Laboratory Testing Procedures for Checking Adulterants in Milk and Milk Products

Laboratory Testing for Dairy Products

Milk is a product that is always in demand and when supplies cannot be met then adulterators indulge in dangerous techniques by using cheap alternatives to increase the volume of milk. Sometime back, The Food Safety and Standards Authority of India (FSSAI) had released a document which described the procedures that laboratories can use to analyze adulterants in food products.
The below information would be useful while testing Milk & Milk Products:
Vanaspati in milk: Place 3ml of milk in a test tube. Add 10 drops of hydrochloric acid and one teaspoonful of sugar to the milk and mix thoroughly. If the mixture turns red after 5 minutes it means vanaspati is present in the milk.
Vanaspati in sweet curd: Place one teaspoonful of sweet curd in the test tube. Add 10 drops of hydrochloric acid. Gently shake the test tube to mix contents thoroughly. If curd changes colour and becomes red after five minutes it means vanaspati is present in the curd.
Vanaspati or margarine in ghee: Take a test tube with a stopper. Place one teaspoonful of melted ghee with an equal quantity of concentrated hydrochloric acid in the test tube and add a pinch of salt to this. Shake for one minute and let it stand for five minutes. The presence of vanaspati or margarine is indicated if the lower acid layer turns crimson.
Formalin in milk: Formalin is used by adulterators as a preservative. Test requires that you place 10ml of milk in test tube. Carefully add 5ml of concentrated sulphuric acid to the milk by pouring it along the side of the test tube. Ensure that the milk and sulphuric acid do not mix. The appearance of a blue or violet ring at the point where the two liquids meet indicates the presence of formalin in the milk.
Salt in milk:
Place 5ml of silver nitrate reagent in a test tube. Add 2-3 drops of potassium dichromate reagent to this. Further add 1ml of milk to this test tube and shake to mix thoroughly.
If the contents of the test tube turn yellow it means the milk contains salt.
If the colour of the mixture changes to chocolate or reddish brown it means there is no salt present in the milk.
Sugar is in milk: Place a 3ml sample of milk in a test tube. Add 2ml of the hydrochloric acid and after that 50mg of resorcinol. Sugar is present in the milk if the colour changes to red.
Ammonium sulphate in milk: Ammonium sulphate is used by adulterators to increase the lactometer reading of milk. Take 5ml of hot milk in a test tube. Add a suitable acid like citric acid. This will curdle the milk, now separate the whey by filtering it. Place the filtered whey in another test tube and add 0.5ml of 5% barium chloride. If there are precipitates, it indicates the presence of ammonium sulphate.
You can also check for ammonium sulphate through a second method. To 5ml milk in a test tube add
5 ml of 2% of sodium hydroxide
5 ml of 2% sodium hypochlorite
5 ml of 5% phenol solution
Heat this mixture for 20 seconds in a vessel of boiling water.
If the light blue mixture changes to a deep blue colour, it means ammonium sulphate is present in the milk.
However, if the colour changes to pink it means there is no ammonium sulphate in the milk.
Hydrogen peroxide in milk: Place 5ml milk in a test tube. Add 3 drops of paraphenylene diamine and shake well. Presence of hydrogen peroxide is confirmed if the colour of the milk changes to blue. In a second testing method place a 10ml milk sample in a test tube, add 10 -15 drops of vanadium pentoxide reagent and mix well. If the milk changes colour to pink or red it indicates the presence of hydrogen peroxide.
Sodium bi-carbonate/neutralize in milk: Place 3ml of milk in a test tube and add
5 ml of rectified spirit to it, now add 4 drops of rosolic acid solution. Sodium bicarbonate will be present if the colour of the milk changes to red/ rosy.
Boric acid in milk: Place 3ml of milk in a test tube. Add 20 drops of hydrochloric acid and shake the test tube to mix up the contents thoroughly. Dip a yellow paper-strip into this mixture and remove the same after one minute. If the colour of the strip changes from yellow to red, followed by a further change of colour from red to green it indicates the presence of boric acid in the milk. Yellow paper strips are prepared by dipping stripped pieces of filter paper in an aqueous solution of turmeric and then dried completely.
Blotting Paper in Rabri: Place a teaspoonful of rabri in a test tube. Add 3ml of hydrochloric acid and 3ml of distilled water. Stir the contents with a glass rod. Remove the rod and examine it. If fine fibres are seen on the glass rod it means there is blotting paper in the rabri.
Use of coal tar dyes in ghee, cottage cheese, condensed milk, khoya, milk powder: Add 5ml of dilute sulphuric acid or concentrated hydrochloric acid to one teaspoonful of food product sample in a test tube. Presence of coal tar dyes is indicated if the colour changes to
Pink in case diluted sulphuric acid is used for the test
Crimson in case concentrated hydrochloric acid is used. If using hydrochloric acid shows no change in colour then dilute it with water to see the colour result.
Fat removed from milk: If fat has been removed from milk then the lactometer will show a reading of above 26 but the milk will appear thick.