Carbohydrate chemistry

Overview

Carbohydrates are organic compounds of natural or artificial mainly carbon, hydrogen and oxygen. They are also called "Carbohydrate" because of their formula: Cn(H2O)n.

Some carbohydrates, soluble in water has a sweet taste and are called sugars. However, it must be noted that there are substances that have a sweetness, sometimes higher than that of carbohydrates and sugar, but they are not carbohydrates. These substances are called sweeteners.

Carbohydrates are the group of organic compounds the most abundant in nature. Mainly synthesized by plants, they are heterotrophic organisms for a source of energy first.

Carbohydrates are classified into two broad categories: monosaccharides and saccharides. The monosaccharides compounds are the simplest carbohydrates. The combination of monosaccharides leads to the formation of a saccharides. They are also divided into two groups: holosides and glycosides. The diholosides (or disaccharides) and polyholosides (or polysaccharides) carbohydrates which are the most important food industries belong to the first group.

Classification des glucides

In their reactions, carbohydrates involve two main functions: the carbonyl group (aldehyde or ketone) and the alcohol. The main carbohydrate is sucrose (or sugar), the starch, dextrin, cellulose and glycogen, substances that form an important part of the diet of humans and many animals.

Monosaccharides

Definition

The monosaccharides, are the simplest carbohydrates. These are polyalcohols with an aldehyde or ketone of a neighboring alcohol, non hydrolysable and endowed with a power reducer. Their molecular formula: Cn(H2O)n.

The simplest monosaccharide is the glyceraldehyde which is a triose formed from carbon atoms 3, 2 alcohol functions and an aldehyde.

Glycéraldéhyde

The best known of monosaccharides are glucose, fructose and galactose gifted a sweetness. This is not the same for the three molecules, glucose and galactose taste much less sweet than fructose.

Glucose galactose et fructose

There are glucose and fructose in most plant sweetness. Corn syrup is a glucose syrup produced from cornstarch. Many sugary foods produced by industry contain invert sugar. It is a mixture of glucose and fructose.

Classifications of monosaccharides

The classification of monosaccharides is based on two factors: the nature of the carbonyl function and the length of the carbon chain.

Depending on the nature of the carbonyl function can be distinguished: the aldose comprising an aldehyde, and ketosis, with a ketone. In nature, there are mostly aldose such as glucose, and ketosis are less abundant. Fructose, fruit sugar, is the response of ketosis.

Aldoses et Cétoses

Depending on the length of the carbon chain can be distinguished: the three (3 carbons), the tetroses (4 carbons), the pentoses (5 carbons), the hexoses (6 carbons) and heptose (7 carbons). Most of the monosaccharides that are found in living beings are pentoses (C5H10O5) or hexoses (C6H12O6).

We find most of the pentoses in nucleic acids (such as DNA for example). The largest are pentoses: ribose and deoxyribose.

The hexoses are the most abundant monosaccharide in nature. The best known are: glucose (or dextrose), fructose (or levulose) and galactose. There are glucose and fructose in most plant sweetness. Glucose is also known as "grape sugar".

Saccharides

A saccharide, is a compound that releases monosaccharides by hydrolysis. This hydrolysis can release, besides monosaccharides, groups of non-carbohydrate called aglycones group.

The classification of saccharides is based on two essential elements: the presence or absence of a group aglycone and the number of monosaccharides molecules constituting the saccharides.

Depending on the presence or absence of an aglycone group, there are holosides and glycosides. Holosides The osides which are not released by hydrolysis of oses. While these are the glycosides that release osides by hydrolysis of oses and one or more groups aglycones.

Depending on the number of molecules oses (molecular weight), we divide the holosides in oligoholosides and polyholosides. The first feature 2, 3 or 4 oses per molecule, the latter contain much more. However, note that some authors consider as part of oligoholosides the holosides up to 10 molecules per molecule oses of osides. Figure No. 1 below mentioned, the class holosides in diholosides (disaccharides) and polyholosides (polysaccharides). This classification is often used in literature.

