Subject: Methylene Blue as a bacterial assay.

We are using methylene blue as a die to detect bacteria in milk. Methylene Blue solution is supposed to indicate the presence of oxygen which would then indicate no microbial activity. If the blue dye disappears quickly, we should assume that oxygen is not present; therefore, bacteria is present because it is using up all of the oxygen. Our problem is that the dye NEVER disappears. Even with milk that has been left out for 24 hours, the methylene blue is still evident. What have we done wrong? Does this work better at a certain temperature ? What strength methylene blue solution should we use? Is there a better method to detect oxygen (or the presence of bacteria) in a liquid?

Thank you for any help that you can give.

Re: Methylene Blue as a bacterial assay.