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IDENTIFICATION TEST FOR CARBOHYDRATES

ID TEST PRINCIPLE TEST FOR POSITIVE RESULT

Molisch Test This test is given by almost all of the carbohydrates. In this test, concentrated sulfuric General test for carbohydrates (+) Purple colored product.
acid converts the given carbohydrate into furfural or its derivatives, which react with The appearance of purple or
α-naphthol to form a purple-colored product. violet ring confirms the
presence of carbohydrates.

Anthrone Test If carbohydrate is present in the form of free carbohydrate as poly- or Anthrone test is a group test (+) All carbohydrates give a
monosaccharide or bound as in a glycoprotein or a glycolipid, the concentrated acid for carbohydrates that positive bluish green color.
in the Anthrone reagent first hydrolyses it into component monosaccharide. Similarly, provides a rapid and
the concentrated acid then catalyzes the dehydration of the monosaccharides to form convenient method for
furfural (from pentoses) or hydroxyl furfural (from hexoses). The furfural or hydroxyl quantification of carbohydrates
furfural condenses with two molecules of naphthol from the Anthrone reagent to form that are either free or bound to
a blue-green complex. any lipids or proteins.

Benedict's Test In an alkaline medium, sodium carbonate converts glucose to enediol and this This test is given by reducing (+) The appearance of red
enediol reduces cupric to cuprous forming cuprous hydroxide. This solution is kept in sugars. precipitate confirms the
sodium citrate and on boiling, red precipitate of cuprous oxide is formed. presence of carbohydrates.

Nylander’s Test The principle of Nylander’s Test revolves around the inherent chemical properties of A chemical test used for (+) The appearance of black
certain carbohydrates, specifically those possessing an aldehyde group or a free detecting the presence of precipitates within a short
ketone. The carbohydrates with either an aldehyde group or a free ketone have the reducing sugars duration posts the test
ability to reduce bismuth subnitrate. As a result of this reduction process, bismuth indicates a positive result. This
subnitrate is converted into its elemental form, black bismuth. This transformation is means that the sample
visually evident as black precipitates form in the solution, serving as a clear indicator contains reduced sugars.
of the presence of such reducing carbohydrates

Barfoed's Test Barfoed’s test is also based on the reduced ability of sugar. However, sucrose also It is a differentiating test to (+) The formation of red
 gives this test positive as it undergoes hydrolysis in the presence of an acid. distinguish between precipitates after the initial first
Monosaccharides give an early positive test while disaccharides give a late positive. monosaccharides and 5 minutes indicates the
Reducing sugars undergo tautomerization in a mildly acidic medium to form enediols. disaccharides. presence of a
These enediols reduce cupric ions to cuprous ions, which form cuprous hydroxide. monosaccharide
This cuprous hydroxide is converted to cuprous oxide on heating, leading to the
formation of precipitates. This principle resembles Benedict’s test, except for the (+) If precipitates are formed
acidic environment. Monosaccharides, being strong reducing agents, give this test after 15 minutes, a
early positive results. disaccharide is present in the
test solution

Fehling’s Test To the aqueous solution of carbohydrate, Fehling's solution is added and heated in a This test is given by reducing (+) The appearance of red
water bath. The formation of red precipitate confirms the presence of reducing sugars. precipitate confirms the
sugars. The copper ions present in Fehling's solution in +3 state is reduced to +2 presence of carbohydrates.
oxidation state and in alkaline medium it is precipitated as red cuprous oxide.

Seliwanoff’s Test This test involves the formation of furfural derivatives by monosaccharides with This test is used to detect (+) A cherry red color forms in
hydrochloric acid. The furfural derivatives formed by sugar with a ketonic functional monosaccharides with a the test tube upon cooling
group condense with resorcinol to form a chromogen with cherry-red color. ketonic functional group. indicating that the given
solution contains a keto-sugar.
It is widely used to differentiate
fructose, a keto sugar, from
glucose and galactose

Bial’s Test It is a general test for carbohydrates and is sensitive only to pentoses. Any It is a general test for (+) The blue color indicates
compound that contains a pentose sugar will give a positive Bial’s test. Pentoses carbohydrates the presence of pentose sugar
form furfural compounds in the presence of concentrated acid. The furfural in the test solution.
compounds formed by pentoses condense with orcinol to form blue-colored
compounds.