The best known of oligoholosides are diholosides including sucrose, maltose and lactose are among them. In contrast, the starch, cellulose and glycogen are polyholosides.

In dietetics, it often divides carbohydrates into sugars (mono and disaccharides), starch and fiber (cellulose).

Diholosides: Disaccharides

A diholoside or disaccharide, a carbohydrate is formed from the union of two oses (two monosaccharides), hexoses generally, by a bond called osidic (also called glycosidic linkages).

Disaccharides and monosaccharides have a sweet taste. If, on an arbitrary scale, is attributed to sucrose (table sugar) a sweetening power of 100, while other sugars are to:

Sucrose 100
NB The perception of sweet taste is something subjective. It varies from person to person.
Fructose ~ 150
Glucose ~ 75
Maltose ~ 40
Galactose ~ 35
Lactose ~ 20

The diholosides or disaccharides are classified diholosides reductive and non reductive. This property is due to the presence or absence of a free reducing the diholoside. Maltose and lactose, for example, are reducing diholosides, while sucrose is a non-reducing diholoside.

Sucrose

Sucrose is a diholoside (disaccharide) non-reducing chemical formula C12H22O11which is formed by the union of a glucose to a fructose. During the reaction, the OH group of one sugar binds to a H of OH of the other sugar. It is formed when a molecule of water. The two sugars are linked through an O. This type of reaction where two molecules combine with simple release of a molecule of water is called condensation reaction (or dehydration synthesis). The reverse reaction is called hydrolysis.

Saccharose

Sucrose is ordinary table sugar, extracted from sugar beet and cane sugar. It is soluble in water and to a lesser extent in alcohol and ether. When crystallized sucrose form of long thin needles made of crystals clockwise (which departs to the right level of polarized light). Subject to hydrolysis, sucrose gives a mixture lévogyre (which deflects to the left of the plane polarized light) of glucose and fructose. The mixture is therefore known as invert sugar because there was a reversal of the sense of optical rotation, and the corresponding process is called inversion.

Inversion du saccharose

The hydrolysis or the inversion of sucrose can be obtained either by chemical (heating in an acid medium) or enzymatically (action of alpha-glucosidase and sucrase as well as beta-fructosidase or invertase). In the intestines of humans, the inversion of sucrose is performed using these two enzymes: the invertase and sucrase.

Note that the invert sugar has a sweetness greater than sucrose. The sweet taste of honey is due to invert sugar.

When heated to temperatures above 180 ° C, the sucrose is transformed into an amorphous substance (not crystallized), known as brown and syrupy caramel.

Maltose

Maltose is a diholoside (disaccharide) reducer molecular formula C12H22O11, formed by the union of two glucose molecules linked by a conjunction osidic alpha 1-4. Maltose is formed by action of the enzyme amylase on starch. It is found in the free state in some plants.

Maltose is soluble in water, slightly soluble in alcohol, crystallises in fine needles and is clockwise. By hydrolysis it gave glucose.

Formation du maltose

Maltose is a sugar easily digestible and is used for preparing food for infants, such as flour and milk beverages. Fermented by yeast, it is important for the brewing of beer.

Lactose

Lactose is a diholoside (disaccharide) reducer molecular formula C12H22O11, consisting of one molecule of glucose and one galactose molecule linked by a conjunction osidic beta 1-4. It is the sugar of milk: there are almost exclusively in the milk of mammals and highly variable levels (10 to 80 g / l depending on species).

Lactose

Lactose is a sugar not fermentiscible by yeast, but it is fermentiscible by other microorganisms. It is hydrolyzable by the émulsine and intestinal lactase.

Polyholosides: Polysaccharides

The polyholosides or polysaccharides are polymers (large molecule formed by the union of many small molecules similar) molecular weight or more months with high hydrolysis and release only oses and derivatives of simple dare.

