Carbohydrates PDF

Food Composition and Chemistry CARBOHYDRATES  Carbohydrates make up most of the organic structure of all plants, as well as, being present to some extent in all animals. Carbohydrates from the major part of the diet and provide 60 – 70 % of the energy in Indian diets. They are f...

Food Composition and Chemistry CARBOHYDRATES  Carbohydrates make up most of the organic structure of all plants, as well as, being present to some extent in all animals. Carbohydrates from the major part of the diet and provide 60 – 70 % of the energy in Indian diets. They are found in a wide range of foods. In plants, carbohydrates are synthesized by the photosynthetic process in the presence of sunlight. All carbohydrates are made up of carbon, hydrogen and oxygen and have twice as many hydrogen atoms as oxygen and carbon. Carbohydrates are sometimes referred as Saccharides. The simplest carbohydrates are called sugars (or monosaccharides) and these may link together to form more complex carbohydrates (oligo- or poly-saccharides).  Classification of carbohydrates CLASS EXAMPLES SOME SOURCES OF FOOD Monosaccharide Glucose bread, rice, pasta, potatoes, vegetables, fruit, sugar Fructose Fruits, honey, vegetables, corn syrup Galactose Occurs only from the digestion of lactose Disaccharide Sucrose Sugar cane, sugar beet maple sugar, small amounts in fruits and some vegetables Maltose Cereal malts and sprouted legumes, fermented doughs Lactose Milk Poysaccharide Starch Grains and grain products, legumes, potato and other root vegetables, green banana Glycogen (animal Liver and muscle of animals starch) Dextrin Partial breakdown of starch by heat or in digestion Cellulose Bran of cereal grains, skins and fibre of fruits and vegetables Pectin Guava, apple  Monosaccharides Monosaccharides are the simplest carbohydrate molecules. The most commonly occurring monosaccharides in food are glucose, fructose and galactose. The formula for glucose is C6H12O6 1. Glucose: It is widely distributed in nature and found in fruits, vegetables and cell sap. It is the most abundant carbohydrate found in corn sugar and is present in corn syrup, honey and molasses. In the animal body it is an end product of digestion of starch, sucrose, maltose and lactose. Glucose is found in the blood of all animals where it serves as source of instant fuel or energy for the body. Glucose is oxidized in the body to produce energy. 2. Fructose: This is also a monosaccharide and is also known as fruit sugar. It occurs in fruits and honey. Fructose consumed in the diet is rapidly converted into glucose in the body. 3. Galactose: Galactose does not occur in the free state in common foods. It occurs in combination with glucose in lactose (a disaccharide) present in milk.  Disaccharides Disaccharides are formed when two monosaccharide molecules join together by glycosidic bonds with the elimination of one molecule of water. They have the general formula C12H22O11. Examples of disaccharides are sucrose (glucose and fructose), lactose (glucose and galactose) and maltose (2 molecules of glucose). Sucrose 1. Sucrose: Sucrose occurs in large amounts in sugarcane, palm juice (palm sugar) and beet root (beet sugar). It occurs along with glucose and fructose in several fruits. On hydrolysis, it gives a mixture of glucose and fructose known as invert sugar. 2. Lactose: It occurs in the milk of mammals (Cow’s milk and buffalo’s milk 4 %). It is formed by the combination of one molecule each of glucose and galactose in the mammary gland. Lactose is hydrolysed in digestive tract into glucose and galactose. 3. Maltose: This occurs in malt prepared from germinated cereal grains. On hydrolyses it gives two molecules of glucose.  Polysaccharides Polysaccharides are made up of many monosaccharide molecules (usually glucose), joined together. They have the general formula (C6H10O5)n where ‘n’ is a large number. Examples of polysaccharides are starch, glycogen, cellulose, beta glucan, pectin etc. 1. Starch: Plants store carbohydrates in the form of starch and it is the main source of nourishment for human race. It contains two polymers composed of glucose units: amylose (linear) and amylopectin (branched). Cereal grains, seeds, roots like potato, tapioca, yam, colocasia and plantain contain considerable amount of starch. On cooking starch absorbs water and it swells and ruptures. This thickening quality of starch is used in cookery to produce a variety of dishes. All cooked starches are broken down into glucose in the digestive system. 