When the bacterial cells are started to increase their numbers in milk it consumes more dissolved oxygen from the milk, as a result, the oxygen gets depleted. Then the Methylene Blue starts acting as an electron acceptor instead of oxygen. The methylene blue gets reduced due to the decreases of oxidation-reduction potential from + 0.06 to 0.01 V.The double-bonded nitrogen atom of  Methylene Blue dye accepts 1 atom of hydrogen as a result the dye is converted into a colorless state. The greater is the number of microorganisms in milk, the greater is the metabolic activity and the faster is the reduction of methylene blue.