.
Source: USDA Nutrient Database
Sugars, brown
Nutritional value per 100 g (3.5 oz)
Energy 1,576 kJ (377 kcal)
Carbohydrates 97.33 g
- Sugars 96.21 g
- Dietary fiber 0 g
Fat 0 g
Protein 0 g
Water 1.77 g
Thiamine (vit. B1) 0.008 mg (1%)
Riboflavin (vit. B2) 0.007 mg (1%)
Niacin (vit. B3) 0.082 mg (1%)
Vitamin B6 0.026 mg (2%)
Folate (vit. B9) 1 μg (0%)
Calcium 85 mg (9%)
Iron 1.91 mg (15%)
Magnesium 29 mg (8%)
Phosphorus 22 mg (3%)
Potassium 346 mg (7%)
Sodium 39 mg (3%)
Zinc 0.18 mg (2%)
Percentages are relative to
US recommendations for adults.
Source: USDA Nutrient Database

Sugar is the generalised name for a class of sweet-flavored substances used as food. They are carbohydrates and as this name implies, are composed of carbon, hydrogen and oxygen. There are various types of sugar derived from different sources. Simple sugars are called monosaccharides and include glucose, fructose and galactose. The table or granulated sugar most customarily used as food is sucrose, a disaccharide. Other disaccharides include maltose and lactose.

Sugars are found in the tissues of most plants but are only present in sufficient concentrations for efficient extraction in sugarcane and sugar beet. Sugarcane is a giant grass and has been cultivated in tropical climates in the Far East since ancient times. A great expansion in its production took place in the 18th century with the setting up of sugar plantations in the West Indies and Americas. This was the first time that sugar became available to the common people who had previously had to rely on honey to sweeten foods. Sugar beet is a root crop and is cultivated in cooler climates and became a major source of sugar in the 19th century when methods for extracting the sugar became available. Sugar production and trade has changed the course of human history in many ways. It influenced the formation of colonies, the perpetuation of slavery, the transition to indentured labor, the migration of peoples, wars between 19th century sugar trade controlling nations and the ethnic composition and political structure of the new world.[1][2]

The world produced about 168 million tonnes of sugar in 2011.[3] The average person consumes about 24 kilograms of sugar each year, equivalent to over 260 food calories per person, per day.[4]

In modern times it has been questioned whether a diet high in sugars, especially refined sugars, is bad for health. Sugar has been linked to obesity and suspected of being implicated in diabetes, cardiovascular disease, dementia, macular degeneration and tooth decay. Numerous studies have been undertaken to try to clarify the position but the results remain largely unclear, mainly because of the difficulty of finding populations for use as controls that do not consume sugars.

Contents

History

Main article: History of sugar
Sugar cane plantation

Ancient times and Middle Ages

Sugar has been produced in the Indian subcontinent[5] since ancient times. It was not plentiful or cheap in early times—honey was more often used for sweetening in most parts of the world. Originally, people chewed raw sugarcane to extract its sweetness. Sugarcane was a native of tropical South Asia and Southeast Asia.[6] Different species seem to have originated from different locations with Saccharum barberi originating in India and S. edule and S. officinarum coming from New Guinea.[6][7]

One of the earliest historical references to sugar cane is in 8th century BC. Chinese manuscripts that also refer to the fact that knowledge of sugar cane was derived from India.[8] It appears that in about 500 BC, residents of present-day India began making sugar syrup and cooling it in large flat bowls to make crystals that were easier to store and transport than the cane itself. In the local Indian language, these crystals were called khanda(Devanagari:खण्ड,Khaṇḍa) which is the source of the word candy.[9]

Different kinds of sugar

Sugar remained relatively unimportant until the Indians discovered methods of turning sugarcane juice into granulated crystals that were easier to store and to transport.[10] Crystallized sugar was discovered by the time of the Imperial Guptas, around 5th century AD.[10] Indian sailors, who carried clarified butter and sugar as supplies, introduced knowledge of sugar on the various trade routes they travelled.[10] Buddhist monks brought sugar crystallization methods to China as they travelled around.[11] During the reign of Harsha (r. 606–647) in North India, Indian envoys in Tang China taught methods of cultivating sugarcane after Emperor Taizong of Tang (r. 626–649) made his interest in sugar known. China then established its first sugarcane plantations in the seventh century.[12] Chinese documents confirm at least two missions to India, initiated in 647 AD, to obtain technology for sugar-refining.[13] In South Asia, the Middle East and China, sugar became a staple of cooking and desserts.

