How it works : Carbohydrate


Carbohydrates are a group of naturally occurring compounds which include a variety of foodstuffs and fibres. Natural sugars, starch (bread, corn, potatoes and many vegetables) and cellulose (cotton, wood and many other natural fibres) are the best known carbohydrates.

Above: a transverse section,about five microns thick,of the stem of the aquatic plant Hippuris vulgaris (Mare's tail).The part stained red is the structure containing cellulose.

Carbohydrates were originally classified together when analysis showed that they contained only three elements, carbon, hydrogen and oxygen, and that the proportion of the hydrogen and oxygen atoms were the same as in water, H2O. As compounds which contain water are known as hydrates and the general chemical formula of this group is Cx (H2O)y, they were originally called hydrates of carbon, or carbohydrates.Later research showed that this formulation is an oversimplification.

Like most organic compounds, carbohydrates have a structure based on a carbon chain, with the other atoms, in this case hydrogen and oxygen, arranged around this chain. In carbohydrates the hydrogen and oxygen are also found as the hydroxyl group (-OH) which is the characteristic group of the ALCOHOL family.

Left: testing the specific gravity (density) of starch suspensions.This is used for checking the progress of the dextrose crystallization process above or for commercial quality control.

Right: a test for estimating sugar in urine either to diagnose diabetes or to check the insulin intake of a diabetic. A reagent tablet is reacted with the urine in solution. After a 15 second wait the colour is compared with a chart. Blue is negative, dark orange shows 2% sugar. A spot test on specimen paper strips is also used.

The simplest carbohydrates are the monosaccharides, which contain only a few carbon atoms. These compounds are generally sweet tasting, hence the name, which comes from the Greek word for sugar, sakkharon. The more complex carbohydrates are formed by linking monosaccharides together. These monosaccharides are divided into different groups according to how many carbon atoms they contain. The best known groups are the pentoses, with five carbon atoms, and the hexoses with six carbon atoms in their molecules. Monosaccharides with other numbers of carbon atoms exist but these are less common.

Above: a pile of topped sugar beets,the second major source of sugar,which is extracted with hot water from the sliced beets.

Glucose (C6H12O6), which is soluble in water and has a sweet taste, is the most abundant hexose. it plays a vital role as an energy source in both animal and plant metabolism. Sunlight is used by plants to make glucose from atmospheric carbon dioxide (CO2) and water (H2O) by a process known as photosynthesis. Rather than storing glucose in solution, it is much more conveniently stored in nature as POLYMERS (long chain molecules) which are largely insoluble in water. These polymers, glycogen in animals and starch in plants, are made by linking glucose molecules together and as each link is formed a molecule of water is eliminated. Although starch and glycogen are formed by the same link between the glucose molecules, there are about twice as many branches in glycogen as there are in the starch chain.

Glucose was first prepared synthetically by Emil Fischer in 1887, but it is so readily available from natural sources that no commercial process for its manufacture has been developed. One of the major difficulties in chemical synthesis of mono saccharides is that for any combination of carbon, hydrogen and oxygen atoms a variety of different structures, each known as an isomer, is possible. Glucose, for example, is one of sixteen possible monosaccharides all with the formula C6H12O6; only three of these isomers exist naturally. Natural synthesis uses specific enzymes (naturally occurring CATALYSTS) which can produce one isomer to the exclusion of all others. Laboratory chemical processes, however, tend to be less specific and mixtures of the different isomers are produced which are difficult to separate. Glucose is prepared commercially by HYDROLYZING starch, that is, breaking down the polymer links and restoring the missing elements of water to reform glucose. Mineral acids can be used to hydrolyze starch but increasingly now a process using commercial preparations of natural plant enzymes is being used.

Above: a molecular model of the soluble hexose sugar,glucose,also known as dextrose. It forms a ring structure enabling various combinations,known as isomers,of hydrogen (blue) atoms and hydroxyl groups (red with blue) above or below the carbon ring.Shown here are a-D-glucose (left) and b-D-glucose (right).