The polyholosides different to each other by the nature of oses and their derivatives used in their creation, their molecular weight, how dare are interconnected and the overall architecture of the chain, which explains the great diversity of natural polyholosides. Most polysaccharides are polymers of glucose. The three polysaccharides are the most known:

  • The starch
  • Glycogen
  • Cellulose

Starch

The starch is sugar reserve of plants. This means that in this form that the plants put aside their excess glucose. There are mainly starch in the roots, seeds and fruits. The starch is particularly abundant in cereals (rice, wheat, corn, etc..) And tubers (potatoes). This is an important raw material in industry: cereal industry, starch-glucose, fermentation industry, industrial adhesives and textiles, pharmaceutical industry.

Microscope, the starch is in the form of small grains within a cell. Their shape and size vary from one species to another.

From chemical point of view, the starch is formed from two types of glucose polymers: the amylose and the amylopectin. The amylose consists of linear chains of glucose united alpha 1-4 while the amylopectin is formed branched chains of glucose are united alpha 1-6. Each molecule may contain from 100 to 20 000 glucose. The amylose-amylopectin ratio generally varies from 1 / 5 to 1 / 4 for the amyloidosis. However, some starch, waxy said, very low amylose (0 to 6%). This is the case of certain varieties of corn, barley and rice. These products are in demand for certain food preparation. In the presence of iodine, the starch turns blue.

Amylose and amylopectin

Broadly speaking, the starch is insoluble in cold water. But after a crushing Therefore, aiming to crush the grain, much of the starch back in solution. It is also insoluble in usual organic solvents but soluble in other less common: the ammonia, piperidine, as well as in solutions of certain salts such as calcium chloride, such solutions are used for the determination of 'starch.

With hot water you get a paste. It is said that the starch is "gelatinized". The product is called using starch. It is a swelling of starch grains leading to a kind of colloidal solution, which can turn into a gel by cooling. The starch can be solubilized by treatment with hydrochloric acid cold or hot glycerol. It is, in fact, in these cases not the soluble starch but a product of degradation or less thrust that keeps some properties of the starch as its blue coloration under the action of iodine, but he lost others such as the possibility of forming the starch by heating starch.

By enzymatic action, starch hydrolysis in the presence of the amylase. There are two main amylases: the alpha-amylase and beta-amylase. The alpha-amylase is an enzyme of animal origin (eg salivary amylase and pancreatic amylase) which attacks the osidic alpha 1-4 linkages of starch without order and releases glucose and fragments more or less important chain of starch, known as dextrins. Beta-amylase is an amylase of mainly plant origin (this is the case of maltase extracted from the barley malt), which attack the alpha 1-4 linkages from the end of the chain and non-reductive release of maltose and dextrin. Note also that there is another called gamma-amylase amylase, found in yeast, fungi and in lysosomes of the liver of animals, hydrolysis links osidic alpha 1-4 and alpha 1-6 to starch.

Modified starch

Starch is often used without modification. However, certain uses require special properties and as a result of changes in starch are needed. The product resulting from the modification of starch is known as modified starch.

Glucose syrup (a mixture of glucose, maltose and dextrins) are obtained by acid or enzymatic hydrolysis of starch, the degree of hydrolysis is labeled by a DE (Dextrose Equivalent). Syrups These modified starches are used primarily confectionery.

The need of meats, sweets, etc.. low viscous starch can be met by oxidation of hydroxyl groups and aldehydes of free starch.

The demand for "solution" to be used as stable starch thickener is satisfied by esterification and etherification of the starch.

Glycogen

Glycogen is a polymer of glucose similar to the amylopectin, present in muscle (meat) and liver of animals. It describes the sugar glycogen reserve of animals and is sometimes called animal starch. It was under this form that the agency set aside the surplus of glucose. Glycogen is also found in some plants but with a very low rate. In the presence of iodine, glycogen is colored in red-brown-purple.

Following a meal rich in sugars, the blood may become dangerously too concentrated glucose. We say that the blood sugar rises. Cells (especially liver and muscles) then convert these excess glucose into glycogen molecules which helps to lower blood glucose.

glucose + glucose + glucose + ............ ==> glycogen

 

Conversely, if the blood glucose gets too low (after a few hours of fasting or intense physical activity), glycogen accumulated in the liver may shed new glucose passes into the bloodstream.

glycogen ==> glucose + glucose + glucose + ................

