Tests To Detect Adulteration Of Milk

There are many methods known for detection of adulteration in milk but the methods discussed below are simple but rapid and sensitive methods to detect adulteration.

I. Detection of Neutralizers in milk

1) Rosalic acid test (Soda Test)

In milk neutralizers like hydrated lime, sodium hydroxide, sodium carbonate or sodium bicarbonate are added which are generally prohibited.

How to detect?

Take 5 ml of milk in a test tube and add 5 ml alcohol followed by 4-5 drops of rosalic acid. If the colour of milk changes to pinkish red, then it is inferred that the milk is adulterated with sodium carbonate / sodium bicarbonate and hence unfit for human consumption.

This test will be effective only if the neutralizers are present in milk. If the added neutralizers are nullified by the developed acidity, then this test will be negative. In that case, the alkaline condition of the milk for the presence of soda ash has to be estimated.

How to proceed?

2) Take 20 ml of milk in a silica crucible and then the water is evaporated and the contents are burnt in a muffle furnace. The ash is dispersed in 10 ml distilled water and it is titrated against decinormal (N/10) hydrochloric acid using phenolphthalein as an indicator. If the titre value exceeds 1.2 ml, then it is construed that the milk is adulterated with neutralizers.

II. Test for detection of hydrogen peroxide

Take 5 ml milk in a test tube and then add 5 drops of paraphenylene diamine and shake it well. Change of the colour of milk to blue confirms that the milk is added with hydrogen peroxide.

III. Test for detection of formalin

Formalin (40%) is poisonous though it can preserve milk for a long time.
How to detect?

Take 10 ml of milk in test tube and 5 ml of conc. sulphuric acid is added on the sides of the test tube with out shaking. If a violet or blue ring appears at the intersection of the two layers, then it shows the presence of formalin.

IV. Test for detection of sugar in milk

Generally sugar is mixed in the milk to increase the solids not fat content of milk i.e. to increase the lactometer reading of milk, which was already diluted with water.

How to detect?

Take 10 ml of milk in a test tube and add 5 ml of hydrochloric acid along with 0.1 g of resorcinol. Then shake the test tube well and place the test tube in a boiling water bath for 5 min. Appearance of red colour indicates the presence of added sugar in milk.

V. Test for detection of starch

Addition of starch also increases the SNF content of milk. Apart from the starch, wheat flour, arrowroot, rice flour are also added.

How to detect?

Take 3 ml milk in a test tube and boil it thoroughly. Then milk is cooled to room temperature and added with 2 to 3 drops of 1% iodine solution. Change of colour to blue indicates that the milk is adulterated with starch.

VI. Test for detection of glucose

Usually poor quality glucose is added to milk to increase the lactometer reading. There are two tests available to detect the adulteration of milk with glucose.

How to proceed?

1. Phosphomolybdic or Barford Test

Take 3 ml of milk in a test tube and add 3 ml Barford’s reagent and mix it thoroughly. Then keep it in a boiling water bath for 3 min and then cool it for 2 min by immersing in tap water with out disturbance. Then add 1 ml of phosphomolybdic acid and shake. If blue colour is visible, then glucose is present in the milk sample.

2. Diacetic test

Take a strip of diacetic strip and dip it in the milk for 30 sec to 1 min. If the strip changes colour, then it shows that the sample of milk contains glucose. If there is no change in the colour of the strip, then glucose is absent. In this method the presence of glucose in milk can be quantified by comparing the colour developed with the chart strip.

VII. Test for detection of urea

1. Urea is generally added in the preparation of synthetic milk to raise the SNF value.

Five ml of milk is mixed well with 5 ml paradimethyl amino benzaldehyde (16%). If the solution turns yellow in colour, then the given sample of milk is added with urea.

2. Take 5 ml of milk in a test tube and add 0.2 ml of urease (20 mg / ml). Shake well at room temperature and then add 0.1 ml of bromothymol blue solution (0.5%). Appearance of blue colour after 10-15 min indicates the adulteration milk with urea.