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Food BiotechnologyDr. Kamal E. M. Elkahlout
Food Biochemistry 1 Carbohydrates
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Carbohydrates• Comprised of > 90% of dry matter of plants
• Abundant, widely available and inexpensive
• Common components of food– As natural components and added ingredients
• Commonly consumed in quantities and varieties of products
• Different molecular structures, size and shape– Exhibit variety of chemical and physical properties
• Therefore amendable in both chemical and biochemical modification to improve properties and extending uses
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Chemical nature
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Classification• Monosaccharides
• Disaccharides
• Oligosaccharides
• Polysaccharides– Starch– Dietary fibre
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(Carbonyl)
1
1
2
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Aldose and ketose• Aldose:• An aldehyde
• is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom which is bonded to a hydrogen atom and double-bonded to an oxygen atom (chemical formula O=CH-).
– Also called the formyl or methanoyl group.
• Ketose:– A ketone
• either the functional group characterized by a carbonyl group (O=C) linked to two other carbon atoms or a chemical compound that contains the functional group.
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Glucose
Fructose
Galactose
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Disaccharides• 2 monosaccharide units joined together
– 2 sugar units
– Maltose• 2 glucose units (α 1-4 linkages)
– Cellobiose• 2 glucose units (β 1-4 linkages)
– Lactose• Galactose + glucose (β 1-4 linkages)
– Sucrose• Glucose + fructose (α 1-2 linkages
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Glycosidic Bonds• Formed between the free carbonyl
group of one monosaccharide and a hydroxyl group of another monosaccharide
• Readily hydrolyzed by – Heat and acid– Certain enzymes such as
sucrase,invertase, amalyaes
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Maltose• also known as malt sugar • actual name = 4-O-(a -D-
glucopyranosyl)- a -D-glucopyranose • obtained by enzyme-catalyzed
hydrolysis of starch • composed of two D-glucopyranoses • 1,4’-a -glycoside bond (1,4’-a -
linkage) • reducing sugar, contains hemiacetal
function and can mutarotate • easily digested by humans
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Lactose• milk sugar (both human and cow) • actual name = 4-O-(b -D-galactopyranosyl)-b -
D-glucopyranose • milk sours when lactose is converted to lactic
acid (tastes sour) • composed of D-galactose and D-glucose • 1,4’-b -glycoside bond (1,4’-b -linkage) • digested by most humans, lactose intolerant
individuals often lack the enzyme, lactase, which hydrolyzes the glycosidic linkage. If a person becomes lactose intolerant, they often they quit producing lactase, or it becomes ineffective.
• reducing sugar
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Sucrose• table sugar (from sugar beets or sugar cane) • actual name = 2-O-(a -D-glucopyranosyl)-b -D-
fructofuranoside • composed of one glucose and one fructose molecule • "invert sugar" is produced by the hydrolysis of sucrose.
The hydrolysis produces a 1:1 mixture of glucose:fructose. This is called "invert sugar" because the optical rotation changes from +66.5o to -22.0o upon hydrolysis.
• 1.2’-glycoside linkage (beta-D-fructoside and alpha-D-glucoside)
• not a reducing sugar, no free hemiacetal, this implies both glucose and fructose must be glycosides
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Reducing Sugars• Contain a free carbonyl group• All monosaccharides are reducing sugars
• Disaccharides are reducing sugars only if they contain a free carbonyl group
• Sucrose is not a reducing sugar-carbonyl group of both glucose and fructose is involved in the glycosidic bond.
• Reducing sugars give brown colors to baked goods when they combine with free amino acids-Maillard reaction.
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Oligosaccharides• 3-10 sugar units• Important oligosaccharides
– raffinose and stachyose. – Composed of repeating units of galactose, glucose
and fructose– nutritional importance because they are found in
beans and legumes. – unique glycosidic bonds– raffinose and stachyose cannot be broken down into
their simple sugars. • therefore, they cannot be absorbed by the small
intestine and are often metabolized by bacteria in the large intestine to form unwanted gaseous byproducts.
• commercial enzyme preparations such as Beano can be consumed before a meal rich in beans and legumes in order to aid the small intestine in the breakdown of these oligosaccharides.
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Polysaccharides or complex carbohydrates
• More than 10 units of sugar • Usually monomers and consist of thousands of repeating glucose
units. • Allow for the storage of large quantities of glucose. • Starch is the major storage form of carbohydrate in plants and has
two different types: amylose and amylopectin. • Although digestible alpha glycoisidic bonds link both types of
starch, each type is unique in their branching of glucose. – amylose is a straight chain polymer– amylopectin is highly branched.
• These differences account for the fact that – amylopectin can form stable starch gels which are able
to retain water – while amylose is unable to do so.
» Therefore, amylopectin is often used by manufacturers to produce many different kinds of thick sauces and gravies. Sources of starch include potatoes, beans, bread, pasta, rice and other bread products.
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Amylose• several hundred glucose molecules with
1,4’-a -glycoside bonds• molecular weight of 150,000 to 600,000 • 1000 to 4000 glucose units per molecule
• little or no branching • chains coil in the form of a helix. In the Iodine test, I2 molecules fit into the helix
and causes the intense blue color
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Amylopectin• more complex than amylose
• 1,4’-a -glucose linkages for the main polysaccharide chain
• 1,6’-a glycoside linked branches approximately every 25 units • very complex 3-d structure
• up to 1 million glucose units per molecule
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Indigestible polysaccharides• Dietary fiber and come in many different forms
including cellulose, hemicellulose, pectin, gum and mucilage.
• Cellulose is by far the most abundant biochemical compound on the earth – because it forms part of the structure of many plants. – unique among polysaccharides in that it forms
intramolecular hydrogen bonds between adjacent glucose units as well as beta 1,4 glycosidic bonds present in other carbohydrates.
– these special bonding characteristics allow cellulose to form long, straight chains of glucose and give it strength and rigidity that many plants require for proper growth.
– Cellulose and most forms of hemicellulose are insoluble fibers while pectin, gum and mucilage are all soluble fibers and readily dissolve or swell when mixed with water.
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Tutorial1. Define the following and give an example
of each:a. Monosaccharidesb. Disaccharidesc. Polysaccharides
2.Differentiate between:d. aldose and ketoses.e. Amylose and amylopectin
3. What is lactose intolerance?4. Why can’t we use galactose?5. What is galactosemia?