Identification of Food Constituents in Milk

Topics: Glucose, Disaccharide, Carbohydrate Pages: 3 (775 words) Published: March 18, 2011

Samples of different types of milk
Benedict’s Reagent
Biuret Reagent
Sudan III
Water bath
Test tubes
Microscopic slides and cover slips

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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