A standard solution of resazurin is prepared by dissolving one tablet in 50ml of sterile glass-distilled water. One ml of this solution should be added to 10ml of milk for each test. Solutions must be made freshly each day.
During incubation with milk resazurin undergoes reduction and the colour changes from blue to pink to colourless. The rate of colour change is an indication of the number of microbial organisms in the sample.