Identification of Food Constituents in Milk
Materials
• Samples of different types of milk
• Benedict’s Reagent
• Biuret Reagent
• Sudan III
• Water bath
• Pipettes/syringes
• Test tubes
• Microscopic slides and cover slips
• Microscope
Method (testing for reducing sugars)
1. Add 3cm³ of whole milk, by using a pipette or syringe to the test tube.
2. Add 5cm³ of Benedict’s reagent and place it in the boiling water bath for 8 minutes. Do the same for semi-skimmed milk and skimmed milk.
3. Once all 3 of the test tubes are left to cool in the air, observe the colours. It will be a good idea to set up a range of colour standards from glucose concentrations of 1%, 2%, 3%, 4% and 5% so that you can relate the colours observed to these concentrations.
4. A positive result would be from green to yellow to brick-red colour.
Method (testing for non-reducing sugars)
5. Make up the same solution as step 1 but this time, adding 3cm³ of dilute hydrochloric acid to break the glycosidic bonds between the monosaccharides.
6. Then add 3cm³ of sodium hydroxide solution to neutralise it.
7. Add 5cm³ of Benedict’s reagent and place it in the water bath for 8 minutes.
8. Once it’s left to cool, it should now turn brick-red colour.
9. The concentration of a non-reducing sugar can be estimated by first adding a drop of 10% invertase (sucrase) concentrate to 2cm³ of the solution to be tested and leaving for 30 minutes at room temperature. The solution is tested for the presence of a reducing sugar. This method is preferable to acid hydrolysis.
Method (testing for starch)
10. On each of the three types of milk, just add a few drops of iodine which is dissolved in potassium iodide solution.
11. The sample should change from browny-orange, to a dark, blue-black colour.
Method (testing for proteins)
12. Place 2cm³ of the three different types of milks on each tube.
13. Then add 2cm³ of Biuret reagent and you should see a purple-violet
• Samples of different types of milk
• Benedict’s Reagent
• Biuret Reagent
• Sudan III
• Water bath
• Pipettes/syringes
• Test tubes
• Microscopic slides and cover slips
• Microscope
Method (testing for reducing sugars)
1. Add 3cm³ of whole milk, by using a pipette or syringe to the test tube.
2. Add 5cm³ of Benedict’s reagent and place it in the boiling water bath for 8 minutes. Do the same for semi-skimmed milk and skimmed milk.
3. Once all 3 of the test tubes are left to cool in the air, observe the colours. It will be a good idea to set up a range of colour standards from glucose concentrations of 1%, 2%, 3%, 4% and 5% so that you can relate the colours observed to these concentrations.
4. A positive result would be from green to yellow to brick-red colour.
Method (testing for non-reducing sugars)
5. Make up the same solution as step 1 but this time, adding 3cm³ of dilute hydrochloric acid to break the glycosidic bonds between the monosaccharides.
6. Then add 3cm³ of sodium hydroxide solution to neutralise it.
7. Add 5cm³ of Benedict’s reagent and place it in the water bath for 8 minutes.
8. Once it’s left to cool, it should now turn brick-red colour.
9. The concentration of a non-reducing sugar can be estimated by first adding a drop of 10% invertase (sucrase) concentrate to 2cm³ of the solution to be tested and leaving for 30 minutes at room temperature. The solution is tested for the presence of a reducing sugar. This method is preferable to acid hydrolysis.
Method (testing for starch)
10. On each of the three types of milk, just add a few drops of iodine which is dissolved in potassium iodide solution.
11. The sample should change from browny-orange, to a dark, blue-black colour.
Method (testing for proteins)
12. Place 2cm³ of the three different types of milks on each tube.
13. Then add 2cm³ of Biuret reagent and you should see a purple-violet