Food Constituents Summary

8/13/2019 Food Constituents Summary

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Constituents of Foods



Constituents of Foods

Primary

Carbohydrates

Proteins

Fats

Water

Secondary

Vitamins

Enzymes

Pigments

Flavors



Carbohydrates



Carbohydrates

o Most widely distributed, naturally occurring

organic compounds

o Produced during the process of photosynthesis:

6CO2 + 6H2O+Energy → C6H12O6 + 6O2



What are carbohydrates?

o Definition: polyhydroxyl aldehydes and ketones

o Carbohydrates include monosaccharides,oligosaccharides and polysaccharides as well assubstances derived from monosaccharides by

Reduction of carbonyl → alcohols

Oxidation of carbonyl → carboxylic acids

Replacement of one or more hydroxyl group(s) by a hydrogenatom, an amino group (H2N), a thiol group (SH) or similarheteroatomic groups.



Occurrence of carbohydrates

Examples:

Cellulose

Starch

Chitin

Sucrose Lactose

Glucose



Monosaccharides:

Nomenclature and Structures



Monosaccharides

o Carbon compounds containing 3 to 9 carbons,

and at least one group has a carbonyl group

o It can not be broken down to simpler

carbohydrate molecules by hydrolysis



Classification of monosaccharides

o Size

– Size of base carbon chains

Triose 3

Tetrose 4

Pentose 5

Hexose 6

Heptose 7

Octose 8

Nonose 9



Classification ofClassification of monosaccharidesmonosaccharides

o Location of C=O

– Aldoses: H-(CHOH)n-CHO

Monosaccharides with an aldehydic carbonyl group

– Ketose: H-(CHOH)n-CO-(CHOH)m-H

Monosaccharides with a ketonic carbonyl group



Classification of monosaccharides

o Stereochemistry

– L-form

– D-form

A

E

C*

E

C* BB

A

DD

Mirror



D- and L-forms

Chiral center: asymmetric carbon atom

Carbohydrates: either D or L depending onconfiguration of the highest-numbered chiralcenters, with -OH on the chiral center either to theright (D) or to the left (L).

The D and L structures are the mirror images ofeach other (enantiomers).



Examples

H – C = O H – C = O CH2OH

HO – C* – H H – C* – OH C = O

CH2OH CH2OH CH2OH

L-glyceraldehyde D-glyceraldehyde dihydroxyacetone



Representations of the 2 stereoisomers of

glyceraldehyde



Numbering and abbreviation of sugars

o Fructose - Fru

o Glucose - Glc

o Galactose - Gal

o Mannose - Man

o Ribose - Ribo Xylose - Xyl

o Arabinose - Ara

1CHO

2C OHH

3C HHO

4C OHH

5C OHH

6CH2OH

1CH2OH

2C O

3C HHO

4C OHH

5C OHH

6CH2OH

D-glucose aldose D-Fructose ketose

Numbering of sugars



Stereochemical

relationship isshown here

D-Aldoses with three to six C atoms



D-Ketoses with three to six C

atoms

Sugar is a ketose if the carbonyl group

is a ketone



D refers to configuration at chiral center

most distance from carbonyl carbon



Enantiomers are not the same



oDifferent in the relativeorientations of bonds

oOnly be changed by breakingbonds and remaking them

oOne enantiomer can only be

changed into another enantiomerby chemical reaction

Enantiomers



Enantiomers



Definitions

o Isomers: Different compounds that have the

same molecular formula

o Enantiomers: Mirror-image isomers

o Diastereometers: Isomers that are not mirror-

image of each othero Racemic compounds: Mixtures of both

enantiomers



Epimers

o Two sugars that differ only in the configuration

around one carbon atom



Cyclic monsaccharide structures and anomeric

forms

Glucose (an aldose) can cyclize to form a

cyclic hemiacetal

Fructose (a ketose) can cyclize to form a

cyclic hemiketal

Cyclic form of glucose is a pyranose (six

membered ring with an oxygen)

Cyclic form of fructose is a furanose (five

membered ring)



Fischer vs. Haworth projections



Fischer vs. Haworth projections

Fischer to Haworth projection

Haworth to Fischer projection



Drawing Haworth formulas from the

open form

Easy rules

Ring size: pyranose = 6; furanose = 5

D-sugar: -CH2OH is up

L-sugar: -CH2OH is down

Other OH’s:

– Left in Fischer projection = up in Haworthformula

– Right in Fischer projection = down in Haworthformula

α = anomeric OH is opposite to –CH2OH

β = anomeric OH is same side as –CH2OH



α and β anomers (C-1 epimers) of D-

glucopyranose



Pyranoses and furanoses

anomers



Reactivity

o Carbonyl group of aldehydes is reactive and easily

undergoes nucleophilic attack by the unshared

electrons of oxygen atom of a hydroxyl group to

produce a hemiacetal.

o The hydroxyl group of a hemiacetal can react further

with a hydroxyl group of an alcohol to produce an

acetal and water.

o A carbonyl group of a ketone can go through the

similar reaction to produce a acetal.



Ring structure

o Nature favors the ring structure:

Reactivity

Thermodynamics



o Thermodynamic

– Thermodynamic equilibrium favors 6-

membered ring. Therefore, 6-membered ring

structure is predominant.



Carbohydrate Ring Structures

o Most common 5 carbon ring furanose

Furan is derived from the furan ring

Furan 5 membered ring with 1 oxygen

Ose ending indicates multiple hydroxyls

o

Most common 6 carbon ring pyranosePyranose derived from pyran ring

Pyran ring is 6 membered ring with 1 oxygen

Pyran

Furan



Ring Structure Conformation

oPyranose ring is not planar due to geometry of

saturated C atoms

oAdopt 2 classes of conformation



Derived carbohydrate structures

o Sugar alcohol

o Sugar acids

o Deoxy sugars

o Aminor sugarso Sugar phosphates



Sugar acids

o Obtained when oxidation of the aldehyde or

the primary alcohol group.



Amino sugars

o Obtained when the hydroxyl group is replaced

by an amino group



Sugar phosphates

o Many of the simple trioses, tetroses, and

pentoses do not occur naturally in the free

state but commonly found as phosphate-

ester derivatives.

o

The phosphoesters are importantintermediates in the breakdown and

systhesis of carbohydrates by living

organisms.

Food Constituents Summary

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