The following article will guide you about how to test the quality of milk and to detect the presence of microorganisms in the milk.
Quality control is vital in the production and handling of milk and milk products. Milk is not only an ideal food for man, also a very good food for many species of bacteria which rapidly bring about its spoilage.
The bacteria are present everywhere and unless specific precautions are taken at each step, they are likely to get into the milk or milk products in large numbers. The presence of bacteria is difficult to detect unless elaborate tests are carried out, with the result that without having this information in advance and if the activities of bacteria are not checked, there may be very heavy financial losses to the manufacturer because of the spoilage of the product.
For these reasons, it is extremely important that some tests which will help in following up the quality of milk are carried out as a matter of routine. There are some tests which are called platform tests which can be used at the receiving platform of the dairy to judge the quality of the milk received from each supplier.
Some platform tests are described here:
1. Organolaptic Test:
Soon after milk is received on the platform of dairy, the lid of can is opened and milk is stirred up with a plunger to test for smell whether pleasant or unpleasant. Then, it is tested for appearance (colour) taste and flavour.
2. Acidity Test:
The acidity developed in milk as a result of bacterial activity is measured by titration against standard alkali solutions. The normal acidity of milk ranges from 0.12 to 0.16 % lactic acid. Acidity in excess of about 0.20 % indicates poor quality of milk.
3. Indicator Test:
The acidity developed in milk due to bacterial activity is measured in terms of pH value as indicated by special indicator dyes e.g., brom thymol blue and brom cresol purple.
4. Sediment Test:
Milk is passed through a funnel containing a filter disc and the amount of dirt and dust collected is compared visually or by weight. The test indicates the gross impurities and dirt in milk as a result of unhygienic conditions of production.
5. Clot on Boiling (COB) Test:
This test is used to determine the shelf life of milk samples and its ability to withstand pasteurisation. A small amount of milk is boiled in a test tube over a flame for two minutes. When acidity exceeds beyond 0.2% the milk clots and hence, it is COB positive which means unsuitable to heat treatment.
6. Fat Test:
Pour 10 cc of sulphuric acid (specific gravity 1.82) into the butyrometer and add 11cc of milk gently along the side of butyrometer, so that it floats on acid. Add 1 mL of amyl alcohol (specific gravity 0.81). Rubber stopper is inserted to close it. Mix the fluids in the tube by gently shaking.
The normal specific gravity of pure fresh cow and buffalo milk varies from 1.028 to 1.030 and 1.030 to 1.032, respectively. It is affected by the nature of adulteration. Specific gravity of milk can also be determined by Wesphal balance.
7. Resazurin Test:
Take 10 mL of milk sample in a sterile test tube and add 1 ml resazurin dye solution (0.005%). Mix the content and incubate at 370° C for 10 min. After this the colour is read against a control milk through resazurin disc comparator to judge the quality of milk, which is given in Table 3. This method at the most takes 10 min to test.
8. Specific Gravity Test:
It is the density of milk showing the ratio of heaviness/mass of milk to the water. It depends upon the amounts and nature of solid substances present in milk. The Solids Not Fat (SNF) increase the specific gravity (sp.gr) of milk, whereas fat decreases it.
To determine specific gravity, mix the sample of milk (standard temperature 60°F) in a jar and lower the lactometer gently into milk to float. Leave it for few seconds. When it is stationary and do not touch the side of the jar, then read the scale of lactometer. If the temperature is above or below the standard, then add 0.1 to the lactometer reading. The lactometer reading after correction is known as corrected lactometer reading.
Specific Gravity of milk = (1+ CLR) / 1000
Where, CLR – Corrected Lactometer Reading
9. Bromocresol Purple Test:
This tests alkalinity in milk due to mastitis or colostral milk. A little milk is placed on the strip of bromocresol paper. If the milk turns bright purple, it is an indication of alkalinity in milk. This is quick and effective test.
