Identification of Food Constituents in Milk

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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 colour...
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