Milk and milk products are rich and convenient source of nutrients for people in many countries with only requirement being of it free of any sort of contamination or adulterants or else can be hazardous to health and shall be contrary to the objective of intake. Milk being constituted of high percent of water is prone to adulteration by human for profit. Not only this, due to high nutritious composition of milk they easily get microbial contaminated, as it provides ambient condition for microbial proliferation.
Special care should be taken in primary stages to eliminate distribution of any unfit milk for consumption. Receiving area are known as milk receiving platform or milk reception dock (RMRD) and raw milk are received in cans. RMRD includes operations such as sampling, testing, weighing or measuring and recording, grading, unloading, dumping and pumping. Before unloading of raw milk for further processing it has to pass through rigorous examinations (tests) to make decision regarding its acceptance and making payments. These tests must be easy to perform, should give quick and reliable results and should not require complicated and elaborate equipment.
“Platform Tests” is the term given to all the tests that raw milk go through to assess the quality of the incoming milk before it is accepted and weighed. They are performed on each can/tanker of milk with the objective of detecting milk of inferior or doubtful quality, to prevent it being mixed with high- grade milk. It helps in making quick decision regarding its acceptance/rejection. Sometimes the term ‘Rapid Platform Test’ used to refer mainly to the organoleptic tests, which take very little time to perform. Platform tests are based on organoleptic (sensory) tests, such as those for smell, taste, appearance and touch, acidity, sediment, fat/SNF, etc.
Classification of Tests
- Organoleptic tests: Organoleptic or sensory tests are performed with the help of five sensing organs, viz. eye, nose, tongue, ear and skin, before emptying the transport containers.
- Smell (Odour): For conducting the test, cover of each can is removed, inverted and raised to the nose and checked for any sort of abnormal smell. The top of the milk in the can may simultaneously be noted for smell and test can be repeated by shaking the can vigorously. The milk should be free from any abnormal smell. For tanker milk, it is advisable to go with this test before chilling process as after chilling, any sort of abnormal smell tend to get supressed.
- Appearance: By carefully observing milk in each can for any floating extraneous matter, off-colour or partially churned milk. The milk should be normal in colour, free from churned fat globules and reasonably free from any floating extraneous material. Small white clots or grains, mastitis milk or adulterated with flour and skim milk powder.
- Temperature: Temperature at which milk is delivered is often an indication of its quality. A daily check on the temperature of milk is helpful in grading the milk at the receiving platform. A temperature of 50C or below is satisfactory and advisable.
- Sediment: Presence of any sort of visible foreign matter in the milk is identified by letting specific amount of milk to set for sufficient time. With time all the heavy or dense material, tend to settle down. It need not made daily, but should be made sufficiently often to ensure a clean milk supply. A low sediment is desirable.
- Preliminary tests: these are simple and rapid physio-chemical tests which can be performed easily on the reception dock.
2.1 Clot-on- boiling test (COB test): Clot on boiling (C.O.B) is quick and simple tests for highly acidic milk (pH<5.8) or abnormal milk (e.g. colostral or mastitis milk). If a milk sample fails in this test, milk might be containing acids or acid producing bacteria or abnormally high level of proteins such as colostrum. Such milk turned into curd or clot on heat treatment during milk processing. Thus, this type of milk should be rejected.
2.2 Lactometer test: Milk has specific gravity or density, determined by lactometer. Addition of water in milk to increase its volume for high price can be determined by change in density of such watered milk. If it is tested along with butterfat test, it enables us to calculate the milk total solids (% TS) and solids not fat (SNF).
2.3 Alcohol test: Alcohol coagulation test is simple and rapid. It is based on poor stability of milk proteins in presence of alcohol. When milk contains the high level of lactic acid, rennet, milk albumin or salt concentration as in mastitis, it may fail to qualify the test.
2.4 Acidity Test: Bacteria that normally grow in raw milk ferment lactose and produce lactic acid. In the acidity test, lactic acid is neutralized with 0.1N solution of sodium hydroxide. The amount of sodium hydroxide is measured and from this amount, percentage of lactic acid is calculated.
2.5 Fat content: Fat content of milk /cream of dairy animals is one of the important factors in determining the price of the milk/cream. Hence fat % is calculated by Gerber method. Testing of butterfat is also important to know in order to make correct adjustments of the butterfat percentage in standardized milk and milk products.
2.6 Freezing Point Determination: The freezing point of milk always has a constant value. A small adulteration of milk with water will cause a detectable elevation of its freezing point from its normal values of -0.54ºC. This test is accurate and sensitive to added water in milk. It is therefore used to monitor the adulteration of milk with water.
2.7 Alcohol-Alizarin Test: This test is just like alcohol test. Alizarin is a pH indicator. Alcohol–Alizarin (A-A test) test is more informative. It will indicate the coagulation as well as intensity of acidity.
- Microbiological Tests: These tests provide information on the sanitary condition and keeping quality of milk. The tests are intended to be carried out on samples collected for microbiological analysis. It helps in determining the extent of bacterial contamination and growth in milk.
3.1 Dye-reduction Test (Methylene Blue or Resazurin): The methylene blue reduction test is based on the fact that the color imparted to milk by the addition of a dye such as methylene blue will disappear more or less quickly depending on the microbial load in the sample. The removal of the oxygen from milk and the formation of reducing substances during bacterial metabolism causes the colour to disappear.
Platform tests are necessary for controlling the quality of incoming raw milk as it is collected from various sources via different modes of transportation. A milk processor or handler will only be assured of the quality of raw milk if certain basic quality tests are carried out at various stages of transportation of milk from the producer to the processor and finally to the consumer. This test must be rapid, accurate and reliable as keeping quality of milk deteriorates with time, which further lowers the quality of final product. We know that, in order for any processor to make good dairy products, good quality raw materials are essential.
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