The triumphant progress of Alexander the Great was halted on the banks of the Indus River by the refusal of his troops to go further east. They saw people in the Indian subcontinent growing sugarcane and making granulated, salt-like sweet powder, locally called Sharkara(Devanagari:शर्करा,Śarkarā),pronounced as saccharum (ζάκχαρι). On their return journey, the Macedonian soldiers carried the "honey bearing reeds" home with them. Sugarcane remained a little known crop in Europe for over a millennium, sugar a rare commodity, and traders of sugar wealthy. Venice, at the height of its financial power, was the chief sugar-distributing center of Europe.

[8]

Crusaders brought sugar home with them to Europe after their campaigns in the Holy Land, where they encountered caravans carrying "sweet salt". Early in the 12th century, Venice acquired some villages near Tyre and set up estates to produce sugar for export to Europe, where it supplemented honey which had previously been the only available sweetener.[14] Crusade chronicler William of Tyre, writing in the late 12th century, described sugar as "very necessary for the use and health of mankind".[15]

Modern history

In August 1492, Christopher Columbus stopped at La Gomera in the Canary Islands, for wine and water, intending to stay only four days. He became romantically involved with the Governor of the island, Beatriz de Bobadilla y Ossorio, and stayed a month. When he finally sailed she gave him cuttings of sugarcane, which became the first to reach the New World.[16]

Sugar was a luxury in Europe prior to the 18th century when it became more widely available. It then became popular and by the 19th century it was considered a necessity. This evolution of taste and demand for sugar as an essential food ingredient unleashed major economic and social changes.[1] It drove, in part, colonization of tropical islands and nations where labor-intensive sugarcane plantations and sugar manufacturing could thrive. The demand for cheap and docile labor to perform the hard work involved in its cultivation and processing drove first, the slave trade from Africa (in particular West Africa), followed by the indentured labor trade from South Asia (in particular India).[2][17][18] Millions of slave and indentured laborers were brought into the Caribbean, Indian Ocean, Pacific Islands, East Africa, Natal, north and eastern parts of South America, and southeast Asia. The modern ethnic mix of many nations that have been settled in the last two centuries has been influenced by sugar.[19][20][21]

Sugar also led to some industrialization of former colonies. For example, Lieutenant J. Paterson, of the Bengal establishment, persuaded the British Government that sugar cane could be cultivated in British India with many advantages and at less expense than in the West Indies. As a result, a number of sugar factories were established in Bihar in eastern India.[22]

During the Napoleonic Wars, sugar beet production increased in continental Europe because of the difficulty of importing sugar at times in which shipping was subject to blockade. By 1880, sugar beet was the main source of sugar in Europe though the United Kingdom continued to import the main part of its sugar from its colonies.[23]

Until the late nineteenth century sugar was purchased in loaves, which had to be cut using implements called 'nips',[24] while in later years, bags of sugar became more common. Henry Tate of Tate & Lyle was an early proponent of sugar cubes, which he manufactured at his sugar refineries in Liverpool and London.[25]

Etymology

Ant feeding on sugar crystals

The etymology reflects the spread of the commodity. The English word "sugar"[26] originates from the Arabic word سكر sukkar, itself from the Persian shakar,[27] itself derived from Sanskrit शर्करा sharkara.[28] It most probably came to England by way of Italian merchants. The contemporary Italian word is zucchero, whereas the Spanish and Portuguese words, azúcar and açúcar respectively, have kept a trace of the Arabic definite article. The Old French word is zuchre – contemporary French sucre. The earliest Greek word attested is σάκχαρις (sákkʰaris).[29][30] A satisfactory pedigree explaining the spread of the word has yet to be done. Note that the English word jaggery (meaning "coarse brown Indian sugar") has similar ultimate etymological origins (presumably in Sanskrit).

Types of sugar

Monosaccharides

Glucose, fructose and galactose are all simple sugars, monosaccharides, with the general formula C6H12O6. They have five hydroxyl groups (−OH) and a carbonyl group (C=O) and are cyclic when dissolved in water. They each exist as several isomers with dextro- and laevo-rotatory forms which cause polarized light to diverge to the right or the left.[31]

Glucose, dextrose or grape sugar occurs naturally in fruits and plant juices and is the primary product of photosynthesis. Most ingested carbohydrates are converted into glucose during digestion and it is the form of sugar that is transported round the bodies of animals in the bloodstream. It can be manufactured from starch by the addition of enzymes or in the presence of acids. Glucose syrup is a liquid form of glucose that is widely used in the manufacture of foodstuffs. It can be manufactured from starch by enzymatic hydrolysis.[32]

