Carbohydrates Practical Report

7/24/2019 Carbohydrates Practical Report

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Introduction

Carbohydrates are one of the most important components in many foods.

Carbohydrates may be present as isolated molecules or they may be

physically associated or chemically bound to other molecules. Individual

molecules can be classied according to the number of monomers that theycontain as monosaccharides, oligosaccharides or polysaccharides.

Monosaccharides

Monosaccharides are water-soluble crystalline compounds. They are

aliphatic aldehydes or ketones which contain one carbonyl group and one or more

hydroyl groups. Most natural monosachharides have either ve !pentoses" or si

!heoses" carbon atoms.. The reactive centers of monosaccharides are the carbonyl

and hydroyl groups.

Oligosaccharides

These are relatively low molecular weight polymers of monosaccharides !# $%" that

are covalently bonded through glycosidic linkages. &isaccharides consist of two

monomers, whereas trisaccharides consist of three. 'ligosaccharides containing

glucose, fructose and galactose monomers are the most commonly occurring in

foods.

Polysaccharides

The ma(ority of carbohydrates found in nature are present as polysaccharides.)olysaccharides are high molecular weight polymers of monosaccharides !* $%".

)olysaccharides containing all the same monosaccharides are

called homopolysaccharides !e.g., starch, cellulose and glycogen are formed from

only glucose", whereas those which contain more than one type of monomer are

known asheteropolysaccharides !e.g., pectin, hemicellulose and gums".

Molecules in which the carbohydrates are covalently attached to proteins

are known as glycoproteins, whereas those in which the carbohydrates are

covalently attached to lipids are known as glycolipids. +ome carbohydrates

are digestible by humans and therefore provide an important source ofenergy, whereas others are indigestible and therefore do not provide energy.

umerous tests have been devised for the determination of the properties

and for the dierentiation of carbohydrates . The main ob(ective of this

eperiment is to observe the physical and chemical properties of some

common carbohydrates. &ierences in their properties can be signicantly


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seen through several analytical and eperimental tests. This helps greatly in

the characteriation of the carbohydrates according to their similar physical

and chemical properties. To be more specic this physical and chemical

properties observed throughout the eperiment helps to distinguish between

monosaccharides,disaccharides and polysaccharides.

Results

Benedicts Test

No Sample Observation Interpretation/ 0lucose Reddish orange colour is

formed

It is a reducing

sugar$ 1ructose Reddish orange colour is

formed

It is a reducing

sugar2 +ucrose No changes Remains

blue in colour

It is a non

!reducing sugar3 4actose Reddish orange colour

but "ith a lighter shade

is formed

#lthough it is a

disaccharide$ it

is a reducing

sugar5 +tarch No changes Remains

blue in colour

It is non

reducing sugar6 7ater No changes Remains

blue in colour

It acts as a

negative

control of thee%periment

Seli"ano&s test

No Sample Observation Interpretation' 0lucose No changes

Remains

colourless

(etose is

absent

) 1ructose *rom colourless

it turns tocherry red in a

fast rate+*ast

colour change,

(etose is

present

- +ucrose *rom colourless

it turns to

cherry red "ith

(etose is

absent


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lesser intensity

and slo"er

rate+Slo"

colour change,. 4actose No changes

Remains

colourless

(etose is

absent

/ +tarch No changes

Remains

colourless

(etose is

absent

0 7ater No changes

Remains

colourless

(etose is

absent #cts as

a negative

control of the

e%perimentIodine test

No Sample Observation Interpretation' 0lucose 1ight bro"n

colour is

formed

#mylose is

absent It is a

monosaccharid

e) 1ructose 1ight bro"n

colour is

formed

#mylose is

absent It is a

monosaccharid

e- +ucrose 2ar3 bro"n

colour is

formed

#mylose is

absent It is

disaccharide. 4actose 2ar3 bro"n

colour is

formed

#mylose is

absent It is a

disaccharide/ +tarch Blue!blac3

solution is

formed

#mylose is

present It is a

polysaccharide0 7ater 1ight yello"ish

bro"n solution

is formed

#mylose is

absent It acts

as a negative

control of the

e%periment


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4ydrolysis of 2isaccharides and Polysaccahrides

Sucrose54)

