Food substances are chemical compounds and therefore can be tested chemically.
Test for carbohydrates
Test for glucose (a reducing sugar)
To a little solution of glucose, add small quantities of Fehling’s solutions A and B (equal amount) and heat gently.
A brick-red precipitate shows the presence of glucose.
Equal quantities of glucose solution and Benedict’s solution in a test-tube are boiled and allowed to cool.
Result: Red or orange precipitate is formed showing the presence of glucose.
Test for starch
To a starch solution, add few drops of iodine solution.
Result: Blue black colour shows the presence of starch.
To a surface of potato or yam, add few drops of iodine.
Blue black colour shows the presence of starch.
Test for sucrose/maltose (non reducing sugar)
Put small quantities of sucrose or maltose solution into two different test tubes.
Labelled them A and B. To test-tube A, add Benedict solution and boil gently for three minutes and allow to cool.
Result: No red precipitate.
To test tube B, add a few drops of dilute hydrochloric acid and boil gently for few minutes.
The hydrochloric acid will hydrolyse the sucrose or maltose to glucose sugar. Then add Fehling’s solution A and B or Benedict’s solution to the hydrolysed solution and boil gently for few minutes and cool.
Brick-red precipitate shows presence of reducing sugar in the hydrolysed solution, that is, sucrose or maltose is present.
Test for protein
1) Millon’s test
Add an equal volume of a protein solution (such as the albumen of an egg) and Millon’s reagent and boil for few minutes.
Brick-red precipitate is formed showing the presence of protein.
2) Biuret’s Test
To 1cm3 of protein solution, add 1cm3 of 20% sodium hydroxide and a drop of 1% copper(II) sulphate solution.
Result: Violet or purple colour shows the presence of protein.
To small amount of protein material, add small quantity of concentrated trioxonitrate (V) acid (nitric acid) and concentrated ammonium solution in excess. The mixture is gently warmed and cooled. Red or orange precipitate during warming and on cooling a deep yellow colour shows the presence of protein.
Test for fats and oils
Translucent white paper mark
Put a drop of oil on a white paper or rub a food substance containing fats against a white paper. Hold the paper against any source of light.
Result: A translucent spot indicates the presence of oil or fats.
Sudan III solution test
To small quantity of oil or melted fats, add a few drops of sudan III solution.
Result: A red colour shows the presence of oils or fats.
Osmic acid test
To small quantity of oil or fat in a test tube, add 1% of osmic acid and boil.
Result: A black precipitate shows the presence of oil or fat.