FOOD CHEMISTRY (CARBOHYDRATES)

CARBOHYDRATES

Earlier, it was believed that these are the compounds of carbon, hydrogen and oxygen in the ratio of 1:2:1. But all the available carbohydrates were not in accordance with this ratio, so the definition has been modified.

According to the modern definition, these are optically active polyhydroxy aldehydes or ketones or the compounds producing these molecules when subjected to hydrolysis. General Formula of the carbohydrates is C_X(H₂O)_Y , e.g,. Glucose {C₆H₁₂O₆  or C₆(H₂O)₆ }, fructose, sucrose, etc. These are primarily produced by plants.

Cereals like wheat, maize, rice, corn, oat, barley, potato, beet root, banana etc are the major sources of carbohydrates.

CLASSIFICATION OF CARBOHYDRATES

1- On the basis of their behaviour on hydrolysis

• MONOSACCHARIDES– These are the simplest form and cannot be hydrolysed e.g,. triose (glyceraldehyde), tetrose (erythrose), pentose (ribose), hexose (glucose, fructose, galactose), etc.

• OLIGOSACCHARIDES– They yield 2 to 10 monosaccharide units upon hydrolysis. e.g,. sucrose, maltose, lactose, etc all give two molecules of monosaccharides on hydrolysis, so these are also called Disaccharides.

       SUCROSE= GLUCOSE+ FRUCTOSE

MALTOSE= GLUCOSE+ GLUCOSE

• POLYSACCHARIDES– They yield a large number of monosaccharide units upon hydrolysis. e.g,. starch, cellulose, gums, glycogen, etc. These are usually insoluble in water and extracted from plants.

2- ON THE BASIS OF THEIR PHYSICAL PROPERTIES

• SUGARS– Carbohydrates which are sweet in taste, are called sugars. e.g,. sucrose, lactose, etc.
• NON-SUGARS– Carbohydrates which are not sweet in taste, are called non-sugars. e.g,. all polysaccharides.

3- On the basis of their reducing properties

• REDUCING CARBOHYDRATES– They reduce Tollen’s reagent into silver mirror and Fehling’s solution into red precipitates. It includes all monosaccharides and disaccharides(except sucrose).
• NON-REDUCING CARBOHYDRATES– These are unable to reduce Tollen’s reagent and Fehling’s solution.

SOME COMMON CARBOHYDRATES

1- Glucose (C₆H₁₂O₆)

It is also known as dextrose. It is present in sweet fruits, ripe grapes and honey. Commercially, it is obtained from starch upon hydrolysis. It is the main sugar metabolised by the body for energy. Its concentration in blood stream ranges between 70-115 mg/100 mL. It provides instantaneous energy to the human body.

2- Fructose (C₆H₁₂O₆)

It is also known as fruit sugar. It is present in honey, other fruits and high fructose corn syrup. Natural fructose is also called laevulose. It is used in medicinal syrups, toffies, etc.

3- Maltose (C₁₂H₂₂O₁₁)

It is composed of two glucose units. It is also known as malt sugar and is used in alcohol production.

4- Lactose (C₁₂H₂₂O₁₁)

It is found in milk of mammals. 4-6 % in cow milk and 5-8 % in human milk. It is also called milk sugar. It is composed of galactose and glucose.

5- Sucrose (C₁₂H₂₂O₁₁)

It is obtained from sugarcane and is also known as cane sugar. On hydrolysis, it gives glucose and fructose in equal amounts and the product is called invert sugar. it ia an excellent preservative which is present in many jarred foods like jams.

6- Starch (C₆H₁₀O₅)_n

It is found in cereals, roots, tubers and some vegetables. On digestion, it gives maltose which further decomposes to give glucose. It is a polymer of glucose and it consists of two components i.e,. amylose(water soluble) and amylopectin(water insoluble). It acts as food reserve of plants.

7- Cellulose (C₆H₁₀O₅)_n

It occurs exclusively in plants. It is a straight chain polysaccharide of glucose. It is used for making shatterproof glass.

8- Glycogen

In animal body, carbohydrates are stored as glycogen, so it is known as animal starch. it is present in liver, muscles and brain. When our body needs glucose(energy), enzymes break glycogen into glucose.

IMPORTANCE OF CARBOHYDRATES

1- They form a major portion of our food and supply energy to the body by the process of oxidation. They also form nucleic acids.

2- These are used as storage molecules such as starch in plants and glycogen in animals.

3- Cellulose in the form of wood is used in making furniture.

4- They provide raw materials for textiles(cotton fibres), liquor and beverages.

POINTS TO REMEMBER

• Glucose because of its reducing nature is used for silvering mirrors.
• Fructose is much sweeter than sucrose.
• Starch (amylose) gives blue violet colour with I₂(Iodine) solution.
• Cotton wool obtained from cotton plant is made up of cellulose.
• Galactose is not generally found in nature in large quantities, however it combines with glucose to give lactose in milk.
• Gum is a polymer of more than one type of monosaccharides. These are acidic polysaccharides.
• Cellulose is not digestible by humans because our digestive system does not contain cellulose enzyme. It is present in stomach of ruminant mammals(cow, deer) and termites.