Fructose or fruit sugar occurs naturally in fruits, some root vegetables, cane sugar and honey and is the sweetest of the sugars. It is one of the components of sucrose or table sugar. It is used as a high fructose syrup which is manufactured from hydrolized corn starch which has been processed to yield corn syrup, with enzymes then added to convert part of the glucose into fructose.[33]

Galactose does not generally occur in the free state but is a constituent with glucose of the disaccharide lactose or milk sugar. It is less sweet than glucose. It is a component of the antigens found on the surface of red blood cells that determine blood groups.[34]

Disaccharides

Sucrose, maltose and lactose are all compound sugars, disaccharides, with the general formula C12H22O11. They are formed by the combination of two monosaccharide molecules with the exclusion of a molecule of water.[31]

Sucrose is found in the stems of sugar cane and roots of sugar beet. It also occurs naturally alongside fructose and glucose in other plants, particularly fruits and some roots such as carrots. The different proportions of sugars found in these foods determines the range of sweetness experienced when eating them.[31] A molecule of sucrose is formed by the combination of a molecule of glucose with a molecule of fructose. After being eaten, sucrose is split into its constituent parts during digestion by a number of enzymes known as sucrases.[35]

Maltose is formed during the germination of certain grains, most notably barley, where it is the source of the malt used in the manufacture of beer. A molecule of maltose is formed by the combination of two molecules of glucose. It is less sweet than glucose, fructose or sucrose.[31] It is formed in the body during the digestion of starch by the enzyme amylase and is itself broken down during digestion by the enzyme maltase.[36]

Lactose is the naturally occurring sugar found in milk. A molecule of lactose is formed by the combination of a molecule of galactose with a molecule of glucose. It is broken down when consumed into its constituent parts by the enzyme lactase during digestion. Children have this enzyme but some adults no longer form it and they are unable to digest lactose.[37]

Production

Sugarcane

Sugarcane (Saccharum spp.) is a perennial grass in the family Poaceae. It is cultivated in tropical and sub-tropical regions for the sucrose that is found in its stems. It requires a frost-free climate with sufficient rainfall during the growing season to make full use of the plant's great growth potential. The crop is harvested mechanically or by hand, chopped into lengths and conveyed rapidly to the processing plant. Here it is either milled and the juice extracted with water or the sugar is extracted by diffusion. The juice is then clarified with lime and heated to kill enzymes. The resulting thin syrup is then concentrated in a series of evaporators and then further water is removed by evaporation in vacuum containers. The resulting supersaturated solution is seeded with sugar crystals and the sugar crystallizes out and is separated from the fluid and dried. Molasses is a by-product of the process and the fibre from the stems, known as bagasse, is burned to provide energy for the boiling of the syrup. The crystals of raw sugar have a sticky brown coating and can either be used as they are or can be bleached by sulphur dioxide or treated in a carbonation process to produce a whiter product.[38]

Sugar beet

Sugar beet (Beta vulgaris) is an annual plant in the family Amaranthaceae, the tuberous root of which contains a high proportion of sucrose. It is cultivated in temperate regions with adequate rainfall and requires a fertile soil. The crop is harvested mechanically in the autumn and the crown of leaves and excess soil removed. The roots do not deteriorate rapidly and may be left in a clamp in the field for some weeks before being transported to the processing plant. Here the crop is washed and sliced and the sugar extracted by diffusion. The raw juice is then treated with lime and carbonated in a number of stages in order to purify it. Water is evaporated by boiling the syrup under a vacuum. The syrup is then cooled and seeded with sugar crystals. The white sugar which crystallizes out can be separated in a centrifuge and dried. It requires no further refining.[39]

Refining

Cane sugar requires further processing to provide the free-flowing white table sugar required by the consumer. The sugar may be transported in bulk to the country where it will be used and the refining process often takes place there. The first stage is known as affination and involves immersing the sugar crystals in a concentrated syrup which softens and removes the sticky brown coating without dissolving them. The crystals are then separated from the liquor and dissolved in water. The resulting syrup is either treated by a carbonation or a phosphatation process. Both involve the precipitation of a fine solid in the syrup and when this is filtered out, a lot of the impurities are removed at the same time. Removal of colour is achieved by either using a granular activated carbon or an ion-exchange resin. The sugar syrup is concentrated by boiling and then cooled and seeded with sugar crystals causing the sugar to crystallize out. The liquor is spun in a centrifuge and the white crystals are dried in hot air, ready to be packaged or used. The surplus liquor is made into refiners' molasses.[40]

Producing countries

The five largest producers of sugar in 2010 were Brazil, India, the European Union, China and Thailand. The largest exporters in 2010 were Brazil, Thailand, Australia and India, while the largest importers were the European Union, United States and Indonesia. Currently, Brazil has the highest per capita consumption of sugar, followed by Australia, Thailand and the European Union.[41][42]

Forms and uses

The supersaturation of sugar in water will make rock candy.

