Methylene blue is a dye behaving as a redox indicator that is commonly used in the food industry to test the freshness of milk and dairy products. A few drops of methylene blue solution added to a sample of milk should remain blue (oxidized form in the presence of enough dissolved ), otherwise (discoloration caused by the reduction of methylene blue into its colorless reduced form) the dissolved  concentration in the milk sample is low indicating that the milk is not fresh (already abiotically oxidized by  whose concentration in solution decreases) or could be contaminated by bacteria also consuming the atmospheric  dissolved in the milk. In other words, aerobic conditions should prevail in fresh milk and methylene blue is simply used as an indicator of the dissolved oxygen remaining in the milk.