Carbohydrates is one of the six classes of foods and most of our daily meals are made up of them. Carbohydrates are naturally occurring compounds of carbon, hydrogen, and oxygen in which the ratio of atoms of hydrogen and oxygen is 2:1, as in water molecules. They conform with the general molecular formula Cx(H2O)y.
Carbohydrates are generally polyhydroxyalkanals and polyhydroxyalkanone with at least three carbon atoms. The mode of classification used for carbohydrates in the past is a bit different from the one that is used now.
In the past, Carbohydrates were classified into two broad groups: sugars and non-sugars, based on the following physical properties[taste and solubility in water]. Sugars are crystalline, sweet in flavor, and soluble in water, to give neutral solutions. Examples include glucose, fructose, and sucrose which is the popular table sugar.
Nonsugars are non-crystalline, tasteless, and insoluble in water and examples include starch and cellulose.
Carbohydrates are classified in the modern day into monosaccharides, disaccharides, and polysaccharides. Monosaccharides are carbohydrates that cannot be split into simpler molecules by hydrolysis. They are simple sugars[glucose, fructose, and galactose].
Disaccharides are carbohydrates that can be split into two molecules of monosaccharides by acid hydrolysis. They are also sugars Polysaccharides are polymers of carbohydrates that on hydrolysis split into many molecules of the same or different types of monosaccharides. They are non-sugars and examples include starch, cellulose, and glycogen.
Monosaccharides
Glucose and Fructose are the most common monosaccharides. They are all isomers with the molecular formula C6H12O6.
Chemistry Of Glucose C6H12O6
Glucose occurs naturally in honey and sweet fruits, such as ripe oranges, grapes, and pawpaw, and to some extent, in the blood of animals. Palmwine owns its sweetness due to the presence of glucose.
Methods Of Preparation Of Glucose
There are two methods to prepare Glucose namely from sucrose and from starch. When sucrose solution is boiled with dilute HCl, hydrolysis occurs, to produce an equimolar mixture of glucose and fructose, which can be separated by fractional crystallization:
C11H22O11(aq) + H2O → C6H12O6(aq) + C6H12O6(aq)
From Starch
When starch, a polymer of glucose(e.g potatoes) is boiled with dilute HCl in an alcoholic solution, hydrolysis occurs to give a mixture, from where glucose can be crystallized as a white solid.
(C11H22O11)n(aq) + nH2O → C6H12O6(aq) + nC6H12O6(aq)
Physical Properties Of Glucose
Glucose is a white solid that crystallizes with a water molecule, C6H12O6.H2O, and melts at 85oC. Its anhydrous form melts at 145oC. Its high melting point is due to the extensive intermolecular hydrogen bonds between the molecules. Another physical property of glucose is that it has a sweet taste and it is very soluble in water and alcohol.
Chemistry Of Fructose C6H12O6
Fructose occurs together with glucose in honey and in juices of sweet fruits such as mangoes, pawpaw, and pineapple. Fructose is prepared by boiling sucrose with dilute HCl or H2SO4 .
Disaccharides
Disaccharides are sugars formed by the condensation reaction between two molecules of monosaccharides, which can be the same or different. The most common disaccharide is sucrose, maltose, and lactose. They are all structural isomers with C12H22O11 as their molecular formula. Generally, when a disaccharide is boiled with a dilute mineral acid, or in the presence of a suitable enzyme, it undergoes hydrolysis to produce monosaccharides.
Chemistry Of Sucrose C12H22O11
Sucrose is the pure form of the popular table sugar that is obtained from sugar cane. When sugar cane is crushed, and then squeezed to obtain the raw juice. The crushed sugar canes are then sprayed with water, and gain squeezed repeatedly to obtain the impure raw juice. After the raw juice is obtained, it is boiled with calcium oxide(quick lime) and the precipitated impurities are filtered before the filtrate is concentrated by partial evaporation of water, and the juice is allowed to crystallize.
After crystallization, the mother liquor called molasses is further concentrated and then crystallized in order to obtain more crystals of sugar, together with the dilute molasses which is the filtrate. The raw sugar is refined by the addition of a calculated quantity of water, in order to wash off the molasses.
After being washed off, it is then dissolved in water to obtain brown syrup, which can be decolorized by the use of animal charcoal to give colorless syrup. The colorless syrup is evaporated partially and allowed to crystallize before then filtered and ten dried to obtain white granulated sugar. The granulated sugar is moistened with a small quantity of water, compressed into cubes, and sold under various trade names like St Louis.