You’re standing in front of the fridge, staring at a half-empty carton of cremini mushrooms. They’ve been there for... how long? Maybe five days. Maybe a week. You see a little dampness on the surface. They look a bit darker than they did at the store. You wonder if you can just cook the "bad" out of them. Honestly, we've all been there, and usually, the instinct is to just toss them to be safe. But mushrooms are weird. They are fungi, after all. Knowing how to tell if a mushroom is bad isn't just about avoiding a stomach ache; it's about understanding the biology of decay so you don't waste perfectly good food or, conversely, end up with a nasty case of food poisoning.
Mushrooms don't rot like a piece of fruit. They don't usually get a "bruise" that you can just cut off. When a mushroom goes, it goes.
The Sliminess Factor: Why Texture is Everything
If you touch a mushroom and your finger slides off a layer of film, stop. Just stop. A fresh mushroom should feel dry, firm, and slightly velvety. The moment they start developing a "slime" or a slick coating, it’s a sign that bacteria have begun to break down the cellular structure. This isn't just "extra moisture." It’s the beginning of the end.
Sometimes you’ll see people online saying you can wash the slime off. Don't listen to them. While a tiny bit of tackiness on a shiitake might be okay if the flesh is still rock-hard, for your standard white button or portobello, slime is the universal "no" signal. Bacteria like Pseudomonas tolaasii thrive in the damp environment of a mushroom package. Once that biofilm starts forming, the mushroom is effectively a petri dish.
It’s gross. It feels slippery. It’s a hard pass.
Wrinkles and Shriveled Skin
Look at the cap. Is it smooth and plump? Or does it look like a thumb that’s been in the bathtub too long? Mushrooms are about 80% to 90% water. As they age, they dehydrate. A few wrinkles aren't necessarily a death sentence—they might just be a bit dry. You can often use slightly wrinkled mushrooms in a long-simmered soup where they’ll rehydrate. However, if the wrinkles are accompanied by a soft, spongy texture, the structure has collapsed. It’s spent.
The Smell Test: Trust Your Nose
Mushrooms should smell like earth. They should smell like a damp forest floor or, at the very least, like nothing at all. If you open that container and get a whiff of something sweet, sour, or—worst of all—ammonia, throw them away immediately.
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I’ve talked to chefs who describe the smell of a "gone" mushroom as "cloying." It’s a scent that sticks in the back of your throat. That ammonia smell is a byproduct of protein breakdown. It's nitrogen being released as the mushroom literally decomposes. If your nose wrinkles before you even get the mushroom up to your face, you already have your answer on how to tell if a mushroom is bad.
Dark Spots and Gill Discoloration
Flip the mushroom over. Look at the gills—those little ridges underneath the cap. In fresh Agaricus bisporus (the common white mushroom), the gills should be tight and relatively light in color. As they age, the gills open up and turn dark brown or black. Now, this is where it gets tricky. A portobello mushroom naturally has dark gills. That’s just its vibe. But if a white button mushroom has gills that are turning into a black slurry, it’s toast.
- Spotting: Small brown spots on the cap are common. They are basically "age spots." If there are only a few, you can peel the skin or just cook them.
- Total Discoloration: If the entire mushroom has turned from white to a murky tan or grey, the oxidation has gone too deep.
- The Stem: Check the base of the stem. If it’s black or mushy, the rot is traveling upward.
The Hidden Danger of Store Packaging
Why do mushrooms go bad so fast? Usually, it's the plastic wrap. Stores wrap them in plastic to keep them looking "pretty" and to prevent weight loss from evaporation. But mushrooms are alive. They respire. They breathe. When you trap them in plastic, they "sweat," and that trapped moisture is exactly what triggers the slime.
The best thing you can do when you get home is take them out of that blue or white plastic tray. Put them in a brown paper bag. The paper absorbs the excess moisture while still allowing the mushrooms to breathe. It can literally double their shelf life.
What about "Fuzz"?
Sometimes you’ll see a white, fuzzy growth on the stem or cap. People panic and think it’s mold. Often, it’s actually just "mycelium"—the mushroom trying to grow again. If the fuzz is pure white and the mushroom is otherwise firm and smells fine, it’s usually safe. But if that fuzz is green, blue, or black? That’s mold. That’s a different story. Throw it out.
