food chemistry by dr boominathan ph.d. m.sc.,(med. bio, jipmer), m.sc.,(fgs, israel), ph.d (nus,...
TRANSCRIPT
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FOOD CHEMISTRY
BY
DR BOOMINATHAN Ph.D. M.Sc.,(Med. Bio, JIPMER), M.Sc.,(FGS, Israel), Ph.D (NUS, SINGAPORE)
PONDICHERRY UNIVERSITY
1/August/2012
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Food Science/Chemistry
• Food science is an interdisciplinary subject involving primarily bacteriology, chemistry, biology, and engineering.
• Food chemistry, a major aspect of food science, deals with the composition and properties of food and the chemical changes it undergoes during handling, processing, and storage.
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Carbohydrates
Copyright © 1999-2008 by Joyce J. Diwan. All rights reserved.
Molecular Food Biochemistry
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Carbon Chemistry• Carbon atoms can form single, double or triple bonds
with other carbon atoms.• Carbon can form up to 4 bonds• This allows carbon atoms to form long chains, almost
unlimited in length.
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Macromolecules
• “GIANT MOLECULES”• Made up of numerous of little molecules.• Formed from a process known as
polymerization, in which large molecules are produced by joining small ones together.
• The small units (monomers), join together to form large units (polymers)
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Where Do Carbohydrates Come From?
• Plants take in • Carbon dioxide (CO2)
and water (H2O) + heat from the sun and make glucose.
• C6H12O6
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Carbohydrates
• As the name implies, consist of carbon, hydrogen, and oxygen.
• Hydrate=(water) hydrogen and oxygen.• The basic formula for carbohydrates is C-H2O,
meaning that there is one carbon atom, two hydrogen atoms, and one oxygen atom as the ratio in the structure of carbohydrates
• What would be the formula for a carbohydrate that has 3 carbons.
• C3H6O3
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Carbohydrate
• Fancy way of saying sugar. • Carbohydrates are energy packed compounds,
that can be broken down quickly by organisms to give them energy.
• However, the energy supplied by carbohydrates does not last long, and that is why you get hungry every 4 hours.
• Carbohydrates are also used for structure.
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Saccharides
• Scientist use the word saccharides to describe sugars.
• If there is only one sugar molecule it is known as a monosaccharide
• If there are two it is a disaccharide• When there are a whole bunch, it is a
polysaccharide.
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Glucose is a monosaccharide
• Notice there is only one sugar molecule.
• Glucose is the main fuel for all living cells.
• Cells use glucose to do work.
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Disaccharide
• Maltose is an example of a disaccharide
• Notice it is two sugar molecules together.
• Glucose + Glucose = Maltose
Maltose
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The most common disaccharide is Sucrose
• Sucrose is glucose + fructose and is known as common table sugar.
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Polysaccharide
• Polysaccharides are a whole bunch or monosaccharides linked together.
• An example of a polysaccharide is starch.
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Polysaccharide
• Polysaccharides are a whole bunch or monosaccharides linked together.
• An example of a polysaccharide is starch.
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Polysaccharide
• 90% of the considerable carbohydrate mass in nature is in the form of polysaccharides.
• Polysaccharides can be either linear or branched. • The general scientific term for polysaccharides is glycans.• Homoglycan & Hetroglycan• Homoglycan: glycosyl units are of the same sugar type.
Eg., Cellulose and Starch amylose (linear) * Starch amylopectin (branched) • Hetroglycan:
two or more different monosaccharide units
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* Diheteroglycans:
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Most of the names of carbohydrates end in -ose
• Glucose-What plants make • Maltose- used in making beer (disaccharide)• Fructose – found in fruit (monosaccharide)• Sucrose- Table sugar (disaccharide)• Lactose – In milk (disaccharide)
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Isomers
• Glucose• C6H12O6
• Fructose• C6H12O6
• Fructose sweeter than glucose because of its structure.
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Glucose can be found in a ring structure or linear structure
• In Water
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Dehydration Synthesis
• Sounds technical but all it really means is taking out the water and making some thing new.
• Dehydration is what happens to you when you don’t drink enough water.
• Synthesis means “making some thing new”
• In this case we are taking out water and connecting glucose with fructose to make sucrose (table sugar)
Fructose
Sucrose
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Hydrolysis Hydro=water lysis= break apart
• Hydrolysis breaks down a disaccharide molecule into its original monosaccharides.
• Hydrolysis, it means that water splits a compound.
• When sucrose is added to water, it splits apart into glucose and fructose.
• It is just the opposite of dehydration
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What do we do with all the sugar?
• Plants store glucose in the form of polysaccharides known as starch in their roots .
