Mushrooms

Mushrooms keep about a week refrigerated, and the storage rule is unusual: use a paper bag, not plastic.

Mushrooms are alive and respiring, and they are mostly water. Sealed in plastic they sweat, the trapped condensation collects on the caps, and they turn slimy within days. A paper bag absorbs that moisture while still slowing dehydration. If they came in a shrink-wrapped punnet, either leave it sealed and unopened, or transfer them to paper once opened.

Do not wash mushrooms until you use them. They absorb water readily, and wet mushrooms both spoil faster and refuse to brown in a pan. Brush off dirt, or rinse briefly and dry thoroughly just before cooking.

Raw mushrooms freeze badly — they turn to mush. Sautéed mushrooms freeze very well for several months, which is a good use for a punnet about to turn.

Mushrooms are fungi, not plants, and they behave differently from everything else in the produce aisle.

They have no waxy cuticle and no skin — the cap surface is directly exposed fungal tissue. They are around 90% water, they continue respiring vigorously after harvest, and they readily absorb any moisture around them. This combination is why they are so sensitive to how they are bagged.

The darkening you see on aging mushrooms is enzymatic browning, the same polyphenol oxidase reaction as in a cut apple. Slightly browned mushrooms are perfectly good and often more flavorful, having lost some water and concentrated.

The line to watch is slime. A slimy, tacky film on the cap means bacterial growth on the surface and the mushroom should go. Similarly, mushrooms that have gone wet and dark, developed a sour or fishy ammoniac smell, or turned soft and squishy are past it. Dry, shriveled mushrooms are merely dehydrated and are fine to cook.

For cultivated mushrooms — button, cremini, portobello, shiitake, oyster — there is no toxicity concern from age; the risk is ordinary bacterial spoilage. Wild foraged mushrooms are an entirely different matter and depend wholly on correct identification.