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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
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.
Molecular Food Biochemistry
Carbohydrates
Copyright © 1999-2008 by Joyce J. Diwan.
All rights reserved.
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.
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)
Where Do Carbohydrates Come From?
• Plants take in
• Carbon dioxide (CO2)
and water (H2O) +
heat from the sun and
make glucose.
• C6H12O6
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
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.
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.
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.
Disaccharide
Maltose
• Maltose is an example
of a disaccharide
• Notice it is two sugar
molecules together.
• Glucose + Glucose =
Maltose
The most common disaccharide is
Sucrose
• Sucrose is glucose +
fructose and is known
as common table
sugar.
Polysaccharide
• Polysaccharides are a
whole bunch or
monosaccharides
linked together.
• An example of a
polysaccharide is
starch.
Polysaccharide
• Polysaccharides are a
whole bunch or
monosaccharides
linked together.
• An example of a
polysaccharide is
starch.
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
* Diheteroglycans:
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)
Isomers
• Glucose
• C6H12O6
• Fructose
• C6H12O6
• Fructose sweeter than
glucose because of its
structure.
Glucose can be found in a ring structure or
linear structure
• In Water
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
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
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.
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.
Chitin
Polysaccharides are used in the shell
of crustaceans like crabs and lobsters.
Carbohydrates also serve as structural
elements.
• The chains sticking out of the proteins in the
cell membrane are polysaccharides known as
cell markers(glycoproteins).
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.
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.
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
Carbohydrates
•
•
•
•
•
Cx(H2O)y
70-80% human energy needs
>90% dry matter of plants
Monomers and polymers
Functional properties
– Sweetness
– Chemical reactivity
– Polymer functionality
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
Nomenclature: Classification of Carbohydrates
Number of carbons
Functional group
Ketone
Aldehyde
4
Tetrose
Tetrulose
5
Pentose
Pentulose
6
Hexose
Hexulose
7
Heptose
Heptulose
8
Octose
Octulose
9
Nanose
Nanolose
Table 1
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
CHO
CHO
H
OH
CH2OH
D-glyceraldehyde
ENANTIOMERS
HO
H
CH2OH
L-glyceraldehyde
Glucose
• Fisher projection
• D-series sugars are built on Dglyceraldehyde
• 3 additional chiral carbons
• 23 D-series hexosulose sugars
(based on D-glyceraldehyde)
• 23 L-series based on Lglyceraldehyde
• D-Glucose is the most
abundant carbohydrate
H
O
C-1
H
OH
C-2
H
C-3
H
OH
C-4
H
OH
C-5
H
OH
C-6
HO
H
Original D-glyceraldehyde carbon
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
H2C CH3
O
HO CH
HC OH
HC OH
C OH
H2
Carbohydrates (glycans) have the following
basic composition:
(CH2O)n
I
or H - C - OH
I
 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.
Monosaccharides
Aldoses (e.g., glucose) have an
aldehyde group at one end.
H
Ketoses (e.g., fructose) have
a keto group, usually at C2.
O
CH2OH
C
C
O
HO
C
H
OH
H
C
OH
OH
H
C
OH
H
C
OH
HO
C
H
H
C
H
C
CH2OH
CH2OH
D-glucose
D-fructose
D vs L configuration
CHO
CHO
D & L designations are
H C OH
based on the
CH2OH
configuration about
the single asymmetric D-glyceraldehyde
C in glyceraldehyde.
HO
H
C
OH
CH2OH
D-glyceraldehyde
H
CH2OH
L-glyceraldehyde
CHO
The lower
representations are
Fischer Projections.
C
CHO
HO
C
H
CH2OH
L-glyceraldehyde
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
C
H – C – OH
HO – C – H
H – C – OH
H – C – OH
CH2OH
D-glucose
O
H
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
L-glucose
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
C
H – C – OH
HO – C – H
H – C – OH
H – C – OH
CH2OH
D-glucose
O
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
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).
H
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.
H
C
H
O
+
R'
OH
R'
O
R
OH
R
aldehyde
alcohol
hemiacetal
R
C
C
R
O
+
"R
OH
R'
ketone
"R
O
C
R'
alcohol
hemiketal
OH
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.
1
H
HO
H
H
2
3
4
5
6
CHO
C
OH
C
H
C
OH (linear form)
C
OH
D-glucose
CH2OH
6 CH2OH
6 CH2OH
5
H
4
OH
H
OH
3
H
O
H
H
1
2
OH
-D-glucose
OH
5
H
4
OH
H
OH
3
H
O
OH
H
1
2
OH
-D-glucose
These representations of the cyclic sugars are called
Haworth projections.
H
CH2OH
1
HO
H
H
2C
O
C
H
C
OH
C
OH
3
4
5
6
HOH2C 6
CH2OH
D-fructose (linear)
H
5
H
1 CH2OH
O
4
OH
HO
2
3
OH
H
-D-fructofuranose
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.
6 CH2OH
6 CH2OH
5
H
4
OH
O
H
OH
3
H
H
2
OH
-D-glucose
H
1
OH
5
H
4
OH
H
OH
3
H
O
OH
H
1
2
H
OH
-D-glucose
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 OH
H OH
4 6
H O
HO
HO
H O
HO
H
HO
5
3
H
H
2
H
OH 1
OH
-D-glucopyranose
H
OH
OH
H
-D-glucopyranose
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.
Sugar derivatives
CHO
COOH
CH2OH
H
C
OH
H
C
OH
H
C
OH
CH2OH
D-ribitol
H
C
OH
HO
C
H
OH
H
C
OH
OH
H
C
OH
H
C
OH
HO
C
H
H
C
H
C
CH2OH
D-gluconic acid
COOH
D-glucuronic acid
 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.
Sugar derivatives
CH2OH
CH2OH
O
H
H
OH
H
H
OH
H
OH
OH
H
NH2
-D-glucosamine
O
H
H
H
O OH
OH
H
N
C
CH3
H
-D-N-acetylglucosamine
amino sugar - an amino group substitutes for a hydroxyl.
An example is glucosamine.
The amino group may be acetylated, as in
acetylglucosamine.
N-
H
O
H3C
C
O
NH
R
H
COO
H
R=
OH
H
HC
OH
HC
OH
CH2OH
OH
H
N-acetylneuraminate (sialic acid)
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.