Granulated sugars are used at the table to sprinkle on foods and to sweeten hot drinks and in home baking to add sweetness and texture to cooked products. They are also used as a preservative to prevent micro-organisms growing and perishable food from spoiling as in jams, marmalades and candied fruits.[43]

Milled sugars are ground to a fine powder. They are used as icing sugar, for dusting foods and in baking and confectionery. [44]

Screened sugars are crystalline products separated according to the size of the grains. They are used for decorative table sugars, for blending in dry mixes and in baking and confectionery.[44]

Brown sugars are granulated sugars with the grains coated in molasses to produce a light, dark or demerara sugar. They are used in baked goods, confectionery and toffees.[44]

Sugar cubes are white or brown granulated sugars pressed together in block shape. They are used to sweeten drinks.[44]

Liquid sugars are strong syrups consisting of 67% granulated sugar dissolved in water. They are used in the food processing of a wide range of products including beverages, ice cream and jams.[44]

Invert sugars and syrups are blended to manufacturers specifications and are used in breads, cakes and beverages for adjusting sweetness, aiding moisture retention and avoiding crystallization of sugars.[44]

Syrups and treacles are dissolved invert sugars heated to develop the characteristic flavours. Treacles have added molasses. They are used in a range of baked goods and confectionery including toffees and licorice.[44]

Low calorie sugars and sweeteners are often made of maltodextrin with added sweeteners. Maltodextrin is an easily digestible synthetic polysaccharide consisting of short chains of glucose molecules and is made by the partial hydrolysis of starch. The added sweeteners are often aspartame, saccharin, stevia or sucralose.[45]

Polyols are sugar alcohols and are used in chewing gums where a sweet flavor is required that lasts for a prolonged time in the mouth.[46]

Consumption

Sugar is an important part of worldwide human food balance sheet. After cereals and vegetable oils, sugar derived from sugar cane and beets provided more kilocalories per capita per day on average than other food groups.[47] According to FAO, about 24 kilograms of sugar, equivalent to over 260 food calories per day, was, on average, consumed annually per person of all ages in the world in 1999. Even with rising human population, sugar consumption is expected to increase to 25.1 kilograms per person by 2015.[4]

The per capita consumption of refined sugar in America has varied between 27 to 46 kilograms in the last 40 years. In 2008, American per capita total consumption of sugar and sweeteners, exclusive of artificial sweeteners, equalled 61.9 kilograms per year (136.2 pounds). This consisted of 29.65 kg (65.4 lb) pounds of refined sugar and 31 kg (68.3 lb) pounds of corn-derived sweeteners per person.[48][49]

Health effects

Some studies involving the health impact of sugars are effectively inconclusive. The WHO and FAO meta studies have shown directly contrasting impacts of sugar in refined and unrefined forms [50] and since most studies do not use a population who are not consuming any "free sugars" at all, the baseline is effectively flawed (or as the report puts it, the studies are "limited"). Hence there are articles such as Consumer Reports on Health that said in 2008, "Some of the supposed dietary dangers of sugar have been overblown. Many studies have debunked the idea that it causes hyperactivity, for example."[51] Despite this, the article continues to discuss other health impacts of sugar. Other articles and studies refer to the increasing evidence supporting the links between refined sugar and hyperactivity.[52] The WHO FAO meta-study suggests that such inconclusive results are to be expected when some studies do not effectively segregate or control for free sugars as opposed to sugars still in their natural form (entirely unrefined) while others do.[50]

Blood glucose levels

Sugar, because of its simpler chemical structure, may raise blood glucose levels more quickly than starch. This finding suggests that this basic differentiation between starch and sugar is insufficient reason to segregate these two substances for controlling blood glucose levels in diabetics, the idea behind carbohydrate counting.[53] A more effective distinction could be that suggested by multiple meta-studies between free sugars and naturally-occurring sugars which suggest that they have different impacts on health.[50][54]

