How do I devein a prawn, and do I actually have to?

There is a second, paler line along the underside — that one is the actual blood vessel and nobody removes it. The dark one on the back is the hindgut, and how objectionable it is depends entirely on size. A cold-water prawn's gut is invisible. A big warm-water tiger prawn's can be a black cord full of sand.

What you'll need

• A small sharp knife or a paring knife
• A cocktail stick or skewer (optional)

Step by step

1. Decide whether it is worth doing.Under about 15cm, or any small cold-water prawn, the gut is negligible and deveining costs you flesh and time for nothing. On king and tiger prawns, look at one: if the line is a distinct dark cord, take it out.
2. Choose shell-on or shell-off.Shell-off is easier. Shell-on keeps far more flavour and stops the prawn drying, so for grilling or barbecuing, cut through the back of the shell instead and leave the shell in place.
3. Hold the prawn curved, back uppermost.The gut follows the outside of the curve, just under the surface, running from the head end to the tail. Working with the natural curve rather than flattening it keeps the cut shallow and even.
4. Score along the back, two to three millimetres deep.One shallow pass with the knife tip from the head end to where the tail fan begins. You are opening a channel, not cutting the prawn in half. Deeper than about a third of the thickness and it will fall open when it cooks.
5. Lift the gut out.Slide the knife tip or a cocktail stick under the dark thread and pull it away in one piece. If it breaks, pick out the rest — leaving fragments of a gritty gut is worse than leaving the whole thing.
6. Rinse briefly and dry hard.A quick rinse of the channel under cold water, then dry both sides thoroughly on kitchen paper. A wet prawn steams instead of searing, and a scored prawn holds water in the channel.
7. Cut deeper if you want them butterflied.Take the same cut about two-thirds through and open the prawn out flat. Butterflied prawns cook faster, curl into an attractive shape and hold marinade in the channel, which is the real reason to do it.

Tips

• A prawn deveined through the shell keeps almost all the moisture and flavour of a shell-on prawn while losing the grit. It is the best of both for a barbecue.
• Keep the heads and shells. Ten minutes in hot oil or simmering water gives you a stock or bisque base far better than anything from a jar.
• Frozen raw prawns are much easier to devein when only partly thawed — firm flesh, and the gut pulls clear in one piece.
• Cooked prawns can be deveined too, but the flesh tears more easily. Do it before cooking wherever you have the choice.

Common mistakes

• Cutting deep along the back on prawns you are going to pan-fry — A deep cut opens up under heat, the prawn splays and dries out, and you lose the plump texture that is most of the point of a big prawn. Shallow unless you actively want a butterfly.
• Soaking prawns to rinse the channel out — Prawns absorb water fast and it comes straight back out in the pan as steam. Rinse for a second, then dry properly.

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