Principles of Food Science and Nutrition CARBOHYDRATE PART

Principles of Food Science and Nutrition CARBOHYDRATE PART I Dr. Nahlaa Khalifa

{1} Carbohydrates

The elements which make up carbohydrates are: l Carbon (C) l Hydrogen (H) l Oxygen (O)

Classification of carbohydrates l * Classification based on the number of sugar units in total chain: l monosaccharides Single sugar unit l disaccharides Two sugar units l*oligosaccharides 3 to 10 sugar units l. Polysaccharides More than 10 units not studied

Carbohydrates - Saccharides Carbohydrates Simple Carbohydrates Monosaccharide Disaccharide Complex Carbohydrates Glycogen Starch Fibers

MONOSACCHARIDES l l l Is a simple carbohydrate The sweet taste of edible sugars especially fructose Dissolves in water Straight chain or ring structure Monosaccharides are seldom found free in nature , but are linked into di- and polysaccharides forms. The only three hexose monosaccharides which can be absorbed by humans are, glucose , galactose and fructose

Simple Carbohydrates - Monosaccharides Glucose Galactose Fructose

Important Monosaccharides (Glucose) • Glucose, from the Greek word for sweet • Glucose, sometimes called dextrose or blood sugar, was • • isolated from raisins in 1747 and by hydrolysis of starch in 1811. Its structure was determined in 1900 by Emil Fischer. Is the best known and most widely distributed sugar in nature , although it is seldom consumed in its monosaccharide form In polymer form, glucose is present as starch and cellulose and found in all edible disaccharides

Important Monosaccharides (Glucose) • Glucose linked with fructose as the disaccharide , sucrose , makes up a large fraction of the total solid content of fruits and vegetables. • The end product of its digestion is glucose • In fruits and vegetables the contents of glucose and fructose depend on species , ripeness and state of preservation • The concentration of glucose in human blood is fairly constant; in the range of 70– 100 mg per 100 m. L of blood. Cells use this glucose as a primary energy source. b-D-glucose

Important Monosaccharides (Galactose) l l l Galactose, from the Greek word for milk, is similar to glucose. D-Galactose, synthesized in the mammary glands, is incorporated into lactose, the sugar in milk. Is rarely free in nature. Most dietary galactose is produced from lactose (milk sugar ) by hydrolysis during the digestive process D-Galactose is present in the chemical markers that distinguish the various types of blood: A, B, AB, and O.

Important Monosaccharides (Galactose) b-D-glucose b-D-galactose Note difference on carbon 4

Galactosemia • One baby out of every 18, 000 is born with a genetic defect of not being able to utilize galactose. Since galactose is part of lactose (milk sugar), it builds up in the blood and urine. • Undiagnosed, it may lead to mental retardation, failure to grow, formation of cataracts, and in severe cases death by liver damage. • The treatment for the disorder is to use a baby formula based upon sucrose (table sugar) rather than milk with lactose. The dairy-free diet is only necessary in infancy. Other enzymes are developed later in life that can metabolize galactose, but galactose intake must still be kept low.

Galactosemia

Important Monosaccharides (Fructose) • • • Fructose, from the Latin word for fruit, . Also known as levulose, it is found in honey and fruits. It is the sweetest-tasting of all sugars. Fruits contain 1% to 7% fructose Fructose makes up about 3% of the dry weight in vegetables and about 40% in honey (1 tablespoon contain 64 kcal , whereas an equal amount of sugar contains 46 kcal). As fruits ripens , it become sweeter because the sucrose it contains is enzymatically cleaved to glucose and fructose. The end product of its digestion is fructose

Important Monosaccharides (Fructose) b-D-fructose

Disaccharides Glucose Monosaccharides = Sucrose lactose Maltose

Disaccharides Maltose Sucrose Lactose

Disaccharides

Disaccharides (Maltose) Maltose, also called malt sugar, is composed of 2 glucose units joined with an a(1 4) glycosidic linkage. a-D-glucose maltose