Unlike the starch, glycogen is soluble in water and the cell protoplasm, and it is insoluble in alcohol. The solubility of glycogen in water is due to its globular structure. In this structure, the hydrophilic hydroxyl groups are placed outside the mesh cells which makes them available.

Glycogen is more resistant to the action of concentrated alkali same. This property which does not starch is used to isolate the glycogen of animal tissues by extraction with potash and hot. The glycogen was then precipitated by adding ethanol.

Cellulose

Cellulose is the most abundant polysaccharide in the plant kingdom. Indeed, over 50% of plant carbon is in the form of cellulose. If the timber contains only 40 to 50% (relative to dry weight) of cellulose in plant fiber are much richer, as is the case for example of cotton which is made up almost 90% (of its total mass ) cellulose.

Like all the polysaccharides, cellulose is a polymer of glucose. It consists of long linear chains of glucose linked together. Against by the links between glucose are different from those of the starch or glycogen these links beta 1-4 can not be broken by digestive fluids of most animals. Result, we can not digest cellulose. It travels throughout the digestive tract intact and emerges at the other end. However, some herbivores also harbor in their intestines of colonies of bacteria that can digest them for a portion of the cellulose they eat. The enzymes responsible for this hydrolysis are called cellulases.

Cellulose is equipped with a high mechanical strength and chemical inertness: insoluble in water and organic solvents. Alkalis and dilute acids have no action on cellulose even hot. This resistance to the action of alkalis and acids is the basis for separation of cellulose in a mixture of polysaccharides.

However, cellulose is soluble in the liquor Schwitz (ammoniacal solution of cupric hydroxide). This property is the basis for the manufacture of artificial fibers: cellulose attacked by the liquor Schwitz gives a colloidal solution which is precipitated by the action of acids, bases or salts and gives "copper silk.

Cellulose is hydrolysed by hot concentrated acids. Next the acid used, the hydrolysis may be total (H2SO4) or partial (HCl). In the first case produces glucopyranose and the latter produces cellubiose, cellotriose counterparts and superiors.

The cellulose is used for various purposes: textile industry, paper, explosives industry (nitrocellulose), plasticisers industry materials (cellulose acetate), etc.. It is also an important part of what is called dietary fiber (non-digestible parts of foods) and plays an important role in food.

The glycosides

The glycosides are the osides that release by hydrolysis of oses and one or more groups aglycones. Chitin, the amygdalin and nucleic acids are examples of glycosides.

Chitin is a polymer similar to cellulose except that it is composed of amino glucose (glucose is a bound to an amine group NH2). Chitin is rigidified and hardened by deposits of calcium carbonate (CaCO3). Chitin forms the exoskeleton (carapace) of arthropods (spiders, insects, crustaceans).

The amygdalin result of the combination of gentiobiose and mandélonitrile (cyanohydrin of benzaldehyde). It is found in bitter almonds and cherry pits. Its hydrolysis releases hydrogen cyanide, hence the bitterness of the almonds and their toxicity.

Nucleic acids are combinations of ribose, of phosphoric acid (as phosphate) and amino bases (known purine and pyrimidine). The monomers are nucleotides, which are combined to form the RNA and DNA. A nucleotide is also the reserve of energy: ATP or adenine-ribose-triphosphate.

Chemical reactions of carbohydrates

Mutarotation

For most oses, a freshly prepared solution has an optical rotation unstable it evolves towards a constant value for each dare. This is the phenomenon of mutarotation. The mutarotation is not a chemical reaction but rather a property of Chemical oses.

For example, a fresh solution of glucose has a regular optical rotation of 112.2 ° to 20 ° C, it gradually evolves to meet a few hours worth of 52.5 ° to 20 ° C. This is due to the formation of two anomers alpha-D-glucose and beta-D-glucose after cyclisation (mutarotation) of glucose.

mutarotation of glucose

Hydrolysis

The hydrolysis of carbohydrates is the reaction of degradation osides small molecules in molecular weight: oses, disaccharides, dextrins, aglycones, and so on. This reaction is catalyzed by heat, acids, alkalis and / or enzymes. The products of the reaction are different depending on the osides and the catalyst used.