Tauber’s Test / Tauber's benzidine test: It is more specific for pentoses. It can detect up to 0.01 mg Test for Ketopentoses (+) Appearance of Bright-
Aminoguanidine Test of pentose either in its free or combined form. It’s a test for ketoses like Seliwanoff reddish purple color.
and it is known to be very sensitive and the most specific test for Ketopentoses.

Phloroglucinol Test This test also involves the formation of furfural derivatives in the presence of This test is specifically to (+) The change of color to red
concentrated HCL. The furfural derivatives formed by galactose then condense with detect galactose and lactose in indicates the presence of
the phloroglucinol to form a red-colored compound. a solution. galactose in the solution.
 Tollen’s Test Carbohydrates react with Tollens reagent and form a silver mirror on the inner walls This test is given by reducing (+) The appearance of a silver
of the test tube. This confirms the presence of reducing sugars. Silver ions are sugars. mirror confirms the presence
reduced to metallic silver. of reducing sugars.

Maillard’s Reaction The Maillard reaction is an organic chemical reaction in which reducing sugars react This test is given by reducing (+) The appearance of brown
with amino acids to form a complex mixture of compounds. This reaction is sugars. in food confirms for the
responsible for the characteristic flavor and aroma of browned food. presence of reducing sugars

Osazone Formation It gives you the final inference about the type of carbohydrate present in the solution. It is a confirmatory test for (+) Osazone crystals formed
/ Kowarsky Reaction Osazone derivative of carbohydrate forms specific crystals that are characteristic of carbohydrates. when viewed under the
it. The shape of the crystal tells us about the nature of the carbohydrate present. microscope have different
Phenyl hydrazine reacts with reducing sugar in an acidic environment at high shapes depending on the type
temperatures to form phenylhydrazone. Phenyl hydrazine further reacts with the of carbohydrate present.
sugar on heating and forms crystals of osazone specific to that carbohydrate.
(+) Needle-shaped/ broom-
stick crystals are formed in 5
minutes indicating glucose

(+) Needle-shaped/ broom-

stick crystals are formed in 2
minutes indicating fructose

(+) Cotton ball-shaped crystals

are formed by lactose in 30
minutes

(+) Sunflower-shaped crystals

are formed by maltose in 30 to
40 minutes

Mucic Acid Test Mucic acid test is a test that is highly specific and is used for the detection of the Mucic acid test is a test that is (+) The formation of crystal at
 presence of galactose and lactose. It is also termed galactaric acid that is named highly specific and is used for the bottom of the tube
after the product of the reaction. Monosaccharides treated with potent oxidizing the detection of the presence indicates a positive result
agents like nitric acid undergo oxidation to yield saccharic acids (dicarboxylic acids). of galactose and lactose. which means that the sample
Nitric acid can oxidize both aldehyde and primary alcoholic groups present at C1 and solution has galactose or its
C6 of galactose, producing an insoluble precipitate of mucic acid at higher derivatives.
temperatures. Lactose also yields mucic acid due to the hydrolysis of the glycosidic
bond between glucose and galactose subunits. Glucose, another monosaccharide
with a similar structure, forms a water-soluble precipitate under room temperature
when treated with nitric acid.

Iodine Test / Lugol’s Test Starch reacts with iodine and forms a complex blue color solution. On heating the This test is only given by (+) The appearance of blue
blue color disappears and on cooling the blue color reappears. starch and glycogen colour solution confirms the
presence of starch.

REFERENCES:

Admin. (2019, June 4). Tests of Carbohydrates - Chemistry Practicals Class 12. BYJUS; BYJU’S. https://byjus.com/chemistry/tests-of-carbohydrates/

Tests For Carbohydrates | Types, Principles, Apparatus. (2020, April 16). A Level Biology. https://alevelbiology.co.uk/notes/tests-for-carbohydrates/

‌https://www.facebook.com/sag.micro. (2022, April 18). Anthrone Test- Definition, Principle, Procedure, Result, Uses. Microbe Notes. https://microbenotes.com/anthrone-test/

MN Editors. (2023, October 22). Nylander’s Test For Carbohydrates Principle, Procedure, Result. Microbiology Note – Online Biology Notes.

https://microbiologynote.com/nylanders-test/

Admin. (2019, September 30). What is the Maillard Reaction? - Mechanism with Examples and Illustrations. BYJUS; BYJU’S. https://byjus.com/chemistry/maillard-reaction/

https://www.facebook.com/sag.micro. (2022, May 19). Mucic Acid Test- Definition, Principle, Procedure, Result, Uses. Microbe Notes. https://microbenotes.com/mucic-acid-

test/#-result-and-interpretation-of-mucic-acid-test