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Putting Moldy Food Into Compost


March 13, 2012

Question posed by Master Composter WR: I have a friend who recently bought a compost bin, the kind that rotates. She said the people who sold it to her said not to put moldy food into it because it would introduce bad bacteria. I’ve certainly never heard of such a thing. (For one thing, isn’t mold a fungus, not a bacteria?) It seems like moldy food in your fridge just has a head start in the composting process, and I wouldn’t hesitate to compost my own moldy food. Any thoughts about this? Was she given bad advice or am I wrong about this?


Answer by JE (a microbiologist): Mold is a fungus and not bacteria. However, if something smells bad, then it is probably from anaerobic bacteria. The bacteria that spoil food don’t make us sick as much as they gross us out. Most of the bacterial food pathogens don’t smell at all! The deceptive little buggers. I agree that putting spoiled food into any compost pile means the microbes are already getting started at the process.

Answer by PB: I agree with this too. Just spoke to a group this morning and the subject of mold came up. I have them basically the information WR and JE have stated above.

Response from Original Questioner:  Thanks a lot JE and PB for your thoughts. I did a google search for “don’t put moldy food in compost” (without the quotes) and most that came say it’s fine to compost moldy food. The one person I found who said not to compost moldy bread also said to not compost bread at all as it “adds nothing to the compost heap”. Of course, I don’t agree with that at all, so don’t consider that person to be a credible source. A major reason to compost is to keep organic waste out of the landfill. Also, bread contributes plenty of organic matter to the pile, per volume. The only reason I wouldn’t compost bread would be if I were concerned about dogs or other large animals eating it. Which wouldn’t be a problem in a tumbler bin.

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