Disaccharides (Lactose) Lactose, also called milk sugar, is composed of one galactose and one glucose unit joined with a b(1 4) glycosidic linkage. b-D-galactose a-D-glucose + H 2 O lactose

Lactose Intolerance Digestion of lactose is facilitated by the lactase enzyme. A deficiency of this enzyme makes it difficult to digest lactose and causes abdominal discomfort (nausea, cramps, bloating, gas, and diarrhea). l Lactose intolerance is a genetic trait. l

Disaccharides (Sucrose) Sucrose, also called table sugar, is composed of one glucose and one fructose unit joined with an a, b(1 2) glycosidic linkage. a-D-glucose b-D-fructose

Disaccharides (Sucrose) l Sugar beet (a vegetable) and sugar cane (a grass) naturally contain particularly large amounts of sucrose. l Certain insects, particularly honeybees, have enzymes called invertases that catalyze the hydrolysis of sucrose to a glucose-fructose mixture. Honey, in fact, is primarily a mixture of these three sugars.

Sugar cane Sugar beet

Complex carbohydrates l Polysaccharides contain thousands of monosaccharides linked together. Because of their size, they are not water soluble. l Three important polysaccharides, all of which are polymers of D-glucose, are starch, glycogen, and Dietary fiber.

Complex carbohydrates • • • Starch is the principle food reserve of plants. Glycogen functions as a carbohydrate reserve for animals. Fibers (Cellulose) serves as structural material in plants.

Polysaccharides (Starch) l l l Plant store carbohydrate as starch granules Starch is stored in plastids (storage vesicles) called leukoplasts. All the starchy foods derived from plants. The giant starch molecules are packed in grains such as rice, wheat in root crops such as yam and potatoes and in legumes such as peas and beans. Grains are the richest food source of starch providing much of food energy for people all over the world – rice in Asia , Wheat in Canada , the United Sates and Europe , corn in much of Central and South America and millet , barley and oats elsewhere.

Polysaccharides (Starch) l l l There are two types of starch. Amylose and amylopectin Amylose (10– 20%) is a smaller , linear molecules with unbranched chains with a(1 4) glycosidic linkages Amylopectin (80– 90%) has a larger size and highly branched polymer with a(1 4) and a(1 6). glycosidic linkages Amylose abbreviated structure

Specific starch characteristics l Starch granules are encased by rigid cellulose walls and so are inaccessible to digestive enzymes . This accounts for the poor digestibility of the raw potato and grains . Cooking causes the granules to swell , gelatinize the starch softens and rupture the cell wall and makes the starch more digestible. l Resistant starch resists enzyme breakdown from cooking and is poorly digested.

Specific starch characteristics l l l Starch from different plant sources , such as corn, rice , potato, are all glucose polymers with the same chemical composition. Their unique characteristics are determined by the number of glucose units in straight and branched configuration Waxy starch is obtained from corn , rice When it dissolve in water , waxy starch forms a smooth paste that does not gel unless in very high concentration Dextrin are large linear glucose polysaccharides of intermediate lengths cleaved from starch by amylase.

Polysaccharide( Glycogen) l l Structurally, glycogen is very similar to amylopectin. but the branches in glycogen are shorter , more highly branched Found in small amount in meat but hydrolyzed after slaughter. Not found in plants The human body stores much of its glucose as glycogen – many of glucose molecules linked together in highly branched chains. This arrangement permits rapid hydrolysis when the hormonal message “Release energy” message arrive storage sites in the liver or muscle cell, enzymes respond by attacking many branches of each glycogen simultaneously , making a surge of glucose available.

Polysaccharide( Glycogen) l Because it is a carbohydrate , glycogen contain water molecules . The additional water makes glycogen large molecule unsuitable for long term energy storage. The 70 -kg man stored only 18 - hours fuel supply as glycogen , compared to a 2 month supply as fat. So if all human energy were glycogen , it has been estimated that humans would need a lot of kg in his body.

Starch & Glycogen