Instructions

1. Wipe the caps with a damp cloth rather than rinsing them. A mushroom is a sponge, it takes on whatever water you wash it in, and it turns grey on the tray — this single choice decides how the finished product looks.
2. Twist the stems out instead of cutting them off. A twisted stem brings its plug of tough fibre with it, where a cut one leaves that fibre in the cap to dry hard.
3. Scrape the dark gills out with a teaspoon if powder is the plan. Gills carry spores that turn a pale powder muddy, though they are worth keeping when the mushrooms are headed for stock.
4. Leave small caps whole and slice large ones, but slice them thick. Mushrooms give up around nine-tenths of their weight, and a thin slice ends as a wafer that disintegrates in the soup.
5. Set the caps gill side down with space around them. They shrink dramatically, so a tray that looks crowded now will look half empty by the end of the run.
6. Dehydrate at 52 °C for 13 hours. Portabellos carry far more water than a button mushroom and this is a long one, so a thick cap may want more than a tray of slices beside it.
7. They are finished when a piece snaps like a crisp instead of bending, including through the thickest part of the cap. Anything leathery is still holding water and will mould in the jar.
8. Keep them whole in an airtight jar in a dark cupboard for a year. Whole pieces hold their smell far longer than powder does, so grind only what you are about to use.
9. For powder, blitz the dry pieces, sieve them and use it within three months. A spoonful stirred into a stew gives a savoury depth that fresh mushrooms cannot produce at all.