Sieving

Sieving is the last step of a purée and the one most cooks skip. You blend a soup until it looks smooth, taste it, and stop. What a blender has actually done is chop the fibre finer and hold it in suspension, which is not the same as taking it out. Skins, seeds and stringy threads survive a blade. They are also the pieces that matter most on this diet, so the sieve is not a refinement here. It is the safety step.

What you need

A fine-mesh sieve, the sort with a woven wire mesh rather than punched holes. A conical chinois is worth owning if you make a lot of soup. A colander will not do, because its holes are sized for draining pasta. You also want a ladle, because the back of a ladle presses more evenly than a spoon.

How to do it

Blend first, and blend for longer than feels necessary. Set the sieve over a clean pan and work in two or three batches, so the mesh never clogs into a dam. Press with the back of the ladle in slow circles rather than stirring, and stop every so often to scrape the underside of the mesh, where a surprising amount of good purée hangs. Keep everything hot. A cooled purée stiffens and refuses to pass.

The cue that tells you it is done

Look at what stays behind. After a leek soup you should have a pad of pale green threads, dry to the touch. After raspberries, a cap of seeds. That residue is the argument for the whole exercise. The sieved liquid falls from the ladle in a sheet rather than dropping in lumps, and it coats the back of a spoon evenly.

Common mistakes

Scraping the residue back into the pan at the end, out of thrift. Throw it away. Using a sieve so coarse that nothing is caught, which tells you nothing. Pushing so hard the mesh bows, which forces fibre through rather than holding it back. And forgetting that a clear stock wants the same treatment before it becomes the base of anything else.