Milk Testing – Two Minutes Resazurin Test
Objectives:
To determine extent of bacteriological quality of milk.
Relevant Information:
The majority of the organisms in milk are capable of reducing and decolorizing the resazurin dye. When bacteria grow in the milk they utilize oxygen, the rate of remove or reduction is proportional to the keeping quality. This test is also based on the same principles as M.B.R. (Methylene Blue Reduction) test, but dye is Resazurin which is much more sensitive that the Methylene blue. For this reason this test provides a rapid measure of the keeping quality of milk.
During incubation, the dye undergoes reduction very largely through the metabolic activity of the organisms present. The greater the number of organisms present in milk, the more quickly the dye is reduced. The reduction takes place in two distinct stages. Resazurin is blue at the reaction of milk. In the first stage dye is changed to pink colour and in second stage pink colour is changed to colourness. The cells present in the milk may also influence the reduction of Resazurin and for that reason; the test may also measure physiologically or pathologically abnormal milk.
Precaution:
The test should be carried out aseptically.
Material Required:
i Milk sample
ii. Resazurin colour/solution 0.05%.
Apparatus:
i. All purpose lovibond comparator.
ii. One Resazurin colour disc from blue to white.
iii. Water path, thermostically controlled to maintain temperature of 37 0 C.
iv. Test tube 10 ml.
v. Pipette 10 ml. and 1 ml.
Procedure:
i. Milk the sample thoroughly by inverting from one to another container.
ii. Pour 10 ml. of milk sample in to previously sterilized test tube.
iii. Add quickly 1 ml. of Resazurin solution in the test tube.
iv. Mix the milk and dye thoroughly by inverting 2-3 times.
v. Place the tubes in the water bath at the temperature of 37.5 0 C only for two minutes.
vi. The tubes are then removed from the water bath.
vii. Compare the colour to test tube with standard disc until the colours are matched under comparator.
viii. Record the number of disc/colour of disc. If colour falls between two disc numbers record half value.