Module 10. Quality assurance in milk processing

Lesson 42

42.1 Introduction

Though addition of any kind of preservative is not legally permitted in India, suppliers add various kinds of preservatives (e.g. hydrogen peroxide, formalin/formaldehyde etc.) to milk in order to increase its shelf life. Similarly, several neutralizing agents (e.g. sodium/potassium hydroxide, sodium carbonate/bicarbonate etc.) are added to mask the developed acidity in milk. Various adulterants such as starch, cane sugar, maltodextrin etc., which are not native to milk, are added to increase or maintain the prescribed levels of SNF and TS. There are several tests designed to detect the presence of such extraneous materials in milk.

42.2 Detection of Common Preservatives

42.2.1 Boric acid and borates

When a strip of turmeric paper is dipped into adulterated milk sample that has been acidified, it turns into characteristic red colour indicating the presence of boric acid and or its salt.

42.2.2 Formalin or formaldehyde

Formalin is added in milk as a preservative, as it checks the rise in acidity. Acidified milk containing formalin or formaldehyde forms characteristic violet colour with ferric salts and other oxidizing agents. There are mainly two tests namely Hehner and Lech tests that are commonly followed. Hehner test

To 10 ml of the milk sample, 0.5 ml of 10% ferric chloride solution is added. Thereafter, 5 ml of concentrated sulphuric acid is added carefully down the side of test tube to form a separate layer without mixing with milk. Presence of a violet coloured ring at the junction of two liquids indicates the presence of formaldehyde. Lech test
To 5 ml of milk in a test tube is added equal volume of concentrated hydrochloric acid containing 1 ml of 10% ferric chloride solution. The contents are heated over a flame for 5 min and the tube rotated or shaken to break the curd. Development of violet colour indicates the presence of formaldehyde.

42.2.3 Hydrogen Peroxide

Hydrogen peroxide acts as an antimicrobial agent thereby, checking the development of acidity. Addition of 2 drops of 2% freshly prepared aqueous solution of paraphenyl diamine hydrochloride to the adulterated milk sample gives intense blue colour, thus indicating the presence of hydrogen peroxide.

42.3 Neutralizers

Chemicals such as carbonates or bicarbonates are added to milk in order to disguise developed acidity. To detect their presence, equal quantities of the milk sample and ethyl alcohol are mixed to which are added, few drops of aqueous rosalic acid solution. Presence of rose red colour indicates addition of sodium hydroxide, potassium hydroxide and/or calcium hydroxide while pink colour points to the addition of sodium bicarbonate, potassium carbonate or calcium carbonate.

42.4 Detection of Common Adulterants

42.4.1 Sugar

Resorcinol produces red colour with sucrose in an acidic medium. To 10 ml of milk in a test tube, one ml of concentrated hydrochloric acid is added and mixed well, followed by addition of one g resorcinol powder. After mixing thoroughly, the tube is placed in a boiling water bath for 5 min. Development of red colour indicates presence of cane sugar in milk.

42.4.2 Starch

Iodine solution gives intense blue colour with starches. Three ml of well-mixed milk is boiled over a Bunsen burner. After cooling, one drop of 1% iodine solution is added and mixed well. The appearance of an intense blue or bluish violet colour indicates the presence of added starch.

42.4.3 Urea

There are two methods by which added urea can be detected in milk. To 5 ml of well mixed milk sample, 5 ml of para-dimethylamino benzaldehyde solution is added and mixed. The development of an intense yellow colour indicates the presence of urea.

In the second method, 5 ml of 24% TCA solution is added to 5 ml of well-mixed milk sample in a test tube. The contents are filtered through Whatman No. 42 filter paper, and 2% of NaOH solution and 0.5 ml of 2% sodium hypochlorite solution are added to one ml of the filtrate. After thorough mixing, 0.5 ml of 5% phenol solution is added. The development of blue or bluish-green colour indicates the presence of urea.

Recently a strip test for detection of urea has been developed by CCS Haryana Agricultural University, Hisar. This test involving mixing a drop of the test reagent with a drop of milk on a paper strip makes it convenient to detect urea in milk even at household level.

42.4.4 Glucose

Glucose is increasingly being used as an adulterant in milk and for the manufacture of synthetic milk. To 1 ml of milk in a test tube, one ml of Barford reagent (24 g cupric acetate dissolved in 450 ml boiling distilled water, 25 ml of 8.5% lactic acid added to the hot solution, cooled and diluted to 500 ml, filtered) is added. The mixture is heated in a boiling water bath for 3 min and cooled for 3 min under ice cold water. On addition of one ml of phosphomolybdic acid reagent (35 g of ammonium molybdate, 5 g of sodium tungstate in a one litre volumetric flask, 200 ml of 10% NaOH and 200 ml of distilled water added, boiled vigorously for 20-40 minutes and cooled followed by addition of 125 ml of concentrated phosphoric acid, volume made up), the formation of deep blue colour indicates presence of glucose.

42.4.5 Pond water

Several unscrupulous milk vendors dilute the milk with unclean water waterways such as ponds. To detect its presence, a clean test tube is rinsed with 5 ml of milk sample and decanted. Along the side of test tube, 1-2 ml of 2% diphenylamine solution is added. The appearance of blue colour indicates presence of pond water.

42.5 Synthetic Milk

In recent times raw milk has sometimes been adulterated with the so called ‘synthetic milk’ usually prepared out of vegetable oil emulsified with the help of commonly available commercial detergents and other compounds such as urea, glucose etc. A test has been developed at National Dairy Research Institute, Karnal to detect the presence of anionic detergent as an indicator of synthetic milk in genuine milk.

Selected Readings

FAO. 1998. Milk Testing and Quality Control. Milk Processing Guide Series. Volume 2. FAO/TCP/KEN/6611 Project. Training Programme for Small Scale Dairy Sector and Dairy Training Institute – Naivasha. Kenya.
IS: SP : 18 (Part XI), 1981. Handbook of Food Analysis. Part XI., Dairy Products. Bureau of Indian Standards, New Delhi.
Marshall, R.T. 1993. Ed. Standard Methods for the Examination of Dairy Products. 16th ed. American Public Health Association, Washington, DC.
MIF. 2005. Analysis of Milk and Its Products: A Laboratory Manual. Milk Industry Foundation (U.S.). Daya Books, India.
Last modified: Wednesday, 10 October 2012, 5:26 AM