How to Tell if a Mushroom is Bad: Varieties Matter
Different mushrooms die in different ways. A Chanterelle won't look like an Oyster mushroom when it turns.
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Oyster Mushrooms: These are incredibly delicate. When they go bad, they get "wet" looking and start to smell like old fish. Because of their high surface area, they rot much faster than a thick-capped cremini. If they feel limp or look transparent in spots, they're gone.
Shiitakes: These are the hardy survivors. They tend to dry out rather than get slimy. If a shiitake is dry and leathery, it’s actually still fine to use in a stock. But if the gills turn white or fuzzy (and not in a good way), or if the cap gets "pitted," toss them.
Enoki: These little noodle-like mushrooms should be white and snappy. If they look yellow, slimy, or are matted together in the bag, they are definitely past their prime.
Risk Assessment: Can You Just "Cook it Out"?
You might think that high heat will kill any bacteria on a slightly sketchy mushroom. While heat does kill most bacteria, it doesn't necessarily destroy the toxins those bacteria produced while they were feasting on the mushroom. Scombroid-like poisoning or general gastric distress doesn't care if you sautéed the mushroom at 400 degrees.
If you have a compromised immune system or you’re cooking for kids, don't take the risk. A $5 carton of mushrooms isn't worth a night in the bathroom. Or worse.
Storage Hacks to Prevent Spoilage
- Don't wash until you use: Water is the enemy of storage. If you wash them and put them back in the fridge, they’ll be slimy by morning.
- The Paper Bag Method: As mentioned, paper is king.
- Crisper Drawer: Keep them in the main part of the fridge or a very "low humidity" crisper.
- Freezing: If you realize you won't eat them in time, sauté them in butter first, then freeze. Raw mushrooms don't freeze well because of that high water content; they turn into mush.
Expert Insight: The 7-Day Rule
Generally speaking, most store-bought mushrooms have a "best by" window of about 5 to 7 days from the time they were harvested. Since you don't know how long they sat on the truck or the shelf, a good rule of thumb is to use them within 3 to 4 days of purchase.
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If you're foraging? That’s a whole different ballgame. Foraged mushrooms should be processed or eaten within 24 to 48 hours. They often carry more hitchhikers (bugs) and have higher moisture levels than cultivated varieties.
Summary Checklist for the Kitchen
- Look: Are there dark sunken pits or widespread discoloration?
- Touch: Is it slimy or overly "squishy"?
- Smell: Is there a sharp, sour, or ammonia-like odor?
Honestly, your senses are evolved to detect decay. If your brain is giving you a "maybe not" signal when you look at that mushroom, listen to it. Evolution spent thousands of years teaching humans how to avoid bad fungi. Don't let a "waste not, want not" attitude override your lizard brain's warning system.
Actionable Steps for Your Next Grocery Trip
Next time you're at the store, don't just grab the first package you see. Flip it over. Look at the bottom of the mushrooms through the clear plastic. If you see liquid pooling at the bottom, leave it there. That batch is already on its way out. Look for the "packaged on" date if available, and always opt for the mushrooms that look the "driest" and most matte in finish.
Once you get home, move them to paper immediately. If you find one or two mushrooms in a pack that are starting to get slimy, remove them immediately. Decay is contagious in the mushroom world. One bad apple—or one slimy cap—really does spoil the bunch.
Keep your mushrooms away from strong-smelling foods like onions or garlic in the fridge, too. Mushrooms are like little sponges; they will absorb the odors of everything around them, which can make it harder to do the "smell test" later on.
Final Thoughts on Safety
If you've already eaten a mushroom and you're worried it was bad, watch for symptoms like severe cramping, vomiting, or diarrhea within 6 to 24 hours. Most "bad" store mushrooms cause standard food poisoning. However, if you're eating wild mushrooms and feel ill, seek emergency medical attention immediately. There is no "tell" for toxicity in the wild that is as simple as slime or smell. But for the grocery store variety? Stick to the dry, earthy-smelling, firm ones, and you'll be golden.