Fresh mushrooms contain a lot of water, and once harvested they continue to respire and gradually break down.  The mushrooms you buy at the supermarket usually come in a closed or plastic-covered carton.  The moisture released by the mushrooms becomes trapped inside.  That creates a damp environment that produces a slippery or slimy surface.

A few things accelerate it:

• Condensation: Temperature changes during shipping, refrigeration, or the trip home cause water to condense inside the package.
• Poor ventilation: Plastic film holds humidity around the mushrooms.  Once relative humidity gets very high, their surfaces stay wet.
• Natural deterioration: As mushroom cells age, their structure begins to break down and release moisture.
• Microbial growth: Bacteria and yeasts naturally present on mushrooms multiply much more rapidly on a wet surface.  The slime is partly the result of this microbial activity and tissue breakdown.
• One wet mushroom affects the others: Mushrooms touching a deteriorating mushroom tend to become wet and deteriorate more quickly themselves.

So what’s the best way to store mushrooms?  A paper bag.

Take the mushrooms out of the plastic-covered carton and put them in a paper bag in the refrigerator, loosely folded over at the top.  The paper absorbs excess moisture while still allowing some air exchange.  Don’t wash the mushrooms before storing them; wash or wipe them just before cooking.

If you want to maximize shelf life further, put a dry paper towel in the bag with them. Replace it if it becomes damp.

Pantry by Clare has a “Vegetables (Fresh)” category but mushrooms are a good example of why you might want to add a category for specific food items: refrigerated mushrooms generally deteriorate considerably faster than many vegetables such as carrots, cabbage, celery, or peppers.