O

Sucrose 546l Starch54)7 Starch546l

Iodine test 1ight bro"n

colour

6olourless Blue blac3

solutionformed

6olourless

solution

8enedict9s

test

1ight blue

solution

+Negative

result,

Reddish

orange

solution is

formed

+Positive

result,

1ight blue

solution+Negat

ive result,

8ery little

red

precipitate

is formed

:ydrolysis

product

present

No

hydrolysis

too3 place

4ydrolysis

too3 place

Sucrose is

hydrolysed

into glucose

and fructose

No hydrolysis

too3 place

Starch

might be

hydrolysed

into

maltose

4ence this

culd be a

false

positive

result

Barfoeds Test

No Sample Observation Interpretation

' 9lucose Red precipitate is formed ina faster rate

It is amonosaccahrid

e) *ructose Red precipitate is formed in

a faster rate

It is a

monosaccharid

e- Sucrose No red precipitate is Not a


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formed monosaccharid

e. 1actose No red precipitate is

formed

Not a

monosaccharid

e

/ Starch No red precipitate is

formed

Not a

monosaccharid

e0 :ater No red precipitate is

formed

Not a

monosaccharid

e #cts a

negative

control of the

e%periment

Osa;one Test

No Sample Observation Interpretation' 0lucose Broomstic3 li3e

shaped crystals

are formed

It is a

glucosa;one

) 1ructose Broomstic3 li3e

shaped crystals

are formed

It is a

fructosa;one

- 4actose Po"derpu& li3eballs of crystals

are formed

It is alactosa;one

. Maltose *lo"er shaped

crystals are

formed

It is a

maltosa;one

/ 7ater No crystals are

formed

#cts as a

negative

control of the

e%periment


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Bials test

No Sample Observation Interpretation

' 0lucose


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2iscussion

Post lab =uestions

7hen the Cu$; in 8enedict9s solution is reduced , the copper is reduced

from ;$ to ;/. The glucose and galactose are monosaccharides consisting of

aldehyde group. 7hen the 8enedict9s solution is reduced the sugar will be

oidied where the aldehyde group in the sugar will be oidied into a

carboylic acid group.

In +eliwanao test, 1ructose will form a red color more rapidly since it is aketose sugar. +eliwanouivalent of amylopectin and is found in liver

and muscle. 4ike starch it is an energy storage compound. In short, =mylose

is a type of starch stored in plant cells, while glycogen is used to storeenergy in animal cells. The iodine testis used to see if a substance contains

starch or not.

= reducing sugaris any sugar that is capable of acting as a reducing agent

because it has a free aldehyde group or a free ketone group. =ll

monosaccharides are reducing sugars, along with some disaccharides,

oligosaccharides, and polysaccharides.=ll monosaccharides are reducing

sugars because all monosaccharides have an aldehyde group !if they are

aldoses" or can tautomerie in solution to form an aldehyde group !if they

are ketoses".The eamples of common monosaccharides are like galactose$glucose$ glyceraldehyde$ fructose$ ribose$ and %yloseMany

disaccharides, like cellobiose$ lactose and maltose, also have a reducing

form, as one of the two units may have an open-chain form with an aldehyde

group. 8esides that glucose polymers such as starch and starch-derivatives

like glucose syrup$ maltode%trin and de%trinare also reducing sugar.


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Moreover, glycogenis also another eample of reducing sugar but only with

one reducing end.

= non-reducing sugar is a sugar that cannot donate electrons to other

molecules and therefore cannot act as a reducing agent. Sucroseis the

most common nonreducing sugar. The linkage between the glucose andfructose units in sucrose, which involves aldehyde and ketone groups, is

responsible for the inability of sucrose to act as a reducing sugar. Trehalose

and starchare also another eamples of non-reducing sugar

= ketose is a monosaccharide containing one ketone group per

molecule.7ith three carbon atoms, dihydroyacetone is the simplest of all

ketoses and is the only one having no optical activity. ?etoses can isomerie

into an aldose when the carbonyl group is located at the end of the molecule.

+uch ketoses are reducing sugars. @amples of ketose sugars are fructose ,

ribulose, ylulose and erythrulose .

8enedictuently change color. = positive 8enedict


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The principle of the 8enedict

Carbohydrates Practical Report

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