1. Drying and Rehydrating
Drying works especially well for boletes, morels and agarics, whose flavors can actually be enhanced and intensified by the process. Other excellent mushrooms for drying include lepiota, russula, puffballs, and the caps of shiitake and honey mushrooms, among others. I don't recommend drying for any mushroom whose flesh gets woody or fibrous with age. Polypores, oyster mushrooms, and coral fungus often become unusably hard when dried, and unpleasantly chewey upon rehydrating. If you try to dry shaggy manes (coprinus comatus) they will simply dissolve into inky slime. Other mushrooms rehydrate poorly, or the process of rehydrating creates a blanching fluid that is difficult to use. Chantrelles, for example, are often sold dried in the market, but the liquid that results from rehydrating tastes like old dirty fruit, and doesn't mix with many dishes. Dried blewits seem to suffer the same problem.
The basic drying process is very simple, if somewhat slow. You can place thin-sliced mushroom pieces on wire racks (I use cookie cooling racks), set them in a warm dry place with plenty of air circulation, and wait a week. Or you can thread them with string, leaving some space between each slice, and hang them up in a window. However, to save time, I almost always use the oven. I set the oven to the lowest "warm" setting, place the cooling racks inside, and prop the door open about an inch to allow the warm air to escape. The mushrooms are usually crispy dry within 12-24 hours.
You can store the dried mushrooms in jars, cannisters or plastic bags for over a year. I often insert a little packet of dessicant into each container to help absorb atmospheric moisture. (I salvage the dessicant from vitamin bottles and shipping cartons.) Be sure to check your collection every few months to make sure that you don't have any unwanted critters living in your containers. Moths, beatles and gnats can be attracted to your supply if it isn't kept totally air-tight.
To use dried mushrooms, put them in a bowl and barely cover them in warm water. Soak for about an hour. Be sure to keep the water that you soaked them in - it's the best part! If you wish to use the mushrooms in a dish without much liquid (as you would have used them fresh) you can boil the liquid with the mushrooms in a pan until the liquid has been mostly absorbed or evaporated. This method will return the flavor from the liquid back to the mushroom itself, although the resulting mushrooms will be more chewy than when they were fresh. In some cases, especially with boletes and suillus, this increased firmness may be an advantage.
A duxelles is a crumbly, semi-dry paste made of finely chopped mushrooms, herbs, onion, butter or olive oil, and other ingredients, cooked slowly on low heat until most of the moisture has evaporated. Duxelles are perhaps my favorite method for preserving mushrooms, particularly because they are so easy to use, and because they concentrate the mushroom flavors in the most delectable way. They are also a very good method for preparing mushrooms that are bug-damaged, over-aged or otherwise aesthetically challenged. Any mushroom afficionado can attest to the frequency of such imperfect specimens, and we come to value any technique that allows us to serve them without apologies.
You can prepare a duxelles from just about any variety of mushroom, but to me it seems most well suited to agaricus, boletus and chantrelles. It takes a lot of mushrooms to make a duxelles, so you may prefer to save the precious specimens (especially morels) to eat fresh or to dry. Since chantrelles don't dry very well, they are well suited to making duxelles; however, when I find an abundance of the large bland California variety I often prefer to pickle them instead.
The texture of a duxelles will differ depending upon how finely you chop the ingredients. Often, when the mushrooms and onions are chopped by hand, the resulting duxelles will by dryer and more distinct, the ingredients more discernable. Since I'm a lazy cook, I usually use a food processor to chop the ingredients, and the resulting duxelle comes out the consistency of pesto. You can also make duxelles with chopped reconstituted dried mushrooms to make a drier mixture, and you can add ingredients like bread crumbs to thicken the texture.
A duxelles can take up to an hour of slow cooking to get the mixture sufficiently dry, with frequent stirring to avoid burning. In order to speed up the dehydration process, I have developed the habit of repeatedly spreading the mixture accross the surface of the frying pan (actually I usually use a wok with a very low flame because it seems to heat more evenly.) By thinly spreading the mixture, then stirring and re-spreading every five minutes or so, you can maintain a large surface area for the water to escape.
A typical duxelles will contain about one onion for every pound of mushrooms, with about 1/2 teaspoon of salt and 4 tablespoons butter or olive oil. There's plenty of room for variation and additions, though, and I provide a few recipes elsewhere.
3. Vinegar & Oil Pickling
I prefer this technique for firm young specimens, and for mushrooms that retain a strong earthy (dirty) flavor. The acid from the vinegar tends to neutralize the base earthiness. This method is especially suited to chantrelles, which excell at absorbing the taste of their surroundings, especially when the mushrooms have been rained on after fruiting. Vinegar pickling can even improve the flavor of some California chantrelles, which grow huge and often lose the exceptional fruitiness found elsewhere in smaller varieties.
Start with a wide, deep pan. Use a mixture of 50% vinegar and 50% water, about one inch deep in the bottom of the pan. I usually find that apple cider vinegar (5% acidity) works best, as it complements the fruitiness of chantrelles, and brings out the muskiness of boletes or agarics. Plain white vinegar also works well. Add about 1 teaspoon salt to the vineger solution, and bring this mixture to a rapid boil.
