Food Testing for Sugar and Starch

Benedict’s test for reducing sugars

REDUCING SUGARS

All the monosaccharides and many of the disaccharides are REDUCING SUGARS

Benedict’s test is used to determine the reducing properties of the different sugars

Benedicts solution is a turquoise liquidcontaining copper ions and sodium

hydroxide; the copper ions exist as Cu2+

in this reagent

If a sugar is a reducing sugar then the Cu2+

ions are reduced to Cu+ which, in the presence of alkaline sodium hydroxide,

form copper oxide

Copper oxide is insoluble and precipitatesout of the solution as a brick-red

precipitate

REDUCING SUGARS

When Benedicts test is performed with the disaccharides maltose andsucrose, the following result is obtained

SUCROSERESULT

MALTOSERESULT

Sucrose is anon-reducing

sugar

Maltose is a reducing sugar

REDUCING SUGARS

Why is sucrose a non-reducing sugar?

CH OH2

HH

OHOH

H OH

H

O

CH OH2

HH

HO

H

OOH

H OH

H

O

1

23

4

5

6

1

23

4

5

6

MALTOSE1

1

2

2

3

3

4

4

5

5

6

6

SUCROSE

Sugars reduce Benedicts solution when the anomericcarbon atom is made available to reduce the copper

ions in the solutionThe anomeric carbon atom is the carbon of the

carbonyl group present in the straight chain form of the sugar

The anomeric carbon atom for glucoseis carbon 1

C

C

C

C

C

C

H

H

H

O

O

O

H

H

H

H

H

O

O

O

H

H

H

H

1

2

3

4

5

6

REDUCING SUGARS

Why is sucrose a non-reducing sugar?

CH OH2

HH

OHOH

H OH

H

O

CH OH2

HH

HO

H

OOH

H OH

H

O

1

23

4

5

6

1

23

4

5

6

MALTOSE1

1

2

2

3

3

4

4

5

5

6

6

SUCROSE

Sugars reduce Benedicts solution when the anomericcarbon atom is made available to reduce the copper

ions in the solution

The anomeric carbon atom for fructoseis carbon 2

C

C

C

C

C

C

H

H

O

O

O

H

H

H

H

H

H

O

O

O

H

H

H

H

1

2

3

4

5

6 Fructose bonds to glucose to formsucrose

Why is sucrose a non-reducing sugar?

CH OH2

HH

OHOH

H OH

H

O

CH OH2

HH

HO

H

OOH

H OH

H

O

1

23

4

5

6

1

23

4

5

6

MALTOSE1

1

2

2

3

3

4

4

5

5

6

6

SUCROSE

When maltose is boiled with Benedict’s reagent, theregion of the ring containing the anomeric carbon

atom (carbon 1) may open exposing a carbonylgroup capable of reducing Benedicts reagent – ONLY

AN ANOMERIC CARBON ATOM THAT IS NOTINVOLVED IN THE FORMATION OF THEGLYCOSIDIC BOND MAY BE EXPOSED

This potential anomeric carbon atom is available to reduce

Benedict’s reagent

This potential anomeric carbon atom is unavailable

The one available anomeric carbon atom is sufficientfor this molecule to reduce Benedict’s solution and

thus MALTOSE is a reducing sugar

1

1

2

2

3

3

4

4

5

5

6

6

SUCROSE

C

C

C

C

C

C

H

H

O

O

O

H

H

H

H

H

H

O

O

O

H

H

H

H

1

2

3

4

5

6

Sucrose is formed when glucose forms a glycosidic bondwith fructose

glucose

fructose

glycosidicbond

The anomeric carbonatom for fructose

is carbon 2

C

C

C

C

C

C

H

H

H

O

O

O

H

H

H

H

H

O

O

O

H

H

H

H

1

2

3

4

5

6

The anomeric carbonatom for glucose

is carbon 1

As both the anomeric carbon atoms are involved in formingthe glycosidic bond when glucose and fructose join,

there are no potentially free anomeric carbon atoms availableto reduce Benedict’s solution

SUCROSE IS A NON-REDUCING SUGAR

TEST FOR SUCROSE

In order to determine if sucrose is present in a sample or solution then the following procedure is performed;

The sample or solution under consideration is boiled for at least fifteen minutesin hydrochloric acid

Boiling in acid breaks glycosidic bonds – the glycosidic bond is hydrolysed

This procedure is called ACID HYDROLYSIS

The solution is then neutralised by adding drops of alkali while testing with pH paper

Benedict’s test is now performed on the resulting solutionIf a brick-red precipitate forms then sucrose was present in the original solution

Acid hydrolysis breaks the glycosidic bonds in the sucrose moleculesreleasing free glucose and free fructose into the solution

Glucose and fructose are both monosaccharides and therefore reducing sugars

If no precipitate is obtained then sucrose was not present in the original sample

The need to neutralise the solution following acid hydrolysis is due to thefact that the Benedict’s test requires an alkaline medium

Iodine test for starch

REACTION BETWEEN STARCH AND IODINE SOLUTION

When iodine in potassium iodide solution is added to starch, the iodine moleculespack inside the amylose helix to give a blue-black colour

When iodine reacts with the starch in thispiece of bread, the blue-black colour

develops

Instructions

Carry out tests for reducing sugars, non-reducing sugars and starch on as many food samples as possible.

Write the methods out in your practical book.Record your results in a suitable table.

Testing for Starch and Sugars

Reducing sugars:Add a small quantity of Benedict’s solution to a sample of the food

(add water if the food is a solid). Place in a boiling water bath for 5 minutes.If the solution stays blue, test for non-reducing sugars:

Add a small quantity of dilute HCl to the food sample (add water if the food is a solid). Place in a boiling water bath for 5 minutes, then add an equal quantity of NaHCO3 (or NaOH) before testing with Benedict’s as above.Results: green → yellow → orange → brick-red precipitate

Low sugar content → → → → → High sugar contentStarch:

Add a few drops of iodine solution to a sample of food in a spotting tile.Results: Blue-black colour indicates starch is present.

Mono- and disaccharides