Why does methylene blue decolorize in stale milk, and what would happen if sterilized milk was used instead?

d) Inference: The decolorization of methylene blue indicates the presence and activity of reductase (dehydrogenase) enzymes. These enzymes, produced by microorganisms in the stale milk, remove hydrogen from the milk's components and transfer it to the methylene blue, causing it to become colorless. f) No, I would not expect to obtain the same results if sterilized milk was used instead of stale milk. Sterilized milk has been treated (e.g., by heating) to kill most or all microorganisms. These microorganisms are the source of the reductase enzymes that cause methylene blue to decolorize. Without these active enzymes, the methylene blue would not be reduced, or its reduction would be significantly slower, compared to stale milk which is rich in these microorganisms and their enzymes. That's 2 down. 3 left today — send the next one.