Prepare the mushrooms by cleaning off the dirt, using a brush with minimum water. Cut larger mushrooms into roughly one inch chunks. Boil the mushrooms vigorously in the salted vinegar for 5-10 minutes (depending on the type and texture of mushroom). Remove them from the vinegar with a slotted spoon or spatula, and squeeze lightly with another slotted spatula to drain the excess vinegar back into the pan. For large batches, keep some extra vinegar and water on hand to add to the pot. Place the cooked mushrooms on paper towels on a cooling rack, and let stand for about 30-60 minutes, allowing the towels to absorb some of the moisture.
If you wish, you can also boil some whole garlic cloves in the vinegar, to add to the preserved mushrooms for aditional flavor.
Use small sterile canning jars and pack the mushrooms tightly, a few at a time. Add sprinkles of parsley, thyme, oregano or other herbs in layers as you go up, and carefully fill all of the air gaps with canola oil or any unflavored vegetable oil. (You can use olive oil if you wish, but in the end it will dominate the flavor of the mushrooms.) Add the oil in increments as you fill the jar, so that you can press out the air bubbles between the mushrooms. Add a few cooked garlic cloves with each jar for flavoring if desired. Top off to the brim with oil, and seal tightly.
The flavor of the mushrooms in the jar will improve over time, as the herbs mix with the oil and permeate the mushrooms. Try waiting at least a month before tasting. If properly topped off and sealed, they should last for several years. If there is too much air or an improper seal, the oil can turn rancid. Otherwise, this is a fairly safe canning technique, and you need not be concerned with extreme sanitary measures.
4. Blanch and Freeze
When you have a huge supply of fresh wild mushrooms, you might be tempted to toss some of them in the freezer to wait until you have time to get around to cooking them. Most mushroom varieties will turn to slime if you do this without blanching first. Since water expands as it freezes, the ice crystals will break up the cellulose structure of the fungus and render it into icky pudding.
To prepare mushrooms for freezing, begin by cleaning thoroughly with a brush or dry scouring pad. Chop large specimens into one inch chunks or leave smaller mushrooms whole. Bring a large pot of water to a fast boil. Place mushroom chunks, a handful at a time, into a metal strainer or collander, and immerse the collander in the boiling water for about 3 minutes. Use less time for tender species, more time for tough ones. This should barely cook the mushrooms but leave them firm enough to hold their shape. Press out the excess water back into the pot as you remove the collander. (you might consider keeping the water to use a soup base later on.) Now air-dry the cooked mushrooms for 30 minutes, with paper towels underneath them to absorb extra moisture. Place them in heavy freezer bags and freeze for up to six months. The frozen mushrooms will probably stick together in a block within the bag, so try to keep the quantity of each feezer bag small enough to use all at once. This will spare you a lot of effort with hammer and chisel.
Frozen fungus, once thawed out, will have lost some of the appealing texture that the fresh fungus may have had. However, of all the preserving methods, freezing probably stays closest to the original flavor of the mushroom. You will inevitably lose some of the flavor into the water when blanching, but the quality of the flavor won't be changed, as it is in drying, salting, or pickling.
5. Other Methods
You can preserve mushrooms with other techniques, including salting and canning, but I tend to avoid these techniques personally, due to their various drawbacks.
The standard canning procedure for mushrooms differs from the vinegar/oil method that I describe above, in that it employs low-acid conditions under a sterile environment. I avoid low-acid canning techniques because of the sanitary precautions required to prevent botulism contamination. For canning information, you should consult a more experienced source than this one.
Salting involves blanching the mushrooms, then packing them in a concentrated brine solution. I am deterred from the salting technique by the long desalination period required to render the mushrooms usable before cooking with them. Typically, to desalinate a salt-preserved mushroom, you must soak it in cold water for up to two days, changing the water occasionally.
I once used a variation of the salting technique to preserve a windfall of chantrelles and boletes that I found once while travelling. I was driving across the country in August, returning with my companion, Dixie, from some concerts in the Midwest. We were passing through the Rocky mountains, stopping occasionally for short hikes. On one of these hikes we chanced upon a few boletus edulis, and then a few more, and clusters of chantrelles. . . so much that I ended up using my sweater as a makeshift sack, tying the sleeves and neck to keep the bounty from falling out. Of course, we were still five days from home, spending our nights in hotels. Luckily, Dixie had packed an electric hot-pot for making tea. We stopped at a market to buy some salt. That night in our hotel I boiled the mushrooms down in their own juices, then added about a half-cup of dry salt in hopes of preventing their decay. We stored them in several plastic containers which had originally held munchies. Days later, when we got home, I was able to use our hastily preserved stash by simply rinsing off the mushrooms, boiling them in water for about 5 minutes, draining, then cooking them into a huge rice dish. The rice thinned out the remaining saltiness and absorbed the savory combination of mushroom flavors, creating enough for several memorable meals.