Methylene blue reduction time. In the reaction, protease breaks down milk protein causing the blue methylene dye, widely used as a redox indicator, to decolourise. Test tubes show the start, middle and end point and this has to be timed. To get a reliable figure the experiment should be repeated several times. The pH value of the milk will effect the action of protease. This test can be part of the assessment of the quality of raw milk for its suitability for further industrial processing.