Food test

Question 1

FOOD TEST

Answer 1

Substance to be Tested | Test | Observations |
|————————–|———|——————|
| Reducing sugars | (i) Benedict’s test: 2 cm³ of the solution to be tested is put into a test-tube. 2 cm³ of Benedict’s solution is then added. The mixture is shaken and brought gently to the boil. | The initial blue color of the mixture turns green, then yellow, and may form a brick-red precipitate. |
| | (ii) Fehling’s test: 2 cm³ of the solution to be tested is put into a test-tube. 1 cm³ of Fehling’s A is added. 1 cm³ of Fehling’s B is then added. The mixture is shaken and brought gently to the boil. | Same as Benedict’s test. |
| Non-reducing sugars | 1 cm³ of the solution is put into a test-tube and 1 cm³ of dilute hydrochloric acid (HCl) is added. The mixture is boiled for 1 minute. 1 cm³ of aqueous NaOH solution is added, followed by 2 cm³ of Benedict’s solution. The mixture is then shaken and boiled gently. | A red-brown precipitate results as the sucrose is hydrolyzed to fructose and glucose by the acid. Fructose and glucose are reducing sugars, so Benedict’s test then can be carried out. |
| Starch | 2 cm³ of 1% starch solution is added to a test-tube. A few drops of iodine in potassium iodide (I₂KI) solution are added. | A blue-black precipitate results. |
| Protein | Biuret test: 2 cm³ of protein solution is put into a test-tube, 2 cm³ of 5% potassium hydroxide (KOH) is added. The mixture is stirred and 2 drops of 1% copper sulfate (CuSO₄) are added. | A mauve or purple color slowly develops. |
| Fats | Ethanol test: 2 cm³ of fat solution or oil is put into a test-tube. 2 cm³ of absolute ethanol is added. The mixture is shaken vigorously and 3 cm³ of water is added. | A cloudy white suspension can be seen when the water is added. |
| | Grease spot test: a drop of the sample is dropped onto a piece of paper. | A permanent translucent spot is seen on the paper. |