That's the title of this post since that's more or less the number one question we get about mushrooms! The answer to this question is probably "yes," but that's just part of how to test your mushrooms for freshness!
Here's our list of test questions:
Are my mushrooms moldy? (first and last question)
Mold is a microscopic fungi, but very different from the mushrooms we eat. According to the USDA some of these cause allergic reactions and respiratory problems. This should be a real concern, especially for those with a diagnosed mold allergy or respiratory problem, but often mushrooms that look problematic are perfectly safe and tasty. That's because mushroom mycelium can look somewhat like mold! So you should look more closely at you mushrooms before you discard them. Please look through the rest of these questions before you give up on them!
Are my mushrooms slimy?
Most mushrooms should look slightly dry and crisp, and certainly firm. Older mushrooms tend to get soft and slimy, and you should discard mushrooms like this, which aren't safe to eat because of higher levels of bacteria in them. However, be aware that some mushrooms - especially Philiota genus like Chestnut and Nameko, have a tendency to be slimy even when just harvested! When evaluating mushrooms like these they should still be firm - and they should smell good.
Are my mushrooms wrinkled and shriveled?
Older mushrooms will often start to dehydrate over time. If they look too dry but still smell good you can use them, but they don't have long!
Do my mushrooms have a soft texture?
Most mushrooms have a firm stem, if not cap, but this will soften as they begin to decay. Of course you should be aware that mushrooms like oysters are fairly soft even when just harvested!
Are my mushrooms discolored?
Discoloration is another sign of decay, but you should be aware of natural color variations as you evaluate your mushrooms. Discoloration means something different for different types of mushroom. For example lion's mane mushrooms are white, but they often have a faint pink or orange hue - even when freshly harvested.
Do my mushrooms smell good?
This is one of the most reliable indicators of freshness - and therefore safety of your mushrooms to eat! Mushrooms should have a fresh earthy smell, if they smell like anything at all. You should discard any mushrooms that have stong smell - especially a fishy or ammonia-like smell.
Are my mushrooms moldy - and what if there is white fuzz on my mushrooms?
Back to this question! Given enough time, mushrooms will mold - especially if stored in a moist environment. All of our packaging (such as sugarcane clamshells and hole-punched plastic) is at least somewhat porous, but eventually mushrooms will start to mold - though generally after a few weeks from harvest. Again, moldy mushrooms should be discarded.
If your mushrooms look good as you've gone down this list, but have white fuzz on them, then what you are probably seeing is mushroom mycelium, rather than mold. Some mycelium (the mushroom organism) like chestnut or oyster tend to grow on the mushrooms they produce and not just on growing media. So the white fuzz you see on mushroom stems is almost certainly mushroom mycelium, rather than mold. This is completely harmless, and won't effect the taste of your mushroom dish.
Below are some photos of chestnut mushrooms - first in the grow room, then just harvested (both front and back). You'll see that when they are as fresh as can be, they still have this white fuzz at the base of stem. Note that they are firm and have almost no smell, and will have a shelf life of about two weeks from harvest.
A last few notes about mushroom safety
This is not a comprehensive guide to mushroom safety, but here are a few other suggestions:
You should also make sure that the mushrooms you're eating are edible - so that you don't suffer from mushroom poisoning.
Even with well-known mushroom varieties, you may have a negative reaction to one or another. We recommend eating modest amounts of a new mushroom - should you prove intolerant!
You should keep mushrooms below 40 degrees to maintain mushroom freshness
If you must mushrooms at room temperature for more than a short time, never store them in a sealed container - since this may encourage botulism or other dangerous anaerobic bacteria!
We recommend that you cook all mushrooms before eating. This will neutralize to some extent bacteria in your mushrooms, and especially in mushrooms that aren't at peak freshness! And they taste great sauteed or roasted anyway!
