No.10 Food Tests

Carbohydrates (Sugars and Starches)

Reducing Sugars. The reducing sugars include all monosaccharides, Such as glucose and fructose, and some disaccharides, such as maltose. Use 0.1 – 1% sugar solutions.

Test for reducing sugar: Benedicts test: Add 2 cm3 of a solution of the reducing sugar to a test tube. Add an equal volume of benedicts solution. Shake and bring gently to the boil, shaking continuously to minimise spitting. Observation: The initial blue coloration of the mixture turns green, then yellowish and may finally form a brick red precipitate. Basis of test: Benedicts solution contains copper sulphate. Reducing sugars reduce soluble blue copper sulphate containing copper (II) ions (Cu2+) to insoluble red - brown copper oxide containing copper (I). The latter is seen as a precipitate.

Non- Reducing sugars. Non reducing sugars, the most common non reducing sugars is sucrose, a disacchriade. If reducing sugars have been shown to be absent (negative result in a above test) a brick red precipitate in the test below indicates the presence of a non reducing sugar. If reducing sugars have been shown to be present, a heavier precipitate will be observed in the following test than with the reducing test if non reducing sugar is also present.

Tests for non reducing sugar: Add 2 cm3 of sucrose solution to a test tube. Add 1 cm3 dilute Hydrochloric acid. Boil for 1 minute. Carefully neutralise with sodium hydrogencarbonate (Check with pH paper) Care is required because effervescence occurs. Carry out benedicts test.
Observation:
As benedicts test.
Basis of test: Disaccharide can be hydrolysed to it’s monosaccharade constituents by boiling with dilute hydrochloric acid. Sucrose is hydrolysed to glucose and fructose, both of which are reducing sugars and give the reducing sugar result with the benedicts