The key thing to look for when choosing oyster mushrooms or king oyster mushrooms is caps that are unfurled at the edges and free of moisture. On oyster mushrooms, the thin gills under the cap shouldn't be broken; on king oyster mushrooms, the stem should feel firm and resistant when handled.
The second thing to check is the cut surface: on a fresh mushroom, the spot where the stem was cut looks moist and pale; a darkened or dried-out cut face means it's been sitting for a few days.
How do you check colour and texture?
Oyster mushrooms can be grey, yellowish or light brown depending on the variety, but the caps should always look matte and smooth; a shiny, sticky surface is a sign the mushroom is starting to lose moisture.
On king oyster mushrooms, the stem should be ivory white and the cap light brown; brown spots or soft patches on the stem mean the mushroom has lost its freshness.
What do you check on the cap and gills?
An oyster mushroom's cap edges should be straight and curled slightly inward; edges curled far outward and drying out mean the mushroom has been sitting a while. The gills under the cap should be intact and closely packed — gills that are falling apart or darkening are a sign of lost freshness.
On king oyster mushrooms, the cap should be small and in proportion to the stem; mushrooms with an oversized, thinning cap tend to have a more fibrous texture.
How do you clean oyster mushrooms?
Oyster mushrooms shouldn't be washed — trim off the tough base of the stem with a knife and pull the caps apart into clusters by hand. Washing lets the porous structure soak up excess water, which then leaks out again during cooking.
If there's dirt or grit, wipe it off gently with a damp paper towel; very dirty pieces can be briefly dipped in cold water and dried immediately, but that should only be a last resort.
How do you clean and slice king oyster mushrooms?
The stem of a king oyster mushroom can also be wiped clean with a damp cloth; its thick, meaty structure holds up to brief washing better than oyster mushrooms, though it still shouldn't sit in water for long.
For grilling or pan-frying, slice the stem lengthwise about 1 to 1.5 centimetres thick — that thickness lets the centre cook through while the surface still browns.
How long do they keep?
Both types keep for 4 to 5 days in the fridge in a paper bag that isn't fully sealed; stored in a plastic bag, they sweat and turn soft much faster. Mushrooms that have already been washed and left wet lose about half of that shelf life.
To freeze them, sauté the mushrooms briefly first to cook off their moisture before freezing; mushrooms frozen raw turn spongy once thawed. Because of that, a large batch that won't be used right away should always get a brief sauté before it goes into the freezer. Spreading the sautéed mushrooms in a single layer on a tray to freeze first, then transferring them to a bag, keeps the pieces from clumping together into one solid mass. Prepared this way, they can be scooped out a portion at a time and kept comfortably frozen for about 2 to 3 months, with no noticeable loss of texture or flavour once properly thawed and ready for the pan.