• Animals store glucose in the from of a polysaccharide known as glycogen in our liver and muscle cells.
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Cellulose
• The most abundant organic molecule on earth.
• Gives trees and plants structure and strength.
• Most animals can not break the glucose linkage by normal means of hydrolysis. Need special enzymes.
• We need cellulose (fiber) to keep our digestive tracts clean and healthy.
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Polysaccharides are used in the shell of crustaceans like crabs and lobsters.
Chitin
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Carbohydrates also serve as structural elements.
• The chains sticking out of the proteins in the cell membrane are polysaccharides known as cell markers(glycoproteins).
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How Sweet It Is
• The human tongue has four basic taste qualities.
• Bitter• Salty• Sour• Sweet• We perceive taste qualities
when receptors on our tongue send a message to our brain.
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Its all about how tightly the molecules fit into the receptors on the tongue.
• The chemical structure of a compound determines its shape, which in turn will determine how well it will fit into a receptor.
• Compounds that bind more tightly to “sweet” taste receptors send stronger “sweet” messages to the brain.
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TASTE• Taste buds: mostly on tongue• Two types
– Fungiform papillae (small, on entire surface of tongue)– Circumvallate papillae (inverted “V” near back of tongue)
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• Taste buds of 50-100 epithelial cells each
• Taste receptor cells (gustatory cells)
• Microvilli through pore, bathed in saliva
• Disolved molecules bind & induce receptor cells to generate impulses in sensory nerve fibers
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Carbohydrate Structure
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Carbohydrates
• Cx(H2O)y
• 70-80% human energy needs• >90% dry matter of plants• Monomers and polymers• Functional properties
– Sweetness– Chemical reactivity– Polymer functionality
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Simple Sugars
• Cannot be broken down by mild acid hydrolysis
• C3-9 (esp. 5 and 6)• Polyalcohols with aldehyde or ketone
functional group• Many chiral compounds• C has tetrahedral bond angles
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Nomenclature: Classification of Carbohydrates
Ketone Aldehyde
4 Tetrose Tetrulose
5 Pentose Pentulose
6 Hexose Hexulose
7 Heptose Heptulose
8 Octose Octulose
Num
ber o
f car
bons
Functional group
Table 1
9 Nanose Nanolose
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Chiral Carbons• A carbon is chiral if it has four different groups• A chiral carbon atom is one that can exist in two
different spatial arrangements (configurations). • Chiral compounds have the same composition but
are not superimposable (two different arrangements of the four groups in space (configurations) are nonsuperimposable mirror images of each other)
• Display in Fisher projection
CH2OH
H OH
CHO
CH2OH
OH H
CHO
D-glyceraldehyde L-glyceraldehyde
ENANTIOMERS
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Glucose• Fisher projection• D-series sugars are built on D-
glyceraldehyde• 3 additional chiral carbons• 23 D-series hexosulose sugars
(based on D-glyceraldehyde)• 23 L-series based on L-
glyceraldehyde• D-Glucose is the most
abundant carbohydrate
H O
H
OHH
HOH
OHH
OHH
OHH
Original D-glyceraldehyde carbon
C-1
C-2
C-3
C-4
C-5
C-6
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D-Fructose
• A ketose sugar found abundantly in natural foods
• One less chiral carbon than the corresponding aldose (only 3)
• Sweetest known sugar• 55% of high-fructose corn
syrup • and about 40% of honey
CH2
CH
CH
CH
CH2
OH
OH
OH
OH
O
CH3
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Monosaccharides - simple sugars with multiple OH groups. Based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose.
Disaccharides - 2 monosaccharides covalently linked. Oligosaccharides - a few monosaccharides covalently
linked. Polysaccharides - polymers consisting of chains of
monosaccharide or disaccharide units.
I (CH2O)n or H - C - OH
I
Carbohydrates (glycans) have the following basic composition:
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Monosaccharides
Aldoses (e.g., glucose) have an aldehyde group at one end.
Ketoses (e.g., fructose) have a keto group, usually at C2.
C
C OHH
C HHO
C OHH
C OHH
CH2OH
D-glucose
OH
C HHO
C OHH
C OHH
CH2OH
CH2OH
C O
D-fructose
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D vs L configuration
D & L designations are based on the configuration about the single asymmetric C in glyceraldehyde.
The lower representations are Fischer Projections.
CHO
C
CH2OH
HO H
CHO
C
CH2OH
H OH
CHO
C
CH2OH
HO H
CHO
C
CH2OH
H OH
L-glyceraldehydeD-glyceraldehyde
L-glyceraldehydeD-glyceraldehyde
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Sugar Nomenclature
For sugars with more than one chiral center, D or L refers to the asymmetric C farthest from the aldehyde or keto group.
