Milk and dairy product adulteration came into global concern after breakthrough of melamine contamination in Chinese infant milk products in 2008. However, history of milk adulteration is very old.
Milk is considered to be the ‘ideal food’ because of its abundant nutrients required by both infants and adults. It is one of the best sources for protein, fat, carbohydrate, vitamin and minerals. Unfortunately, milk is being very easily adulterated throughout the world.
Possible reasons behind it may include- demand and supply gap, perishable nature of milk, low purchasing capability of customer and lack of suitable detection tests. The motivation for food fraud is economic, but the impact is a real public health concern.
The situation is significantly worse in developing and underdeveloped countries due to the absence of adequate monitoring and lack of proper law enforcement.
Qualitative detection methods:
Qualitative detection of adulterants in milk are simple color based chemical reactions. These can be performed in any Biosafety Level 1 Laboratory with availability of chemical reagents and necessary precautions.
Major drawbacks of these techniques are the facts that these are valid for a limited range of concentrations and are not sufficiently precise. However, qualitative detection are advantageous because these are simple, rapid and very easy to perform.
There are also some hazardous chemicals added in milk to improve the physical appearances and shelf life. Some of those are very hazardous and can lead to fatal diseases.
Detection methods:
Sugar
Test: Take 5 mL sample in a test tube. Add 1 mL conc. HCl and 0.1 g resorcinol solution. Place the test tube in water bath for 5 min.
Observation: Appearance of red color indicates he presence of added sugar.
Starch
Test: Take 3 mL sample in a test tube. After boiling it thoroughly, cool it to room temperature. Add 1 drop of 1% iodine solution.
Observation: Appearance of blue color indicates he presence of starch.
Glucose
Test: Take 1 ml of sample in a test tube. Add 1 ml of modified Barfoed’s reagent. Heat the mixture for exact 3 min in a boiling water bath. Rapidly cool under tap water. Add one ml of phosphomolybdic acid reagent to the turbid solution.
Observation: Immediate appearance of deep blue color indicates the presence of glucose.
Common salt
Test: Take 5 ml of sample into a test tube. Add 1 ml of 0.1 N silver nitrate solution. Mix the content thoroughly and add 0.5 ml of 10% potassium chromate solution.
Observation: Appearance of yellow color indicates the presence of added salts, whereas, brick red color indicates the milk free from added salt.
Formalin
Test 1: Take 10 mL sample in attest tube. Add 5 mL conc. sulfuric acid with a little amount of ferric chloride without shaking.
Observation: Appearance of violet or blue color at the junction of two liquid layers indicates the presence of formalin.
Test 2: Take about 5 ml of sample in a test tube. Take 1 ml of 10% ferric chloride solution in a 500 ml volumetric flask and make up the volume using concentrated hydrochloric acid. Add 5 mL from this solution to the sample in test tube. Keep the tube in boiling water bath for about 3-4 min.
Observation: Appearance of brownish pink color indicates the presence of formalin.
Test 3: Take 1 mL of sample in a test tube. Take saturated solution of 1, 8- dihydroxynaphthalene-3, 6- disulphonic acid in about 72% sulfuric acid to make chromotropic acid solution. Add 1 mL of chromotropic acid solution to the sample in test tube.
Observation: Appearance of brownish pink color indicates the presence of formalin.
Ammonium sulfate
Test: Take 2 ml. in a test tube and add 0.5 ml NaOH (2%) 0.5 ml sodium hypochlorite (2%) and 0.5 ml phenol (5%) Heat in boiling water bath for 20 sec.
Observation: A bluish colour forms immediately, which turns deep blue afterward. Pure milk shows salmon pink colour which gradually changes to bluish after 2 hours.
Test 2: Take 10 ml of sample in a 50 ml stoppered test tube. Add 10 ml of TCA solution. Filter the coagulated milk through Whatman filter paper Grade 42. Take 5 ml of clear filtrate. Add few drops of barium chloride solution.
Observation: Formation of milky-white precipitates indicates the presence of added sulfates like ammonium sulfate, sodium sulfate, zinc sulfate and magnesium sulfate etc. to milk
Urea
Test 1: Take 5 mL milk sample in a test tube. Add equal volume of 24% TCA to precipitate fat and proteins of milk. Take 1 mL filtrate and add 0.5 mL 2% sodium hypochlorite, 0.5 mL 2% sodium hydroxide and add 0.5 mL 5% phenol solution, then mix.
Observation: A characteristic blue or bluish green colour develops in presence of added urea whereas pure milk remains colourless.
Test 2: Take 5 ml milk in a test tube, add 0.2 ml urease (20 mg/ml) Shake well at room temperature and then add 0.1 ml Bromothymol Blue (BTB) solution (0.5%).
Observation: Appearance of blue colour after 10-15 min. indicates the presence of urea in milk. Normal milk shows faint blue colour due to natural urea present in milk.
Benzoic and salicylic acid
Test: Take 5 mL milk sample in a test tube. Then 0.5% ferric chloride solution is added to it drop by drop. Mix it. 5 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.
Observation: Appearance of buff color indicates the presence of benzoic acid whereas that of violet color indicates salicylic acid.
Author: Muhammad Hassan, M.Sc. (Hons.) Food Technology, University of Agriculture, Faisalabad.