10. Hansa Test:
It is meant for testing adulteration of cow milk with buffalo milk. Skimmed milk is injected into rabbits and the anti-bodies produced are collected and purified to make specific anti-scrum. When anti-scrum is added to buffalo milk, tiny particles can easily be seen.
A drop of milk is placed on a clean glass slide and one drop of scrum is added and mixed. If sample contains buffalo milk, the mixture on slide shows clears flocculation, in case of particles within a minute. However, if there is no change in case, then it is pure cow milk.
11. Test for Heated Milk:
Take 10cc of milk in a test tube and add 3mL of starch solution, 1 mL of potassium iodine solution and 2 drops of hydrogen peroxide in it. Shake the contents well. If the milk is already heated, a pale yellow colour appears, otherwise unheated (raw) milk shows a blue colour. The reagents take 4 to 5 min to react.
12. Guaical Test for Heated Milk:
Take 5 mL milk in a test tube and add 5 mL 1% solution of crystallised guaical and 1 drop of hydrogen peroxide. If the milk is heated, it shows no colour but unheated raw milk gives a pomegranate colour.
Bacteriological Tests of Milk:
It is a method of analysing to estimate the number of bacteria present in the milk.
The following tests are done to detect the presence of microorganisms in the milk:
1. Standard Plate Bacterial Count:
Collect representative sample and pipette out one mL of milk and mix it in a 9 mL priority sterilised normal saline blank in a tube to prepare 1:10 dilution. Similarly, subsequent dilution are prepared (1:100, 1:1000, 1:100000 etc.).
Then, 1 mL of the suitable dilution mixture is pipette out to transfer into a sterilised petriplate and is mixed with melted sterilised nutrient agar medium (15 g agar-agar + 5 g peptone + 3 g beef extract + 5 g sodium chloride + 1000 mL distilled water) and after setting, it is incubated at 37° C for 48 h.
The bacterial colonies developed due to proper temperature and nutrition present in medium are counted with the help of colony counter and only such dilution which show number of colonies between 30 to 300 per plate is considered ideal for greater accuracy of results.
The number of colonies is multiplied by dilution presented as number. Standard plate bacterial count (Spe/ml).
2. Direct Microscopic or Breeds Count:
In this 0.01 mL milk is uniformly spread on 1 sq cm area on clean glass slide. This film is dried by passing it over flame of spirit lam and the film is stained with Newman’s stain. After that it is washed with water and dried. Then, the slide is observed under oil immersion lens of microscope. This gives the total number of bacteria (live and dead) per mL of milk.
3. Methylene Blue Reduction Test (MBRT):
Bacteria in milk utilize oxygen present in milk as a result oxidation reduction potential is reduced from + 02 volts (in normal milk) to –0.02 volts in milk containing methylene blue due to bacterial action and hence milk becomes colourless. Add 10 mL methylene blue dye in the sample.
Mix the content and put two drops of liquid paraffin to prevent air contact. Keep it at 37°C in the water bath and examine every half an hour till it gets decolourised, the time is noted.
4. Resazurin Test:
Take 1 mL milk sample in a sterile test tube and add 1 mL resazurin dye solution (0.05%), and mix the contents. Add two drops of liquid paraffin to seal air entrance and keep it in water bath at 37°C. Compare the colour after every half an hour with standard milk sample in which resazurin is not added.
5. Tetrasolium Test:
Recently, a new type of indicator, viz. triphenyl tetrazolium bromide, has been used for testing milk. Unlike other dyes, it is initially colourless and then becomes red as the oxygen level in milk falls due to bacterial activity. If the milk sample containing this indicator assumes a red colour within half an hour, it indicates that sample is very poor sample.
6. Phosphatase Test:
This test is very important for determining the efficiency of the heat treatment in pasteurisation, and depends on the fact that the phosphatase (an enzyme) present in milk is destroyed at the critical pasteurisation temperature.