Among the many brilliantly colored mushrooms in North America, few stand out like the Frost bolete, also known as the apple bolete and Butyriboletus frostii. Among the Midwest's dizzying amount fungi that stain blue, it's one of the easiest to identify with its distinctly reticulate stem. Mostly appreciated among more serious mushroom hunters, they have a tart taste that makes a novel addition to any foragers repertoire. Today we'll go over background and identification, safety considerations, and how to use them in the kitchen.
Background
The name Frost's bolete comes from mycologist Charles Christopher Frost (1805-1880). A cobbler by profession, he was a self-taught mycologist from New England who amassed a personal herbarium documenting hundreds of species. Impressive work at a time when most naturalists might be doctors, ministers or scientists.
He was part of a wave of 19th century amateur naturalists whose work helped mycologists compare European species with their North American cousins. Frost's bolete is one his most well-known contributions, although the frostii epithet is found sprinkled throughout other genera. In the Southeastern U.S. there's also B. floridanus, which is analogous.
One of the most interesting aspects of this mushroom is that it's well known in Mexico as an edible and often sold fresh in markets. There's a notable amount of interesting common names for it, like strawberry tree mushroom, sour belly and sour bread mushroom, according to Larousse Cocina.
The wide cultural acceptance is different from North America where red-pored, blue-staining boletes are widely regarded with caution. On a related note, if you're interested in other lesser-known fungi eaten in Mexico I recommend reading this essay by David Arora (reprinted with his permission) I posted years ago.
Identification & Habitat
I typically see a few Frost's boletes every year when mushroom season is peaking in August, growing with red oaks in mixed hardwood forests of Northern Wisconsin and Minnesota. If it's peak summer and I'm I look for chanterelles, hedgehogs and black trumpets I usually find a few. Like the others, it's a mycorhizal / symbiotic associate of oaks.
Key ID Characterstics
Reticulation
The ornate netting pattern is found on many bolete mushrooms, but, sans Rusell's bolete covered below, few have a dense, coarsely reticulate pattern comparable to B. frostii.
Yellow Flesh
The flesh of the mushroom is pale yellow, but it can be tricky to show others through an image as they stain blue after being cut.
Like some other blue-staining boletes, they undergo a transformation during cooking, reverting to the original yellow color when exposed to heat.
Glossy, Sticky, Bright Red Cap
The brilliant primary-color red cap sticks out like a sore thumb and is typically tacky, making it easy for debris like leaves, grass and twigs to stick to it. As the mushrooms age the red cap cuticle can become brittle and may peel off.
Guttation
A characteristic of plants and mushrooms not found on many boletes, guttation is small drops of liquid forming on mushrooms that's thought to help regulate internal water levels. Think of it like mushroom sweat. B. frostii is the poster child for it, and the previous name Exsudoporus literally means "to exude on the pores". This is easiest to see in the field immediately after picking.
Look Alike Mushrooms
I include the mushrooms below as examples, but if you keep in mind the hallmark characterstics of B. frostii: growing with oaks, tacky, bright red cap, dense reticulation, blue staining and red pores they're not difficult to pick out.
Boletus satanus
As the name implies, this is a poisonous bolete. Most will know from field guides, but it's important to know that B. satanus is only known from Europe. It has red pores and a more bulbous stem that lacks the ornate reticulation of B. frostii.
Aureoboletus russellii
The most similar in my mind as far as stem reticulation, A. russellii is also edible, but not as good for eating as B. frostii. It has a notably longer stem with rougher, more pronounced reticulation, and yellow instead of dark red. I often see it growing in the same vicinity as B. frostii.
Various Reddish & Yellow Blue-Staining Boletes
The definition of boletes that are difficult to identify, there's a host of blue-staining summer boletes with varying degrees of red and yellow coloration. While some like Boletus bicolor and the Lanmoa carmanipes (?) below are edible, Boletus sensibilis can be very difficult to separate at a glance and is well documented to cause digestion issues or mushroom sickness.
None of the aforementioned mushrooms have red pores or the dense, frilly reticulation of B. frostii.
Cooking
While not in the same league as porcini and other tasty species like cornflower boletes, Frost's bolete has a subtle tart taste that invites a lot of creativity in the kitchen. While some refer to the taste as sour or lemony, I find that to be an exaggeration compared to something like the beefsteak polypore which is more tart.
It's important to know that just like many other mushrooms, some people can't tolerate these and can experience gastro-intestinal distress, while others can eat them with impunity. In addition, unlike the select few boletes like porcini in the Edulis clade, B. frostii should always be thoroughly cooked.
A good experience in the kitchen starts in the field. Like most of boletes, these have a shorter shelf life than many other wild mushrooms and are prone to fungal gnat larvae, although it isn't as bad as some other boletes.
Although the stem is pretty, it's often buggy so I trim it heavily in the field, unless I'm harvesting for pictures. Trimming the stem down until you don't see any holes can help prevent larvae from spreading and extend the lifespan in your fridge.
General Cooking
If you want to taste the tart flavor it's best to keep it simple. There's nothing wrong with a simple saute with salt and butter to try them out. They were perfect this week in a walleye meuniere with a few filets my father brought from the Boundary Waters.
To make it, cook 1.5 oz of ¼ inch sliced caps in a pan with two tablespoons water and a pinch of salt. When the pan is nearly dry, add 2 tablespoons unsalted butter and cook until browned and set aside. Season a filet of white fish with salt and pepper. Dredge the fish in flour and brown in oil. Remove the fish to a plate, quickly heat up the mushrooms again, add a squeeze of lemon and a tablespoon of chopped herbs like thyme, tarragon, chervil and chives. Spoon the mushrooms and butter over the fish and serve.
As I often find these when I'm picking large varieties of mushrooms, I may cook them in a blend. Alongside the walleye above, the best thing I've made with them so far is a mushroom ceviche based off of the recipe in my friend Chad Hyatt's book. I'll post the recipe next week.
It's a perfect chilled appetizer after a hot day in the woods and a fitting nod to fungal culinary traditions of Latin America, which, sans huitlacoche, are grossly underrepresented.
Following that, Mexican mushroom recipes will be some of the most fun to experiment with here. From thick guisado stew to sauteed mushroom tacos and quesadillas, there's many ways to enjoy them.
Last night I cooked the last of mine down, finely chopped with salsa verde and a few handfuls of sheep sorrel for sour mushroom quesadillas. Those are demonstrated in the video at the end of the post.
Preservation
Like most boletes they dehydrate well. The tart taste is much less noticeable to me dried, and the most I've done with them after drying is adding them to a soup made with a blend of dried mushrooms.
They're fit perfectly in a mix of summer wild mushrooms for recipes like my Italian Mushroom Soup.
William
Are they in East Central Minnesota?
Alan Bergo
No you’ll need to go at least an hour north of the Twin Cities. Hinkley, Moose Lake, St Cloud, could all be ok, but ideally more north like Hinkley.
Mark Edward Krebs
What is meant by "blue staining"?
Alan Bergo
When a mushroom is cut, bruised or crushed it stains blue. It’s a useful part of bolete identification as the speed at which it bruises, the shade of blue (blue-black, sky blue) and the overall intensity or lack there of, will differ between species.
Then you also have differences in how each individual part of the mushroom can stain. For example, my frost bolete stems stain a much darker and more even blue when cut compared to the cap and pore layer.
D. Wier
Thank you for posting these when they can be found. I’m excited to find a few that I can try. The sour mushroom quesadillas look amazing.