Paragraph 4 Method of carrying out the test

The sample shall be mixed thoroughly by inverting and shaking and the milk shall be transferred to a test tube up to the 10 ml. mark in such a manner that one side of the interior of the test tube is not wetted with milk. 1 ml. of methylene blue solution shall be added without letting the pipette come into contact with the milk in the tube or with the wetted side of the interior of the tube. After a lapse of 3 seconds, the solution remaining in the tip of the pipette shall be blown out. The test tube shall be closed with a rubber stopper, aseptic precautions being taken, and shall then be inverted twice slowly, so that the whole column of contained air rises above the level of the milk. Within a period of 5 minutes the test tube shall be placed in a water bath. The water in the bath shall be kept above the level of the milk in the test tube, and its temperature, which shall be between 37°C. and 38°C., shall be maintained as nearly uniform as possible by means of a reliable automatic thermo-regulator. The interior of the bath shall be kept completely dark.

A control tube shall be used for comparison with each batch of experimental tubes to indicate when decolourisation is complete. The control tube shall be prepared by immersing in boiling water for 3 minutes a stoppered test tube containing 1 ml. of tap water and 10 ml. of mixed milk having a fat content and colour similar to that of the milk being tested.

The milk shall be regarded as decolourised when the whole column of milk is completely decolourised or is decolourised up to within 5 mm. of the surface. A trace of colour at the bottom of the tube may be ignored provided that it does not extend upwards for more than 5 mm.