Food tests

What is the food test for proteins and explain it

The biuret test. Add about 2cm² of test solution then add equal volume of biuret solution. A blue ring should form on surface of solution, if a protein is present this will change to lilac-purple upon shaking.

Why is there colour change when protein is present in biuret solution

Biuret reagent contains sodium hydroxide and copper sulphate, when nitrogen atoms in the proteins peptide chain interact with Cu²⁺ ions this reaction occurs.

What is the test for lipids and explain it

The emulsion test. To carry it out you shake some of the test sample with about 4cm³ of ethanol, then decant this liquid into a test tube of water leaving anything undissolved behind. If lipids have dissolved in ethanol, a milky white precipitate will form in water.

What is a reducing sugar

A reducing sugar has an aldehyde group, this allows sugar to act as a reducing agent, donates electrons to another molecule to reduce it.

All monosaccharides are reducing sugars and some disaccharides like maltose and lactose are reducing sugars

What is the test for reducing sugars and explain it

The Benedict’s test. To carry this test out place a sample in a test tube and add equal volume of benedict’s reagent, then heat in a water bath at about 80^oC for 3-minutes. It should change from blue to green to orange to brick red.

What is the reasoning for the colour change

Reducing sugars react with copper ions ions in Benedict’s reagent (Copper II sulphate). Reducing sugars adds electrons to Cu²⁺ which is blue and Cu⁺ forms which is red.

How do you test for non-reducing sugars

To test for non-reducing sugars like sucrose, first use Benedict’s test to check if there is any reducing sugars present. Then, using a separate sample, boil the test solution with dilute hydrochloric acid, this hydrolyses the glycosidic bond. Neutralise the solution with solid sodium hydrogen carbonate until it stop fizzing, then if you carry out Benedict’s test again it should be positive.

Why does HCl hydrolyse sucrose back to glucose and fructose

HCl provides H⁺ ions which break the glycosidic bond.

What is the test for starch and explain it

Iodine test. To carry it out a few drops of iodine is dissolved in potassium iodide solution and mixed with a sample. If the solution changes from yellow orange to blue-black starch is present.

What is the use of reagent strips

These can detect the presence of reducing sugars, mostly used for glucose. Can use a colour coded chart for easy concentration determination

What is colorimetry

It turns a qualitative test into a quantitative test. A colorimetry machine is used to measure the absorbance, or transmission, of light by a coloured solution. The more concentrated a solution more light is absorbed and less is transmitted

Describe the procedure of colorimetry when testing for unknown glucose conc.

1. Red filter is placed in colorimeter

2. Colorimeter is calibrated using distilled water

3. Benedict’s test performed on range of known conc. of glucose

4. A centrifuge is used to filter preciptate to the bottom of resulting solution

5. The % transmission of each glucose solution measured

6. Using transmission % plot a calibration curve

7. Repeat steps 3-5 with an unknown conc. of glucose and compare its transmission % to graph to determine concentration