2. Dextrin: When starch is partially broken into fragments either by digestion or by acids the compounds called dextrins are formed. Dextrin on further hydrolysis is broken down into maltose. 3. Pectin: It is a polysaccharide with no nutritional significance. Pectins are water-soluble carbohydrates. These are present in vegetables and fruits and are prepared mainly from waste citrus peel and apple skin. It has jellying characteristic. 4. Glycogen: Animals store carbohydrate in the body as glycogen. It is stored in the liver and muscles. This is the form of immediate energy for the body. It is also known as animal starch. 5. Cellulose: It is an insoluble, indigestible polysaccharide. Cellulose consists of an unbranched (linear) chain of several thousand glucose units joined by β glycosidic linkages. Since the human digestive enzymes cannot hydrolyze them, these are resistant to digestion.  The Functional Role of Sugars in Foods Sweetness  Carbohydrates differ in sweetness based on composition and source and may be ranked in decreasing order of sweetness as fructose, sucrose, glucose, lactose, dextrin and starch.  Raw fruits are bland in taste as they contain starch which is broken down into simple sugars during the ripening process with development of sweet taste.  Texture:  In bakery applications, sugars are used to impart flavour, aroma and colour.  During the mixing process, addition of sugar will ensure that gluten maintains an optimal elasticity, allowing the dough to expand and rise properly. Sugars compete for water with gluten proteins, allowing the dough to rise at an optimal rate during leavening.  The naturally occurring irregular surface texture of the sugar crystals encourages yeast growth and effectiveness by providing an immediate and easily accessible source of nourishment.  In bakery products, sugar is recrystallized as water is removed during baking, resulting in a crisp texture. This crispness is increased by the effects of browning (maillard reaction), which takes place when reducing sugars and nitrogen-containing ingredients(e.g. protein) are heated together.  Thickener:  starch absorbs water and swells up. If the amylose content of the starch is more, its long water soluble chains increase the viscosity and thickens gravies, sauces and pudding. Fermentation:  The production of chemicals by fermenting various sugars has been in practice since long. Ethanol has been made since ancient times by the fermentation of sugars. All beverage ethanol and more than half of industrial ethanol is still made by this process.  Yeast changes the simple sugars into ethanol and carbon dioxide. The fermentation reaction can be represented by the following simple equation. C6H12O6 2CH3CH2OH + 2CO2  Preservation  In many products, sugars play an important role in preservation. The addition of monosaccharides, such as glucose or fructose, to jams and jellies inhibits microbial growth and subsequent spoilage.  Sugars have a great affinity for water, thus slowing moisture loss in foods, like baked foods and extending the shelf-life of these products.  Sugars are added to canned vegetables both to maintain firmness and minimize oxidation when the can is opened. Inhibiting oxidation reactions protects against deterioration of texture and flavour.  Appearance /colour  The browning reactions are complex reactions which occur when foods are processed.  In coffee, the brown crust of bread and all baked goods, potato chips, roasted nuts and many other processed foods controlled browning is necessary.  Two major types of non-enzymatic browning reactions have been recognized to occur in foods during processing. These reactions include: – 1) Maillard Reaction: reaction of aldehyde and ketone groups of sugars with amino compounds (mostly amino acids, peptides, proteins), independent of the presence of oxygen. – 2) Caramelization: the change which occurs in polyhydroxycarbonyl compounds (sugars and sugar acids) when they are heated to high temperature independent of the presence of oxygen.  Effect of food processing on carbohydrates Gelatinization: On heating starch in the presence of water, the crystalline structure of the starch granules is lost irreversibly by a process called gelatinization. It is due to absorption of water by starch granules and turning into a jelly like substance. In this process, amylopectin forms the gel and amylase comes into solution. In the food processing, the starch granules are not completely dissolved however, their gelatinization is sufficient to allow a good part of the starch to be digested rapidly.