How do I know how fresh a mushroom is? When it should be thrown out?

Mushrooms are neither plant nor animal, they need to be treated as their own separate group.

How do I store mushrooms so as to maximize their freshness?

• poster created by the U.S. Mushroom Council
  • "Do not stack other produce items on top of mushrooms as they bruise easily."
  • "Shelf Life: 10 days, under optimum conditions."
  • "Do not rinse until ready to use."
• Harvest to Market Guide > Post-Harvesting Handling — This advice is directed to food producers, not end consumers, but some of it may apply to them too.
  • "Mushrooms should always be cleaned prior to storage. ... Mushrooms, in particular, are unique in that they are mostly water, and are not best cleaned by washing, since it ruins their texture and shelf life of the product. ... Instead, a variety of brushes are used to clean them. ... Additionally, a damp towel can be used to pat mushrooms and clean them lightly, if necessary."
  • "Once mushrooms are clean, they should be stored in food grade containers in refrigeration conditions at 36 – 38 °F. Storage bins with loose fitting lids are ideal; never seal the lid on your storage container. ... Paper bags work well for small amounts."
• Note that the "should you wash mushrooms?" question is a hotly-debated one. [1] [2] [2b] [3] [4]

Indications that mushrooms need to be thrown out

• if they smell fishy at all, they're bad
• How to Tell If Mushrooms are Bad | AllRecipes
  • "The number one rule of thumb when it comes to detecting freshness is that when mushrooms are slimy, they're no longer ideal for use."
  • "If the mushrooms have also developed an unpleasant odor, they should not be used. Bad mushrooms can make you sick due to bacteria that has started to form."
  • "Appearance alone can tell you a lot when looking at the choices of fungi. If the mushrooms have bruises or dark spots, look slimy and wet, or are slightly darker in color than usual, they are starting to go bad."