How to Tell if Mushrooms Are Bad

Written by Cammi Morgan
Updated: May 8, 2023
© Vintagepix/Shutterstock.com
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If you consistently buy or forage fresh mushrooms for culinary use, you’ll want to make sure you can identify the signs of mushroom spoilage. The taste, texture, and smell of mushrooms that are past their prime can be pretty offensive to the senses.

This offense or sense of disgust for decomposing food is actually hardwired into our brains, designed as an evolutionary tool to prevent us from eating spoiled foods that may cause illness or death. Researchers have actually proven that our brains are much more activated by photos of decomposing food than photos of food at the height of freshness.

In this guide, we’ll dive into culinary mushroom spoilage, how spoilage of fresh food occurs, and four signs that your mushrooms may be starting to turn.

Read on to learn more!

Recognizing Baseline Characteristics of Your Mushrooms

natualy slippery slimy mushroom Suillus
Beautiful young naturally “slippery” slimy Suillus luteus

©Krzycho/Shutterstock.com

To be able to discern the signs of mushroom spoilage, it’s important to know what the baseline characteristics are for the fresh species. Some information online may claim that a telltale sign of mushrooms going bad is discoloration or sliminess. While there can certainly be truth to this, consider species of fresh mushrooms that can be a bit slimy or have immediate discoloration when cut.

Some people, especially those in Eastern Europe, consider Suillus luteus (aka the slippery Jack) a great edible. As pictured above, this mushroom comes naturally slimy. It is said that you do need to peel the top skin off of the cap to prevent gastric upset. But I know of quite a few people who have no issues eating them. Other edible slimy mushrooms, like the honey mushrooms (Armillaria mellea), for example, can be quite sticky to slimy after a rain storm, and they also produce a thin slime in the first few minutes of cooking. However, just like the “slippery jack” this is normal for honeys and not an indication that they are rotting.

As with many boletes, another example is the old man of the woods mushroom (Strobilomyces sp.). This mushroom has an interior flesh that stains red to brown-black when cut or bruised. Of course, this means if you slice them up and store them in your fridge for use the next day, the flesh will have discolored from the immediate effects of oxidation, but this does not mean you need to throw the mushrooms out.

The same is true for the delicious black-staining polypore (Meripilus sumstinei), which, as the common name indicates, has flesh that quickly stains black when cut or bruised.

Finally, the common button mushroom you find at most grocery stores (Agaricus bisporus), often lightly stains pink-red and then brown when sliced. While the process of oxidation does contribute to food spoilage, the practically fat-free composition of mushrooms prevents oxidation from causing rancidness nearly as quickly as it does in fatty foods such as avocados.

The bottom line of the above points is to pay attention to the characteristics of the mushroom when fresh. Then you can look for signs later on that it may be starting to turn bad.

Causes of Mushroom Spoilage

Old moldy button mushrooms Agaricus bisporus spoilage
Old moldy button mushrooms (Agaricus bisporus)

©Marc Bruxelle/Shutterstock.com

There are a few main factors that contribute to the spoilage of food, including culinary mushrooms. These factors include:

  • Macro and microscopic organisms such as insects, molds, yeasts, and bacteria.
  • Time
  • Temperature
  • Air
  • Physical Damage

Mushroom Spoilage: Macro and Microorganisms

Especially when consuming foraged mushrooms, it’s important to thoroughly clean them to remove any macro-organisms, such as insects. While most folks aren’t eager to accidentally consume the insects found on wild mushrooms, removing them also prevents these insects from further damaging the mushrooms, which can speed up decomposition processes (although some cultures do eat a variety of insects, some of which boast high protein and citrusy flavors).

Cleaning moist dirt and debris from the mushrooms also helps keep them dry. This delays the process of molds, yeasts, and bacteria from developing on your mushrooms.

Time

Generally, how long you can store fresh mushrooms depends greatly upon the species and the storage method. In porous containers or paper bags, whole, raw button mushrooms usually keep 7-10 days in the fridge while sliced raw buttons typically last between 3-5 days. Of course it’s always better to try and consume them quicker than this. While effective food storage options can delay the effects of time on fresh mushrooms, oxidation, enzymatic actions, and microbial growth will all eventually cause signs of food spoilage.

Temperature

What temperature you keep your mushrooms will play a large role in how fast they succumb to spoilage. This is due to the activity of yeasts, molds, and bacteria at various temperatures. Typically, these organisms thrive and grow rapidly at room temperature. The ideal temperature to store you mushrooms is around 38-42 degrees Fahrenheit.

Air

As we mentioned above, many species of mushrooms show immediate color changes when the interior flesh is exposed to air. These immediate effects aren’t indicators that the mushrooms need to be thrown out, but oxidation will contribute to spoilage over time through oxidative reactions with various compounds in the mushrooms. Over a period of days (range depends on the species), these reactions can result in a nutritional decrease, discoloration, senescence (cell aging and deterioration), and flavor change or quality reduction.

Physical Damage

Additionally, when raw mushrooms become bruised or otherwise damaged, pathogenic or spoilage-inducing microbes are able to colonize the area easier, leading to a faster rate of your mushroom turning bad. As such, handling your fresh mushrooms gently and storing them uncut will delay this process.

4 Signs of Potential Mushroom Spoilage

When mushrooms begin to spoil from one or more of the factors listed above, you should start to notice some changes in the appearance, texture, and smell of your mushroom. It’s crucial to the safe consumption of fresh foods to be able to accurately access when spoilage is beginning to occur. Below, we’ll dive into four of the signs of mushroom spoilage and the characteristics to look out for.

1. Dry Mushrooms Turning Slimy

If your mushrooms are dry-feeling when fresh, such as shiitake or button mushrooms, but are beginning to feel slimy, this can be a sign of spoilage. When assessing the sliminess of normally dry mushrooms, make sure the cause isn’t from someone rinsing them, as this can quickly cause many species to feel quite slimy. Moreover, if you do rinse your mushrooms, you’ll want to immediately dry them as leaving them moist can speed up the process of bacterial and mold development. There is some debate over rinsing your mushrooms as moisture isn’t helpful in the storage process. If you are going to rinse them, I do advise that you cook them soon after.

If your mushrooms have developed a slight tackiness to them, then they may still be good to eat if you go ahead and cook them up that day. It’s best to thoroughly cook your mushrooms at this stage. I like to bread and fry my mushrooms if they’re reaching the end of their freshness. Once they turn truly slimy, however, you’ll want to toss them or add them to your compost bin as they’ve developed a layer of bacteria that could make you sick. They probably won’t taste as good either, so it’s not just getting sick you should be worried about.

2. Mushroom Spoilage: Moldy Spots

If you start to notice fuzzy patches on your mushrooms, this is a sign that they are being colonized by a fast-growing mold. There are a few molds that could be growing in your refridgerator, but the most common culprits are Aspergillus, Penicillium, and Alternaria. Once these molds develop, it’s definitely time to toss the mushrooms.

3. Discoloration Over Time

Especially for mushrooms that don’t immediately or rapidly change color when the flesh is exposed to air, discoloration can be a sign that your mushrooms may be spoiling. You may also start to see discoloration on the exterior of the mushroom as well, which may be the result of bruising or sliminess setting into areas.

4. Mushroom Spoilage: Changing Smell

As we mentioned in the intro, our brains are hard-wired to react with disgust toward rotting food, and this includes our olfactory sense. This reaction occurs in other organisms as well. Researchers studied fruit flies and their hard-wired revulsion toward the smell of geosmin, a compound released by bacteria and mold on rotting fruit. Significantly, a revulsion response from the fruit flies to the smell of geosmin prevents them from eating and laying eggs in food colonized with pathogens.

Finally, for mushrooms, the presence of decomposing microbes often causes a fishy smell that many people can readily detect. This fishy smell is typically caused by the release of alkylamines by various actions of bacteria.


The Featured Image

Old store bought mushrooms
Old mushrooms that should be thrown away or composted
© Vintagepix/Shutterstock.com

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About the Author

Cam Morgan is a queer forest dweller writing about animals, plants, and ecological-centered living from the hollers of Southeast Appalachia where she lives off-grid in her self-built cabin. She shares 20 forested acres with her wonderful partners and pals, an ever-growing pack of rescue dogs, and all the plants and critters who call these woods home.

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