Immune system

According to WebMD, consuming too much sugar suppresses the immune system.[55]

Obesity and diabetes

Studies on the link between sugars and diabetes are inconclusive, with some suggesting that eating excessive amounts of sugar does not increase the risk of diabetes, although the extra calories from consuming large amounts of sugar can lead to obesity, which may itself increase the risk of diabetes.[56][57][57][58][59][60][60][61] Others show correlation between refined sugar (free sugar) consumption and the onset of diabetes, and negative correlation with the consumption of fiber. [62][63][64][65] including a 2010 meta-analysis of eleven studies involving 310,819 participants and 15,043 cases of type 2 diabetes.[66] This found that "SSBs (sugar-sweetened beverages) may increase the risk of metabolic syndrome and type 2 diabetes not only through obesity but also by increasing dietary glycemic load, leading to insulin resistance, β-cell dysfunction, and inflammation". As an overview to consumption related to chronic disease and obesity, the World Health Organization's independent meta-studies specifically distinguish free sugars ("all monosaccharides and disaccharides added to foods by the manufacturer, cook or consumer, plus sugars naturally present in honey, syrups and fruit juices") from sugars naturally present in food. The reports prior to 2000 set the limits for free sugars at a maximum of 10% of carbohydrate intake, measured by energy, rather than mass, and since 2002[50] have aimed for a level across the entire population of less than 10%. The consultation committee recognized that this goal is "controversial. However, the Consultation considered that the studies showing no effect of free sugars on excess weight have limitations."[50]

Cardiovascular disease

A number of studies in animals have suggested that chronic consumption of refined sugars can contribute to metabolic and cardiovascular dysfunction. Some experts have suggested that refined fructose is more damaging than refined glucose in terms of cardiovascular risk.[67] Cardiac performance has been shown to be impaired by switching from a carbohydrate diet including fiber to a high-carbohydrate diet.[68] Switching saturated fatty acids for carbohydrates with high glycemic index values shows a statistically significant positive association with the risk of myocardial infarction.[69] Other studies have found links between high fat and high glycemic index carbohydrates accelerates the development of cardiac pathology and pump dysfunction in hypertension despite no signs of diabetes and only a modest level of obesity, suggesting that the link between obesity and coronary heart disease should be shifted towards macronutrients and the high glycemic load typical of the "junk-food" diet.[70]

The consumption of added sugars has been positively associated with multiple measures known to increase cardiovascular disease risk amongst adolescents as well as adults.[71] Studies are suggesting that the impact of refined carbohydrates or high glycemic load carbohydrates are more significant than the impact of saturated fatty acids on cardiovascular disease.[72][73] A high dietary intake of sugar (in this case, sucrose or disaccharide) can substantially increase the risk of heart and vascular diseases. According to a Swedish study of 4301 people undertaken by Lund University and Malmö University College, sugar was associated with higher levels of bad blood lipids, causing a high level of small and medium low-density lipoprotein (LDL) and reduced high-density lipoprotein (HDL). In contrast, the amount of fat eaten did not affect the level of blood fats. As a side note, moderate quantities of alcohol and protein were linked to an increase in the good HDL blood fat.[74]

Alzheimer's disease

It is suggested that Alzheimer's disease is linked with the western diet. This is characterised by high intakes of red meat, sugary foods, high-fat foods and refined grains. It has been hypothesized that dementia could be prevented by the taking of mono-supplements of specific vitamins or drugs, but studies have shown that this approach does not show appreciable results.[75]

Dietary pattern analysis considers overall eating patterns, comparing diets of people with Alzheimer's disease to diets of healthy controls using factor analysis. This analysis shows a major eating pattern for those with Alzheimer's characterised by a high intake of meat, butter, high-fat dairy products, eggs and refined sugar, while the major eating pattern for those without Alzheimer's was characterised by a high intake of grains and vegetables.[76]

One group of experimenters compared a normal rodent diet (19% protein, 5% fat and 60% complex carbohydrate) with free access to water against the same diet but with free access to a 10% sucrose solution. The experimental results underscore the potential role of dietary sugar in the pathogenesis of Alzheimer disease and suggest that controlling the consumption of sugar-sweetened beverages may be an effective way to curtail the risk of developing the disease.[77]

Macular degeneration

There are links between free sugar consumption and macular degeneration in older age.[78]

Tooth decay

In regard to contributions to tooth decay, the role of free sugars is also recommended to be below an absolute maximum of 10% of energy intake, with a minimum of zero. There is "convincing evidence from human intervention studies, epidemiological studies, animal studies and experimental studies, for an association between the amount and frequency of free sugars intake and dental caries" while other sugars (complex carbohydrate) consumption is normally associated with a lower rate of dental caries.[54] Lower rates of tooth decay have been seen in individuals with hereditary fructose intolerance.[79]

Terminology

Popular

The term sugar usually refers to sucrose, which is also called "table sugar" or "saccharose." Sucrose is a white crystalline disaccharide. It is often obtained from sugar cane or sugar beet.[80] Sucrose is the most popular of the various sugars for flavoring, as well as properties (such as mouthfeel, preservation, and texture) of beverages and food.

