Characterisation of carbohydrates using chemical tests
Carbohydrates contain carbon, hydrogen and oxygen with the general formula C m(H2O)n. The
carbohydrates are divided into 4 groups namely monosaccharide (eg. glucose, fructose, galactose),
disaccharides (eg. Maltose, sucrose, lactose), oligosaccharide (Raffinose) and polysaccharides (eg. Starch,
cellulose). Monosaccharide cannot be hydrolyzed further. Oligosaccharide contains 3-10 monosaccharide
units. Polysaccharide contains larger number of monosaccharide units. There are two types of
carbohydrates namely reducing and non-reducing carbohydrates. The reducing sugar reduce Tollen’s
reagent and Fehling’s solution. All monosaccharide are reducing sugars. Non-reducing sugar don’t reduce
Tollen’s reagent and Fehling’s solution (Sucrose).
1) Benedict’s test
This test determines whether the carbohydrate sample contains free aldehyde groups or not.
Benedict’s reagent oxidizes aldehyde group into carboxylic acid. But it does not oxidize primary
or secondary alcohols. This test changes initial blue color to a red precipitate by reducing copper
2+ resulting copper (I) oxide. Final color of the solution depends on the amounts of aldehyde
initially present. Sugars that give positive result to Benedict’s reagent are called reducing sugars.
But fructose and many other ketoses are reducing sugars, even though, ketones are not oxidized
by Benedict’s reagent. These ketoses rearrange to become aldoses in the presence of basic
Benedict’s test solution. Eg. D-fructose rearranges to become D-glucose or D-mannose where,
each are reducing sugar. They contain aldehyde group.
Procedure
1) Prepare a water bath.
2) Add 0.5 ml of 1% of carbohydrate solution to separate tubes. As a control 0.5 ml of distilled
water can be used.
3) Add 2 ml of Benedict’s reagent to each tube and place them in the boiling water bath for 2-3
min.
4) Remove the tubes and note the results.
A red, brown or yellow precipitate indicates positive result. A precipitate must
form to be positive and ignore color changes without precipitate.
1|P ag e
Carbohydrates contain carbon, hydrogen and oxygen with the general formula C m(H2O)n. The
carbohydrates are divided into 4 groups namely monosaccharide (eg. glucose, fructose, galactose),
disaccharides (eg. Maltose, sucrose, lactose), oligosaccharide (Raffinose) and polysaccharides (eg. Starch,
cellulose). Monosaccharide cannot be hydrolyzed further. Oligosaccharide contains 3-10 monosaccharide
units. Polysaccharide contains larger number of monosaccharide units. There are two types of
carbohydrates namely reducing and non-reducing carbohydrates. The reducing sugar reduce Tollen’s
reagent and Fehling’s solution. All monosaccharide are reducing sugars. Non-reducing sugar don’t reduce
Tollen’s reagent and Fehling’s solution (Sucrose).
1) Benedict’s test
This test determines whether the carbohydrate sample contains free aldehyde groups or not.
Benedict’s reagent oxidizes aldehyde group into carboxylic acid. But it does not oxidize primary
or secondary alcohols. This test changes initial blue color to a red precipitate by reducing copper
2+ resulting copper (I) oxide. Final color of the solution depends on the amounts of aldehyde
initially present. Sugars that give positive result to Benedict’s reagent are called reducing sugars.
But fructose and many other ketoses are reducing sugars, even though, ketones are not oxidized
by Benedict’s reagent. These ketoses rearrange to become aldoses in the presence of basic
Benedict’s test solution. Eg. D-fructose rearranges to become D-glucose or D-mannose where,
each are reducing sugar. They contain aldehyde group.
Procedure
1) Prepare a water bath.
2) Add 0.5 ml of 1% of carbohydrate solution to separate tubes. As a control 0.5 ml of distilled
water can be used.
3) Add 2 ml of Benedict’s reagent to each tube and place them in the boiling water bath for 2-3
min.
4) Remove the tubes and note the results.
A red, brown or yellow precipitate indicates positive result. A precipitate must
form to be positive and ignore color changes without precipitate.
1|P ag e