When two monosaccharides are combined with the elimination of a molecule of water between them a disaccharide is formed. So when glucose and another hexose, fructose, which is found in many fruits, are joined in a specific way, a disaccharide is formed called sucrose. This is commonly known as sugar. Sugar is an essential food ingredient and its consumption in the developed countries exceeds 100 lb (45.4 kg) per head per annum. Sugar is obtained by the extraction of sucrose from sugar cane, sugar beet and sugar maple, all of which contain up to 20% by weight of sucrose.

If sucrose is hydrolyzed, a mixture of glucose and fructose is obtained which is called invert syrup; the name was given to the mixture because a solution of it rotates POLARIZED light in the opposite way to a sucrose solution. Honey has a similar composition to this syrup but obtains its flavour from traces of other compounds present in the plants used by the bees for obtaining nectar which is converted into honey.

Above:Crystallizer tanks of refined high-conversin hydrolzate from which dextrose is crystallized over a period of three days. Maize,the raw material for the process,is first soaked in dilue acid and passed through grinders. This breaks up the husk providing the germ (used for oil),gluten,outer husk and starch slurry. The starch slurry undergoes hydrolysis by acid and enzymes to give a thick hydrolzate syrup. The pure dextrose monohydrate (a form of sugar) is used in foods and beverages, intravenous feeding and so on.

Germinating grain, where the breakdown of the stored starch is incomplete, contains another disaccharide, maltose, which is two glucose molecules linked together. Milk has a small amount of a disaccharide, lactose made from glucose and another hexose, galactose. It is obtained by evaporation from whey, which is a by-product in cheese making.

Polysaccharides are powdery compounds, usually insoluble in water and tasteless. They consist of monosaccharides combined to form compounds of high molecular weight.

Starch and cellulose occur abundantly in plant life. Starch is found stored in the seeds and tubers as a future food supply, while cellulose is the chief constituent of plant cell walls. Both are formed by the polymerization of glucose, that is the linking together of many glucose molecules, but the geometry of polymerization is different in each case.

In starch the link makes the polymer take a helical form so that starch molecules tend to pack together as granules. in cellulose the glucose units adopt a straight chain structure,which gives it a fibrous character. Bread contains about three parts of a branched form of starch (amylopectin) and one part of an unbranched form (amylose).

Starch is also used in a number of industries, for example in the dressing of fabrics and in giving specialized types of finish to paper (known as sizing). Partial breakdown of starch and cellulose provides a range of ADHESIVES, such as dextrin or 'British gum'. Glycogen, or animal starch, occurs in the liver and muscle tissue and is necessary for muscular action.

Cellulose is the main constituent of natural fibres such as cotton, wood, flax and hemp. Several other useful products can be obtained if cellulose is modified chemically. Nitro-celluloses (mote correctly known as cellulose nitrates) are obtained if cotton wool is treated with a mixture of nitric and sulphuric ACIDS. One of these highly nitrated celluloses, guncotton, explodes on impact or when strongly heated. Two high explosives, nitroglycerine and guncotton, are blended together with a little Vaseline to give another explosive -cordite. Cellulose nitrate containing less nitrogen (formerly called collodion cotton) is not explosive and is used to make artiticial silk, imitation leather book binding cloth and in lacquer production. A solution of collodion cotton in a mixture of ether and alcohol is called collodion and is used for coating metals and in medicine for covering wounds. The first artificial silk, Chardonnet silk, was made from collodion in 1884 and the first plastic, celluloid, was made in 1865 by mixing collodion cotton and camphor in alcohol.

Other modified celluloses have been used recently as ingredients in slimming foods. They provide the bulk which makes the slimmer feel full, but as the body is unable to break them down, they pass through the system and have no nutritional value.

The man-made fibre, rayon, is made from cellulose either by forcing a viscous solution of cellulose acetate in acetone through fine holes into warm air where the solvent evaporates leaving tiny threads of cellulose acetate, or, more usually, by the viscose process. Here cellulose and caustic soda react with carbon disulphide to give a xanthate, called viscose. extrusion through fine holes (in a spinneret) into an acid solution produces rayon threads.By extruding the viscose through rollers, in an acid medium, a film, better known as CELLOPHANE, is formed.

Reproduced from HOW IT WORKS p465