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.
VIII.
Test for detection of ammonium sulphate
The presence of sulphate in milk increases the
lactometer reading.
How
to proceed?
5 ml of hot milk is taken in a test tube and
added with a suitable acid for e.g. citric acid and the whey thus separated is
filtered. Collect the whey in another test tube and add 0.5 ml of 5% barium
chloride. Appearance of precipitate indicates the presence of ammonium sulphate
in milk.
IX.
Test for detection of salt
Addition of salt in milk is mainly resorted to
with the aim of increasing the corrected lactometer reading.
How
to detect?
Five ml of silver nitrate (0.8%) is taken in a
test tube and added with 2 to 3 drops of 1% potassium dichromate and 1 ml of
milk and thoroughly mixed. If the contents of the test tube turn yellow in
colour, then milk contains salt in it. If it is chocolate coloured, then the
milk is free from salt.
X.
Test for detection of pulverized soap
Take 10 ml of milk in a test tube and dilute it
with equal quantity of hot water and then add 1 – 2 drops of phenolphthalein
indicator. Development of pink colour indicates that the milk is adulterated
with soap.
XI.
Detection of detergents in milk
Take 5 ml of milk in a test tube and add 0.1 ml
of bromocresol purple solution. Appearance of violet colour indicates the
presence of detergent in milk. Unadulterated milk samples show a faint violet
colour.
XII.
Detection of water in milk
Though the adulteration of milk with water can be
checked by lactometer reading, other adulterations too affect the lactometer
reading. Hence freezing point depression, recognized by AOAC, is usually
adopted.
Percentage of water added = Normal freezing
point – Observed freezing point X 100
Normal
freezing point
Normal freezing point of milk is taken as
–0.55°C. A tolerance level of 3% is given which is equivalent to specifying a
minimum freezing point depression for authentic milk of –0.55°C.
XIII.
Detection of skim milk powder in milk
If the addition of nitric acid drop by drop in to
the test milk sample results in the development of orange colour, it indicates
the milk is adulterated with skim milk powder. Samples with out skim milk
powder shows yellow colour.
XIV.
Detection of vegetable fat in milk
The characteristic feature of milk is its fatty
acid composition, which mainly consists of short chain fatty acids such as
butyric, caproic, caprylic acid; whereas the vegetable fats consist mainly of
long chain fatty acids and hence adulteration
of vegetable fat in milk
can be easily found out by analyzing the fatty acid profile by gas
chromatography.
XV.
Detection of buffalo milk in cow milk
The presence of buffalo milk in cow milk is
tested by Hansa test. It is based on immunological assay. One ml of milk is
diluted with 4 ml of water and then it is treated with 1 ml of antiserum. The
characteristic precipitation reaction indicates the presence of buffalo milk in
the sample taken. (The antiserum is developed by injecting buffalo milk
proteins into rabbits).
XVI.
Detection of benzoic and salicylic acid in milk
Five ml of milk is taken in a test tube and
acidified with concentrated sulphuric acid. 0.5% ferric chloride solution is
added drop by drop and mixed well. Development of buff colour indicates
presence of benzoic acid and violet colour indicates salicylic acid.
XVII.
Detection of borax and boric acid in milk
Five ml of milk is taken in a test tube to which
1 ml of concentrated hydrochloric acid is added and mixed well. Tip of a
turmeric paper is dipped into the acidified milk and it is dried in a watch
glass at 100°C or over a small flame. If the turmeric paper turns red, it
indicates the presence of borax or boric acid.
Confirmation can be made by adding a drop of
ammonia solution on the turmeric paper and if the red colour changes to green,
it shows the presence of boric acid.
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