Food Biotechnology
Dr. Kamal E. M. Elkahlout
Food Biochemistry 1
Carbohydrates
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
Chemical nature
Classification
• Monosaccharides
• Disaccharides
• Oligosaccharides
• Polysaccharides
– Starch
– Dietary fibre
(Carbonyl)
1
1
2
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.
Glucose
Fructose
Galactose
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
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
Maltose
• also known as malt sugar
• actual name = 4-O-(a -Dglucopyranosyl)- 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
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
Sucrose
• table sugar (from sugar beets or sugar cane)
• actual name = 2-O-(a -D-glucopyranosyl)-b -Dfructofuranoside
• 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-Dglucoside)
• not a reducing sugar, no free hemiacetal, this implies
both glucose and fructose must be glycosides
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.
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.
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.
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
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
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.
Tutorial
1. Define the following and give an example
of each:
a. Monosaccharides
b. Disaccharides
c. Polysaccharides
2.Differentiate between:
a. aldose and ketoses.
b. Amylose and amylopectin
3. What is lactose intolerance?
4. Why can’t we use galactose?
5. What is galactosemia?