Chemical

"Sugar" can also be used to refer to water-soluble crystalline carbohydrates with varying sweetness. Sugars include monosaccharides (e.g., glucose, fructose, galactose), disaccharides (e.g., sucrose, lactose, maltose), trisaccharides, and oligosaccharides,[81] in contrast to complex carbohydrates such as polysaccharides. Corn syrup, dextrose, crystalline fructose, and maltose, for example, are used in manufacturing and preparing food.

Baking weight/mass volume relationship

Grainy, raw sugar.

Different culinary sugars have different densities due to differences in particle size and inclusion of moisture.

The Domino Sugar Company has established the following volume to weight conversions: [82]

  • Brown sugar 1 cup = 48 teaspoons ~ 195 g = 6.88 oz
  • Granular sugar 1 cup = 48 teaspoons ~ 200 g = 7.06 oz
  • Powdered sugar 1 cup = 48 teaspoons ~ 120 g = 4.23 oz

Bulk density[83]

  • Dextrose sugar 0.62 g/mL ( = 620 kg/m^3)
  • Granulated sugar 0.70 g/mL
  • Powdered sugar 0.56 g/mL
  • Beet sugar 0.80 g/mL

Purity standards

The International Commission for Uniform Methods of Sugar Analysis sets standards for the measurement of the purity of refined sugar, known as ICUMSA numbers; lower numbers indicate a higher level of purity in the refined sugar.[84]

Chemistry

Sucrose: a disaccharide of glucose (left) and fructose (right), important molecules in the body.
Main article: Carbohydrate

Scientifically, sugar loosely refers to a number of carbohydrates, such as monosaccharides, disaccharides, or oligosaccharides. Monosaccharides are also called "simple sugars," the most important being glucose. Almost all sugars have the formula CnH2nOn (n is between 3 and 7). Glucose has the molecular formula C6H12O6. The names of typical sugars end with "-ose," as in "glucose", "dextrose", and "fructose". Sometimes such words may also refer to any types of carbohydrates soluble in water. The acyclic mono- and disaccharides contain either aldehyde groups or ketone groups. These carbon-oxygen double bonds (C=O) are the reactive centers. All saccharides with more than one ring in their structure result from two or more monosaccharides joined by glycosidic bonds with the resultant loss of a molecule of water (H2O) per bond.[85]

Monosaccharides in a closed-chain form can form glycosidic bonds with other monosaccharides, creating disaccharides (such as sucrose) and polysaccharides (such as starch). Enzymes must hydrolyze or otherwise break these glycosidic bonds before such compounds become metabolized. After digestion and absorption the principal monosaccharides present in the blood and internal tissues include glucose, fructose, and galactose. Many pentoses and hexoses can form ring structures. In these closed-chain forms, the aldehyde or ketone group remains non-free, so many of the reactions typical of these groups cannot occur. Glucose in solution exists mostly in the ring form at equilibrium, with less than 0.1% of the molecules in the open-chain form.[85]

Natural polymers of sugars

Biopolymers of sugars are common in nature. Through photosynthesis plants produce glucose, which has the formula C6H12O6, and convert it for storage as an energy reserve in the form of other carbohydrates such as starch, or (as in cane and beet) as sucrose, with the chemical formula C12H22O11. Starch, consisting of two different polymers of glucose, is a readily degradable form of chemical energy stored by cells, and can be converted to other types of energy.[85] Another polymer of glucose is cellulose which is a linear chain composed of several hundred or thousand glucose units. It is used by plants as a structural component in their cell walls. Humans can only digest cellulose to a very limited extent, though ruminants can do so with the help of symbiotic bacteria in their gut.[86] DNA and RNA are built up of the monosaccharides deoxyribose and ribose respectively. Deoxyribose has the formula C5H10O4 and ribose the formula C5H10O5.[87]

See also

References

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