If we want to study about the food tests of carbohydrates, first, we have to know what reducing sugars are.
Reducing sugar
Some sugars are capable of reducing silver ions to free silver, and copper(II) ions to copper(I) ions, under prescribed conditions. Such sugars are called reducing sugars. This reducing ability, which is useful in classifying sugars and in certain clinical tests, is dependent on the presence of (1) aldehydes, (2) a-hydroxyketone groups such as in fructose, or (3) hemiacettal structures in cyclic molecules such as maltose. These groups are easily oxidized to carboxylic acid (or carboxylate ion) groups; the metal ions are thereby reduced. Several different reagents, including Tollens¡¦, Fehling¡¦s, Benedict¡¦s and Barfoed¡¦s reagents, are used to detect reducing sugars. The Benedict, Fehling and Barfoed tests depend on the formation of copper (I) oxide precipitate to indicate a positive reaction.
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Identification of carbohydrates (Food Test)
Reducing sugars, including all monosaccharides and some dissacharides, such as maltose. Use 0.1-1% sugar solutions.
| Test | Observation | Basis of test |
| Benedict¡¦s
test Add 2 ml of a soluion of the reducing sugar to a test-tube. Add as equal volume of Benedict¡¦s solution. Shake and bring gently to the boil. |
The initial blue coloration of the mixture turns green, then yellowish and may finally form a brick-red precipitate. | Benedict¡¦s solution contains copper sulphate. Reducing sugars reduce soluble bluer copper sulphate, containing copper(II) ions to insoluble red-brown copper oxide containing copper(I). The latter is seen as a precipitate. |
| Fehling¡¦s
test Add 2 ml of a solution of the reducing sugar to a test-tube. Add 1ml of Fehling¡¦s solution A and 1ml of Fehling¡¦s solution B. Shake and bring to the boil |
Non-reducing sugars
| Test | Observation | Basis of test |
| Add 2 ml of
sucrose solution to a test tube. Add 1-ml dilute HCl boil
for 1 minute. Carefully neutralize with sodium
hydrogencarbonate (check with pH paper). Carry out Benedict¡¦s test. |
As above | A disaccharide can be hydrolysed to its monosaccharide constituents by boiling with dilute HCl. Sucrose is hydrolysed to glucose and fructose, both of which reducing sugar result with the Benedict¡¦s test. |
Starch
| Test | Observation | Basis of test |
| Iodine/potassium
iodide test Add 2 ml 1% starch solution to a test-tube. Add a few drops of iodine/KI solution. Alternatively add the latter to the solid form of starch |
A blue-black coloration | A polyiodide complex is formed with starch |