Most naturally occurring sugars are D isomers.
O H O H C C H – C – OH HO – C – H
HO – C – H H – C – OH
H – C – OH HO – C – H
H – C – OH HO – C – H
CH2OH CH2OH
D-glucose L-glucose
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D & L sugars are mirror images of one another.
They have the same name, e.g., D-glucose & L-glucose.
Other stereoisomers have unique names, e.g., glucose, mannose, galactose, etc.
O H O H C C H – C – OH HO – C – H
HO – C – H H – C – OH
H – C – OH HO – C – H
H – C – OH HO – C – H
CH2OH CH2OH
D-glucose L-glucose
The number of stereoisomers is 2n, where n is the number of asymmetric centers.
The 6-C aldoses have 4 asymmetric centers.
Thus there are 16 stereoisomers (8 D-sugars and 8 L-sugars).
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Hemiacetal & hemiketal formation
An aldehyde can react with an alcohol to form a hemiacetal.
A ketone can react with an alcohol to form a hemiketal.
O C
H
R
OH
O C
R
R'
OHC
R
R'
O
aldehyde alcohol hemiacetal
ketone alcohol hemiketal
C
H
R
O R'R' OH
"R OH "R
+
+
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Pentoses and hexoses can cyclize as the ketone or aldehyde reacts with a distal OH.Glucose forms an intra-molecular hemiacetal, as the C1 aldehyde & C5 OH react, to form a 6-member pyranose ring, named after pyran.
These representations of the cyclic sugars are called Haworth projections.
H O
OH
H
OHH
OH
CH2OH
H
OH
H H O
OH
H
OHH
OH
CH2OH
H
H
OH
-D-glucose -D-glucose
23
4
5
6
1 1
6
5
4
3 2
H
CHO
C OH
C HHO
C OHH
C OHH
CH2OH
1
5
2
3
4
6
D-glucose (linear form)
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Fructose forms either a 6-member pyranose ring, by reaction of the C2 keto
group with the OH on C6, or a 5-member furanose ring, by reaction of the C2 keto
group with the OH on C5.
CH2OH
C O
C HHO
C OHH
C OHH
CH2OH
HOH2C
OH
CH2OH
HOH H
H HO
O
1
6
5
4
3
2
6
5
4 3
2
1
D-fructose (linear) -D-fructofuranose
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Cyclization of glucose produces a new asymmetric center at C1. The 2 stereoisomers are called anomers, & .
Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1:
(OH below the ring) (OH above the ring).
H O
OH
H
OHH
OH
CH2OH
H
-D-glucose
OH
H H O
OH
H
OHH
OH
CH2OH
H
H
OH
-D-glucose
23
4
5
6
1 1
6
5
4
3 2
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Because of the tetrahedral nature of carbon bonds, pyranose sugars actually assume a "chair" or "boat" configuration, depending on the sugar.
The representation above reflects the chair configuration of the glucopyranose ring more accurately than the Haworth projection.
O
H
HO
H
HO
H
OH
OHHH
OH
O
H
HO
H
HO
H
H
OHHOH
OH
-D-glucopyranose -D-glucopyranose
1
6
5
4
32
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Sugar derivatives
sugar alcohol - lacks an aldehyde or ketone; e.g., ribitol. sugar acid - the aldehyde at C1, or OH at C6, is oxidized
to a carboxylic acid; e.g., gluconic acid, glucuronic acid.
CH2OH
C
C
C
CH2OH
H OH
H OH
H OH
D-ribitol
COOH
C
C
C
C
H OH
HO H
H OH
D-gluconic acid D-glucuronic acid
CH2OH
OHH
CHO
C
C
C
C
H OH
HO H
H OH
COOH
OHH
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Sugar derivatives
amino sugar - an amino group substitutes for a hydroxyl. An example is glucosamine. The amino group may be acetylated, as in N-acetylglucosamine.
H O
OH
H
OH
H
NH2H
OH
CH2OH
H
-D-glucosamine
H O
OH
H
OH
H
NH
OH
CH2OH
H
-D-N-acetylglucosamine
C CH3
O
H
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N-acetylneuraminate (N-acetylneuraminic acid, also called sialic acid) is often found as a terminal residue of oligosaccharide chains of glycoproteins.
Sialic acid imparts negative charge to glycoproteins, because its carboxyl group tends to dissociate a proton at physiological pH, as shown here.
NH O
H
COO
OH
H
HOH
H
H
RCH3C
O
HC
HC
CH2OH
OH
OH
N-acetylneuraminate (sialic acid)
R =