CARBOHYDRATES - CHEMICAL STRUCTURE

INTRODUCTION

Carbohydrates consist of the elements carbon (C), hydrogen (H) and oxygen (O) with a ratio of hydrogen twice that of carbon and oxygen.

MONOSACCHARIDESNumber of

CarbonsCategory Name Examples

4 Tetrose Erythrose, Threose

5 Pentose Arabinose, Ribose, Ribulose, Xylose, Xylulose, Lyxose

6 Hexose Allose, Altrose, Fructose, Galactose, Glucose, Gulose,

Idose, Mannose, Sorbose, Talose, Tagatose

7 Heptose Sedoheptulose, Mannoheptulose

TETROSES

D-erythrose D-Threose

PentoseD-Ribose D-Arabinose

HEXOSE- 6 carbons

D-Gulose D-Idose

heptose

D-Sedoheptulose

D-Mannoheptulose

DISACCHARIDEDisaccharide Description Component

monosaccharidesucrose common table sugar glucose 1α→2

fructosemaltose product of starch

hydrolysisglucose 1α→4 glucose

lactose main sugar in milk galactose 1β→4 glucose

Sucrose Lactose

Maltose

Polysaccharide- polymers of simple sugar

Many polysaccharides, unlike sugars, are insoluble in water.

Dietary fiber includes polysaccharides and oligosaccharides

that are resistant to digestion and absorption in the human

small intestine but which are completely or partially

fermented by microorganisms in the large intestine.

Starch Starch is the major form of stored carbohydrate in

plants. Starch is composed of a mixture of two substances: amylose, an essentially linear polysaccharide, and amylopectin, a highly branched polysaccharide.

Amylose

Amylopectin: Amylopectin differs from amylose in being highly branched. Short side chains of about 30 glucose units are attached with 1α→6 linkages approximately every twenty to thirty glucose units along the chain. Amylopectin molecules may contain up to two million glucose units.

Glycogen

Glucose is stored as glycogen in animal tissues by the process of glycogenesis. When glucose cannot be stored as glycogen or used immediately for energy, it is converted to fat.

Cellulose

Hemicelluloses

Properties of carbohydrates

It is common for individuals to associate sugar (sucrose) with its

inherent taste properties.

Aside from providing a sweet taste and flavour, sugar imparts

numerous functional properties that make it an important

ingredient in many foods.

Specific functional properties of sugar can be classified into four

broad categories: sensory, physical, microbial and chemical

Sensory Properties

Taste - Sweetness is generally the most recognized functional property of sugar.

Caramelization - is a browning reaction that results from the action of heat on

sugars. At high temperatures, the chemical changes associated with melting

sugars result in a deep brown colour and new flavours.

Flavour - Flavours result when tastes (sweet, sour, bitter, salty) are combined

with sense of smell when food is consumed.

Texture - Sugar makes an important contribution to the way we perceive the

texture of food. For example, adding sugar to ice-cream provides body and

texture which is perceived as smoothness.

Condi., Tenderizer - Sugar acts as an important tenderizing agent in

foods such as baked products. During the mixing process, sugar compete with other ingredients for water.

Maillard Reaction - results from chemical interactions between sugars and proteins at high heat. An amino group from a protein combines with a reducing sugar to produce a brown colour in a variety of foods (e.g. brewed coffee, fried foods, milk & breads).

Appearance - Sugar is responsible for the yellow-brown colours that develop in baked foods. Sucrose itself develops colour through caramelization.

Physical Properties

Solubility - Sugar is easily soluble in water. The ability to produce solutions of varying sugar concentrations is important in many food applications.

A high level of solubility, for example, is essential in beverages to provide sweetness and to increase viscosity to create a desirable ‘mouthfeel'.

Freezing Point - Sugar is effective in lowering freezing points. Freezing point depression is an important property in ice-creams, frozen desserts and freeze-dried foods to ensure the development of fine crystal structure and product smoothness.

Boiling Point - The concentration of sugar in a solution affects the boiling point by raising it. This characteristic is important in candy manufacture as boiling point elevation allows for more sugar to be dissolved in solution, creating a ‘super saturated' and more concentrated solution.

Microbial Properties

Preservation - Sugar plays a role in the preservation of

many food products. The addition of sugar to jams and

jellies, for example, inhibits microbial growth and

subsequent spoilage.

Fermentation - Sugar is extremely important in the baking

and brewing industries. Yeasts use sugars as food to produce

ethanol, carbon dioxide and water through the process of

fermentation.

Chemical Properties

Antioxidant Activity - Sucrose has been reported to exhibit

antioxidant properties which help to prevent the deterioration of

textures and flavours in canned fruits and vegetables.

In addition, the products of the hydrolysis of sucrose (glucose and

fructose) appear to have the ability to block the reactive sites of ions

such as copper and iron and, to a lesser extent, cobalt. This

characteristic of monosaccharides aids in food preservation by delay

catalytic oxidation reactions.