Plain mushrooms ought to be gluten-free after all, they're a fresh vegetable, right? But unfortunately, that's not the end of the story for mushrooms if you're particularly sensitive to trace gluten.
You see, mushroom spores are grown on gluten grains most commonly on rye, but also on wheat and occasionally on a combination of the two grains. And this cultivation method leads to some gluten cross-contamination on the finished fungi.
How much gluten? It's tricky to say, although it almost certainly comes in far lower than the less than 20 parts per million standard that's considered "gluten-free" in the United States and some other countries.
More on sensitivity levels:
- How Much Gluten Can Make Me Sick?
- How Sensitive To Gluten Are You?
- I'm eating gluten-free, so why am I getting glutened?
But Won't Washing Get Rid of the Gluten?
Okay, so mushrooms are grown on gluten grains, which does sound a little scary. But you'd think washing your mushrooms well would get rid of any stray gluten-containing growing medium.
Unfortunately, it doesn't seem to do the job. Washing works for the less sensitive (who might not react to unwashed mushrooms, either). But for those who are sensitive to lower levels of gluten, washing really doesn't seem to help enough to prevent a reaction.
I speak from experience on this: I definitely react (although not badly) to conventionally grown mushrooms, including Portabella mushrooms and the button mushrooms you buy at the grocery store. And yes, they've been washed. When I can source wild mushrooms (a woman at my local farm market sold them for a while), I have no trouble eating mushrooms.
Is this something you need to worry about? Very likely not, unless you react to mushrooms yourself or if you want to eliminate every possible gluten exposure, regardless of whether you react. Most people consume mushrooms just fine.
But if you repeatedly find yourself glutened following a meal that features mushrooms, then you may want to look around for a source of wild mushrooms. Here's a list from About.com's Guide to Gourmet Food: Where To Buy Wild Mushrooms. If you ask around at your local farm markets, you may also have success in finding someone who grows mushrooms on sorghum or millet, two gluten-free grains that occasionally are used to cultivate mushrooms.