The hydrolysis reaction is of great importance in industry. It is used to produce a finished product that has different properties from that of the raw material or to produce carbohydrates that are rare or do not exist in nature from other rich carbohydrates such as starch and cellulose.

For example, the hydrolysis of corn starch is made commercially to produce glucose syrups are used in non-alcoholic beverages and other food products.

Reaction of esterification

The esterification reaction is a reaction of an acid and an alcohol. The product of the reaction is an ester. The reverse reaction is hydrolysis.

Acid + Alcohol <======> Ester + Water

Like all alcohols (carbohydrates have polyalcohols), the functions hydroxides (-OH) of dare react with acids, acid anhydrides and the acid chlorides to give esters. This reaction is called esterification of carbohydrates.

This reaction is used industrially to produce cellulose nitrate, explosives and flammable, from cellulose.

Reaction of etherification

The dare react with alcohols to give real connections ether. In practice this reaction is rather difficult and requires more energy than reactants methanol such as dimethyl sulfate or methyl iodide.

R1--- OH + R2--- OH =======> R1--- O --- R2+ H2O

(Reaction of etherification with OH --- R1is a daring and HO --- R2is an alcohol)

The interest of this reaction is its use to study the structure of oses.

Maillard Reaction

The Maillard reaction is the set of interactions resulting from the initial reaction between a reducing sugar and an amino group. This reaction has an enormous importance in the chemistry of food. It is the main responsible for the production of fragrances, flavors and pigments characteristics
cooked food. It may also give rise to carcinogenic compounds and also reduce the nutritional value of foods by degrading the essential amino acids. The Maillard reaction is developed in part related to chemical weathering reactions of food.

Utilization of carbohydrates

Carbohydrates constitute the majority of the sources of calories for human consumption. They are also used as raw materials or additives for the manufacture of several food products and others. Their use as additives in food products is mainly due to property stabilizing, thickening and gelling of certain carbohydrates.

Caramel is a very popular food product in cooking for its organoleptic and physical properties. This product is obtained by heating sucrose (sugar cooking) at temperatures above 180 °C.

The starch is used in the preparation of several foods such as livestock feed, pectin, gelling agents, production of adhesive materials and emulsions. Corn syrup is produced by a chemical treatment of maize starch in which the starch molecules are broken into glucose molecules. You can also convert a portion of glucose to fructose to increase the sweetness of the syrup. Glucose syrups are used in non-alcoholic beverages and other food products.

Wall polysaccharides of red algae, agar, or agar-agar and carrageenan, are used industrially for their gelling, emulsifying or stabilizing. The agar and its purified derivative, agarose, are used in the laboratory. The first is a basic constituent of solidified culture media for growing microorganisms, bacteria, fungi, micro-algae or plants. The second is support for the separation of macromolecules by electrophoresis. Carrageenan are mainly use in the food industry, particularly in the manufacture of ice cream, but also in the pharmaceutical, textile and printing.

Dextrin, soluble carbohydrate and amorphous, is obtained by the action of acids, heat or enzymes on the starch paste. The first product formed in this reaction is soluble starch, which then hydrolyzes to form the dextrin. Preparing dextrin commercial moistening slightly potato starch with nitric acid low, then by drying and heating the mass to 110 ° C. Dextrin is used in the production of beer, and as a substitute for gum arabic in the printing of cotton fabrics. It is also used as an adhesive.

With a mixture of nitric acid and sulfuric acid, cellulose forms a series of explosives and flammable compounds called cellulose nitrate or nitrocellulose. The pyroxiline is a cellulose nitrate used in various lacquers and plastics; another, collodion, a component of artificial leather and some varnish. Photographers use the alcohol-ether collodion to 0.4% of cellulose nitrate added bromide, chloride or iodide on glass. A third nitrate cellulose, fulmicoton, is an explosive propellant used in the cartridges. La cellulose est utilisée également pour la fabrication du papier, textil et matières plastiques.

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