A flavour built from a sauce, not a shellfish

Prawn cocktail crisps are a curious thing. In most packets there is no prawn to speak of, or so little that it hides behind the catch-all word "flavouring" rather than appearing on the front of the bag. What you are actually eating is a crisp seasoned to taste like Marie Rose sauce: the pink, faintly sweet, faintly sharp mixture of mayonnaise, tomato ketchup, a dash of Worcestershire sauce, lemon juice and a whisper of cayenne that was once spooned over cold prawns in a glass coupe.

So the seasoning is not really about seafood at all. It is about tomato, acid, sugar and savoury depth, arranged in an order that lets your brain fill in the rest. That is why the flavour works even when you know perfectly well there is nothing of the sea in the bag.

What is actually in the seasoning

Read the back of a pack closely and you will find the same architecture repeated again and again. Expect to see tomato powder and paprika for the colour and the base note, sugar or dextrose for the roundness that stops the whole thing turning sour, and a mix of acids — citric, acetic, sometimes spirit vinegar powder — for the tang. Onion and garlic powder, yeast extract and often a little monosodium glutamate supply the savoury spine.

• Whey powder, lactose or buttermilk powder stands in for the mayonnaise, giving that faint creamy edge
• Dried prawn powder appears in some recipes and is entirely absent in others, which tells you how much the fish is doing
• Cayenne or white pepper provides a warmth that arrives a second after the sweetness
• Paprika extract is often there to keep the dust the right shade of coral, since the real thing fades

The trick is balance. Push the acid too far and it tastes like a plain salt and vinegar crisp wearing a disguise; overdo the sugar and it turns cloying by the fourth crisp. Done properly, each mouthful gives you sweet first, then sharp, then savoury, and the flavour lingers pleasantly on the tongue.

The seaside, the shrimp and the starter trolley

Prawn cocktail belongs to a particular British coastal ritual. Along the Lancashire and Cumbrian coast, potted brown shrimp — tiny, sweet, boiled and packed in butter — have been a seaside treat for generations, eaten on the sand with bread and a squeeze of lemon. Further south, the same small shrimp turned up in sandwiches and in the glass dishes of hotel dining rooms.

The dish itself was a mid-century import from across the Atlantic, but Britain made it our own: on the menu at the seaside hotel, on the buffet at the wedding, at the front of the carvery, always with brown bread and butter and a wedge of lemon. The crisp arrived in the mid-1970s, at precisely the right moment. It condensed a whole starter course into something you could eat on a bench overlooking the sea for loose change, out of a bag that fitted in a coat pocket.

Why the nostalgia bites so hard

Nostalgia is rarely about the food itself. It is about the context the flavour unlocks. A bag of prawn cocktail crisps tastes of long car journeys with the windows down, of a packet shared on a sea wall with the wind coming in, of sun cream and vinegar and the smell of a pub with orange carpet. The flavour is an unusually efficient trigger because it hits several senses at once: the sharp-sweet smell the second you open the bag, the orange dust on your fingertips, the crunch, the tang.

There is something else, too. Prawn cocktail flavour is unashamedly retro in a way that few crisp flavours are. Nobody is trying to make it fashionable or to dress it up with hand-foraged ingredients. It exists in its own small, unbothered world, and that is precisely its charm. It is a flavour that has never needed a relaunch.

Serving them well, and mixing your own

A few practical notes, whether you are eating them straight from the bag or doing something slightly more ambitious:

• Pair with dry cider or a pale ale. The acidity cuts the sugar beautifully. A strong cup of tea works surprisingly well too.
• Crush a handful over seafood chowder, a prawn bisque or a bowl of crab soup instead of croutons. The seasoning melts into the hot liquid and gives you crunch and salt in one go.
• Layer them into a prawn cocktail itself as the base, in place of the usual shredded lettuce. Unorthodox, but it makes a genuine difference to the texture.
• Make your own seasoning. For roughly 150g of plain salted crisps, mix half a teaspoon of tomato powder, half a teaspoon of caster sugar, a quarter teaspoon of citric acid, a quarter teaspoon each of onion and garlic powder, a pinch of paprika and a small pinch of cayenne. Add a tiny pinch of MSG if you keep it. Tip the crisps and the mix into a large bag, shake hard for thirty seconds, then leave for ten minutes so the dust settles and clings.
• Store opened bags clipped shut. The seasoning is hygroscopic and will go sticky and dull within a day in damp air.

Eat them soon, ideally somewhere with a view of the sea, and ideally with someone who remembers the same things you do.