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.

Carbohydrates

Copyright © 1999-2008 by Joyce J. Diwan. All rights reserved.

Molecular Food Biochemistry

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 is an example of a disaccharide

• Notice it is two sugar molecules together.

• Glucose + Glucose = Maltose

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.

Polysaccharides are used in the shell of crustaceans like crabs and lobsters.

Chitin

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.

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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)

29

• 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

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

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

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

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

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

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:

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

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

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

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).

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

+

+

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)

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

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

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

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

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

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 =