Practical work 1 - Food tests

Sep 5, 2026

Practical work 1 presents a set of food chemistry tests to detect common nutrients in laboratory samples. It explains the Benedict’s test for reducing sugars based on reduction of copper(II) ions to brick-red copper(I) oxide, the iodine test for starch yielding a blue-black complex, and the Sudan III test for lipids that stains oils red and separates them on the water surface. It also covers the Biuret test for proteins, producing a violet complex in peptide bonds, with clear step-by-step procedures and key observation notes.

LanguageEnglish

CategoryHealthcare

FormatPDF

Pages3

Reading Time8 min

AccessFree to download

Practical work 1

Food tests

Benedict’s test for reducing sugars

Iodine test for starch

Sudan III test for lipids

Biuret test for proteins

Practical work 1 |
1. Food tests | | | --- | --- | | A. Benedict’s test for reducing sugars
Reducing sugars include all monosaccharides (e.g. glucose and fructose) and some disaccharides (e.g. maltose) .
Benedict’s solution contains copper sulphate. Reducing sugars reduce copper(II) ions present in the blue copper sulphate solution to insoluble brick-red copper(I) oxide which is a precipitate.
Procedure
1. Add 2 cm3 of 0. 1% glucose solution to a test tube.
2. Add an equal volume of Benedict’s solution.
3. Shake and boil the mixture.
Note
1. The initial blue colouration of the mixture turns green, then yellow and may finally form a brick-red precipitate. The amount of precipitate gives a rough indication of the amount of reducing sugars present. |
Fig.1 Benedict’s solution
The mixture is likely to bump violently during heating and extra care should therefore be taken. It is safer to use a water bath to heat up the mixture. | 2 Practical work | | B. Iodine test for starch
Starch is only slightly soluble in water and it forms a colloidal solution in water. Starch is a mixture of two polysaccharides, amylose and amylopectin. The relative amounts of the two polysaccharides vary widely indifferent types of starch. Amylose molecules consist of long straight chains of glucose units. Amylopectin has a complicated branched structure. The two polysaccharides combine with iodine to give the characteristic blue-black colour of the standard iodine test.
Procedure
1. Add 2 cm3 of 1% starch solution to a test tube.
2. Add a few drops of iodine solution.
Note
1. A blue-black colouration indicates the presence of starch due to the formation of a complex. | | | --- | --- | --- | | | |
Fig.2 Iodine solution | | C. Sudan III test for lipids
Lipids include oils, fats and waxes.
Procedure
1. Add 2 cm3 of oil to 2 cm3 of water to a test tube.
2. Add a few drops of Sudan III and shake.
Note
1. Oils are stained red with Sudan III. Since they are less dense than water, they separate out as a red layer on the water surface. | | | Practical work 3 |
D. Biuret test for proteins | | | --- | --- | | Egg albumen is a suitable material for Biuret test which tests for the presence of peptide bonds. When mixed with dilute alkaline copper sulphate solutions, nitrogen atoms in the peptide bonds form a violet complex with copper(II) ions.
Procedure
1. Add 2 cm3 of protein solution to a test tube.
2. Add an equal volume of 5% sodium hydroxide solution and mix.
3. Add 2 drops of 1% copper sulphate solution and mix.
Note
1. Violet colouration is developed in the presence of protein.
2. Biuret reagent can be used directly for detecting the presence of protein without heating. | |

FAQs

What does Benedict’s test detect and what color change indicates a positive result?

Benedict’s test detects reducing sugars. The blue solution changes to green, then yellow, and may form a brick-red precipitate, indicating reducing sugars.

How does the iodine test confirm the presence of starch?

The iodine test confirms starch by forming a characteristic blue-black color complex with starch.

What observation confirms proteins in the Biuret test?

In the Biuret test, proteins produce a violet color due to formation of a violet complex involving copper(II) ions and peptide bonds.

Sep 5, 2026

Upload by Margaret