What’s the dirt on mushrooms?

What you see: Dirt in your package of store-bought mushrooms.
What is is: Peat!
Eat or toss: Clean the mushrooms and eat. If you can’t get every speck of the dirt, don’t worry about it. 

The “dirt” on store-bought mushrooms is typically peat

Packaged button mushrooms often have some dirt on their caps, and that’s OK. It’s typically remnants of a peat mixture that’s integral to large-scale mushroom growing. 

The mushrooms are cultivated in dark, damp, cave-like buildings, which you can see examples of on this page from Penn State Extension. Growers distribute mushroom spawn (yep, it’s actually called spawn) on trays filled with a specially developed, nutrient-rich mix known as “compost.” Above the compost layer is the central character in today’s story: peat mixed with ground limestone. 

The peat and limestone mixture, also called “casing” by those in the biz, doesn’t provide nutrients, but it does hold on to moisture and support the developing fungi. As the final level of the mushroom-growing layer cake it sometimes hitches a mushroom cap ride from the grow room to your kitchen, where you might be wondering how thoroughly you should clean your mushrooms and what is the meaning of life, anyway?

Is the dirt on mushrooms manure?

The dirt you see on your mushrooms is most likely the peat mixture we described above. However, it’s also not impossible for some treated manure to be on your mushrooms. But don’t panic! 

As we mentioned, mushrooms growers create a special “compost” that nourishes the growing fungi. But any old kitchen or garden compost won’t do for large scale production of button mushrooms and their cousins, like portobello and cremini mushrooms. Instead, farmers devise their own specialized recipes, often based on byproducts from other industries in their region. Mushroom compost might include: straw; corn cobs; grape crushings from wineries; spent grain from breweries; soybean meal; gypsum; and, yep, you knew this was coming, chicken and horse manure. 

There’s a big difference between raw manure and treated manure, as we discussed in this post about dirt on produce. A central part of creating compost for mushrooms is exposure to high, pathogen-killing temperatures. Luke LaBorde, a food scientist at Penn State University, researched pathogen survival on mushroom compost and found that organisms that cause human disease simply couldn’t live through the pasteurization step. 

“I kept telling people, I would eat this stuff,” he joked.

Mushroom growing

To get a sense of what all this mushroom growing looks like, check out this video from the Mushroom Council. You’ll see the mushrooms rising through the peat (this step is called pinning) and then ballooning into their full mushroom selves.   

And, fun fact: afterward, the compost is repurposed into potting soil and other uses. 

How to clean mushrooms

There’s much debate in the culinary world about the best way to clean mushrooms. Personally, I like a water rinse, supplemented with some rubbing with my fingers to get as much of the dirt as possible. While some folks advocate for wiping them clean with a dry towel, I find this labor intensive and less effective. Water left on the mushrooms doesn’t mess with my cooking process (though I am but a humble home cook).

It’s also worth keeping in mind that mushrooms often have some naturally occurring discoloration on their caps and stems (courtesy of wounds, aging and bacteria on their delicate flesh) which can look like dirt, but, of course, will never wash off. 

LaBorde said you obviously want to clean off mushrooms, but there’s no need to stress about excising every speck of dirt. He pointed out that it’s just a reminder that the mushrooms were recently harvested.

“So consider a dirty mushroom a fresh mushroom and get as much as you can off,” he said.