Advanced Level Biology Food Test

FOOD TEST is a test that often comes for GCE Advanced Levels in Cameroon as well as other countries like India. This implies that the content and procedures will work for you. There classes of food involved in these experiments are: Carbohydrates (Starch), and reduving sugar - Proteins ( million's test And biurets test) Lipids Explanations for these test will be given in details below. They are very simple as they are stuffs we do in our day to day lives without noticing so let's begin. Test for carbohydrates Carbohydrates are a good source of energy for humans. In plants they are starch and in animal, they are stored as glucose. The transformation is caused by enzymes like amylase released from the mouth and pancreas.

 Food substances that contain starch are: 

• maize, 
• cocoyam etc. 
• Cassava

Stach is tasted using iodine.

 Experiment: Test for starch.

•  Add 1ml of iodine solution in 2ml of starch solution and kept for 10 minutes. 

Result: A brown color indicates the absence of startch. A blue-black color indicates the presence of startch.

Carbohydrates can be grouped as reducing or non-reducing sugar depending on the ease with which that sugar (carbohydrate) can act as a reducing agent.

Substances (sugar) that have a free aldehyde or ketone in it's molecular structure e.g maltose and all monosaccharide acts as a reducing agent.

To differentiate between reducing and non-reducing sugar, add Benedicts solution or Fehling's solution A and B and place the test tube in boiling water bath for 5 mins . 

Shake the tube at intervals and observe colour change.

Result: greenish-yellow or orange or red or brick red color.

• Greenish-yellow color = limited amount of glucose in solution (reducing sugar is present)
• Red/brickred = excess glucose (reducing sugar is present)

Test for non-reducing sugar

The substance sucrose, appears to be a non- reducing sugar. The sugar is made from glucose and fructose, which are reducing sugars.

 

Benedict test can be used to test for the presence of reducing sugar.

 Constant blue color= no reducing sugar present 

 to check for presence of sucrose we proceed as following :

• take 2cm3 of sugar solution tubes
•  add 1 cm3 of HCL concentrated to it. on heating, there will be a break down of glycosidic bonds between glucose and fructose which are reducing sugars

•   heat it and then allow it to cool down
• add Na2CO3 to neutralize cHCL 
• add benedict solution and heat in H2O bath up to 90 °C and observe the colour change

 Reducing sugar test: 

•  add 2cm3 of sample 
•  To the solution ,add 2cm3 of benedict reagent 
•  heat the solution in a bath till it boils 

Final results:

 red coloration= reducing sugar is present

If the solution remains blue, then = reducing sugar is not present

Test for Lipids:

Ethanol emulsion of lipid is used to test for lipid: 

• The sample has to be crushed
• To the crushed sample, add alcohol to it to make a solution 
• Then add a little bit of water to it 
• Conclude by shacking the test tube - 

Results:

if the cloudy emulsion forms then it shows presence of lipids

Test for proteins

Biurets test and millon's reagent can be used to test for proteins in the laboratory

 biuret test

•  take a protein solution using the  sample (bean, groundnut peal etc) t
• Crushing the protein or peal is indicative depending of whether it is said in the question paper
•  Collect a 2cm3 solution and place in your test tube.
•  add 2cm3 of biuret solution  

purple indicates the presence of peptide bond 

conclusion= protein are present

Millons test: