Dark spots or discoloration on mushrooms

What you see: Brown areas on your mushrooms that won’t wash off.
What it is: Discoloration from bruising or aging.
Eat or toss: A little discoloration isn’t a big deal. If there’s a lot of discoloration, look out for sliminess or a foul odor, which would suggest that they aren’t fit for eating.

Are dark spots on mushrooms OK to eat?

If your button mushrooms have some mildly discolored areas, don’t worry. Mushrooms injure easily and age quickly, which makes them more likely to sport imperfect exteriors. The brown spots don’t necessarily mean they’re unfit for eating; it’s just a sign that they aren’t youngsters anymore.  

“Once you harvest mushrooms they start to senesce, which means they start to die of old age,” said Luke LaBorde, a food scientist at Penn State University. 

Sounds grim, but it’s par for the course virtually anytime you harvest produce. As mushroom cells lose their youthful elasticity and vigor, they weaken; compounds and enzymes normally kept separate leak and mix together. Add some oxygen from the air and a series of chemical reactions ensues, which results in dark colors.  

In addition to aging quickly, mushrooms bruise easily–injuries also bust open cells, causing those previously separated compounds to mix and produce darker colors. The same reaction causes apples, avocados, potatoes and lots of other fruits and vegetables to darken when cut or bruised and exposed to air (we’ve even written about a similar reaction in shrimp!). 

The color change itself is harmless and doesn’t indicate that there’s anything unsafe or unsavory about the mushrooms. In fact, LaBorde said, it could liberate more flavor.

“The breakdown of tissue during aging, bruising, or cutting releases other enzymes that rapidly form the distinct aroma of mushrooms – which would be a favorable development,” he wrote in an email. 

But there are consequences if you wait too long before you eat your mushrooms.

How to know when mushrooms are bad or starting to spoil

LaBorde said that bacteria on the mushroom’s surface can also damage cells and speed along the discoloration.  

“The more the mushrooms start to age the more susceptible they are to bacterial invasion,” he said.  

If the bacterial armies grow too large, you could see or feel significant sliminess or smell a foul odor, which would signal they weren’t fit for eating. So while you don’t need to worry much about discoloration on your mushrooms, you will want to give them a quick assessment before preparing your meal.

Take the white button mushroom below. LaBorde deemed it too far along to eat raw, but passable for something well cooked (he noted a sheen suggestive of slime on the upper left).

“I think that goes a little beyond my limit, and I personally would not want to eat it raw, but would feel comfortable using it in soup,” he said.  

LaBorde pointed out that even the freshest mushrooms are more vulnerable than produce like apples or avocados because they don’t have an outer skin to protect their soft and delicate flesh.

Some mushrooms are naturally darker

Some mushroom varieties simply come in darker colors. The baby bella below, for example, has a darker cap the same way some people have darker colored hair. The color difference has nothing to do with aging or bruising.

Baby bellas (also called crimini or cremini mushrooms) are actually the same species as button mushrooms and portobellos. The differences come down to pigmentation and when they are harvested.

Whole mushrooms kept cool and dry will last longer

Refrigeration and low-moisture storage slow down the edible fungi’s metabolic processes, which decelerates aging. Colder, drier environments also inhibit bacterial growth. With fewer exposed surfaces, a whole mushroom will last longer than sliced mushrooms.

“Everything we do to extend the shelf life of mushrooms is to slow those reactions down. But we can never stop them,” LaBorde said.

LaBorde recommended using mushrooms within five days of picking them up at the grocery store. SaveTheFood.com says you might be good for up to a week. FoodKeeper says three to seven days. Much will depend on how old your mushrooms are when you buy them, which can be hard for a consumer to know. So, plan wisely and always store mushrooms in the fridge. 

Thanks to Ben Moss, EatOrToss Vice President of Transportation, for this helpful illustration!