Adulteration: Adulterants in milk

Definition: Addition or removal of legally prohibited substances from the milk with the view to increase the quantity and reduce the quality to make extra profit.

Common adulterants include
  1.  Addition of water  
  2. Removal of fat 
  3. Addition of starch 
  4. Addition of milk powder 
  5.  Addition of carbonate and bicarbonate

Detection of adulterants

Detection of water: water is the most common adulterant and its presence can be detected by testing the freezing point of milk. The official  method of AOAC assumes a freezing point  for normal milk of –0.550oC

0.55-xt

The percentage added water = –––––––– x 100 where x is freezing point depression.
A tolerance of 3% is allowed which is equivalent to specifying a minimum freezing point depression for authentic milk of 0.5335oC. The presence of water can also be checked using a lactometer.

Detection of neutralizer in milk

            The difference in coagulation behaviors of milk in the presence of alcohol. Add 5 ml of distilled alcohol (95%) to 5 ml of milk sample, mix the contents thoroughly by shaking and observe the coagulation behavior of the sample. The appearance of fine and uniform sized flakes indicates the presence of added neutralizers in milk whereas the appearance of bigger and unevenly sized flakes indicates their absence.

Rosalie acid test

           Add 4 drops of a freshly prepared alcoholic solution of 1% Rosalic acid to the above mixture and mix gently. The appearance of pink color at the junction of mixture and Rosalie acid indicates the presence of either sodium hydroxide potassium hydroxide or calcium hydroxide added to milk, and that of rose-red color indicates the presence of sodium carbonate or sodium bicarbonate.
          The appearance of brownish color indicates the absence of any of these neutralizers. This test can detect the addition of sodium hydroxide, potassium hydroxide, and calcium hydroxide in milk up to 15-20 mg/100ml and sodium carbonate, sodium bicarbonate, and potassium carbonate up to 25-30 mg/100ml. The higher amounts of the neutralizer can be detected from the appearance of pink rose red color even after thorough mixing. The presence of neutralizers can also be detected by testing ash content

Detection of starch

       Place in a test tube about 3ml of a well-mixed sample. Boil it by holding the tube over a flame. Allow cooling to room temperature. Add a drop of 1% iodine solution. The presence of starch is indicated by the appearance of a blue color that disappears when the sample is boiled and reappears on cooling.

Detection of gelatin

        Gelatin produces a yellow precipitate with a picric acid solution. While cloudiness shows a smaller amount and yellow precipitate a large amount of gelatin in milk.

Detection of cane sugar

          To about 15ml of milk in a test – tube add 1 milliliter of concentrated hydrochloric acid and 0.1g of resorcinol and mix. Place the tube in a boiling water-bath for 5 minutes. In the presence of cane sugar red color is produced.

Detection of saccharin

            Curdle an an aliquot of the diluted sample (about 25ml) with dilute acetic acid. Shake well and filter. Acidity the clear filtrate with 2ml of concentrated hydrochloric acid and extracted with 25ml portion of ether. Draw of adequate layers and wash the combined ether extract with 3 successive portions of 5ml of water. Evaporate the ether extract on a water bath and add a drop or two of water, mix well with a glass rod, and taste little. Characteristic sweet taste indicates the presence of the saccharin. 

Detection of glucose or monosaccharides (Barfoed’s test)


         The reagent is prepared by dissolving 6.5 of crystallized copper acetate in 100 ml of 1% acetic acid solution. For the test heat 5ml of Barfoed’s reagent in boiling water for 3 ½ minutes. Production of a red precipitate of cuprous oxide indicates the presence of monosaccharides.

Detection of sodium chloride


         Take 2 ml of milk and add 0.1ml of 5% potassium chlorinate and 2ml of 0.1 N silver nitrate. The appearance of a red precipitate indicates the presence of sodium chloride.

Detection of urea in milk


        Take 2ml of milk and add 2ml of p-dimethyl amino benzaldehyde reagent (1.6% in ethyl alcohol containing 10% HCI). The development of a distinct yellow color denotes the presence of urea. The pure milk samples show a faint pink color which should be ignored due to the presence of natural urea (up to 50mg/100ml.) This test should be carried out with the control sample. A sample paper strip method has also been developed using the above principle.

Detection of formalin


           Take 5 ml of milk sample in the test tube and add 5ml of concentrated sulfuric acid containing traces of ferric chloride. The formation of a purple ring at the junction indicates the presence  of formaldehyde in milk.

Detection of hydrogen peroxide


       The presence of hydrogen peroxide can be detected by an intense blue color developed on addition of 2 drops of paraphenylenediamine hydrochloride to 10ml of milk.

Detection of buffalo milk in cow milk


          The presence of buffalo milk in cow milk can be detected by the Hansa test, which is based on the immunological assay. A drop of suspected milk after dilution with water (1:4) is treated with a drop of antiserum obtained by injecting buffalo milk proteins into rabbits. The characteristic precipitation reaction indicates the presence of buffalo milk.

Detection of added color


            The chief coloring materials which are considered here are some natural coloring material like annatto, turmeric of coal-tar dyes. Some of these dyes are permitted only in some products. While the use of annatto is prohibited in milk, its use is permitted in butter. To detect annatto the milk fat is shaken with 2% sodium hydroxide and the mixture is poured on filter paper. The filter paper absorbs the color, which remains even after washing with water. When the stain is treated with a drop of 40% SnC12 and dried, a purple color indicated the presence of annatto. Turmeric is detected when the colour aqueous or alkali, extracted is treated with HCL. The resulting orange color is treated with H3BO3 crystals, a red color indicates the presence of turmeric.

        Coal-tar dyes adhere to animal fibers more firmly than natural colors. The curd of pure milk is white when extracted with ether but one containing coal-tar dyes remains orange or yellow; this when treated with concentrated hydrochloric acid becomes pink.

Detection of pulverized soap

      Soaps are generally defined as sodium and potassium salt of fatty acid. Therefore, to detect the presence of pulverized soap, iodine value refractive index, fatty acid composition, salt ratio and ash content are excellent methods. The presence can be judged by the qualitative method. For example, in 10ml milk, 10ml hot water is added followed by 1-2 drops of phenol-pathline indicator solution. The development of pink color indicates the presence of soap in milk.

Detection of vegetable fat


        The adulteration of vegetable fat in milk can easily be detected by the following methods. In the case of synthetic milk, the fat is extracted either by the Rose-Gottleib method or fat extracted in butyro-meter can also be used.
  • Fatty acid composition: Milk fat is characterized by lower chain fatty acids. for example butyric, capric, capralic, etc. whereas most of the vegetable fats do not contain these fatty acids. Therefore, the adulteration of the vegetable fat can easily be detected by analyzing the fatty acid profile by gas chromatography.
  • Detection by measuring different Physico-chemical properties: The adulteration of vegetable fat can also be detected by measuring various Physico-chemical properties. For example, refractive index, RM, and Polenske values iodine value. etc.
  • Hydrogenated vegetable oils like banaspati is a common adulterant in milk fat. Its presence in milk fat can be detected by the fact that sesame oil is added in vanaspati as per the law. The presence of sesame oil can be tested by Baudoin test.

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