I'm looking to make something close-enough at home for a friend. I'm suspecting it doesn't actually have Prawns in it.
Assuming you're talking about Walker's brand, we can see the following ingredients list from Tesco.co.uk (should match what's on your bag):
Prawn Cocktail Seasoning contains: Flavouring, Sugar, Glucose, Salt, Citric Acid, Potassium Chloride, Dried Yeast, Dried Onion, Vale of Evesham Tomato Extract, Colour (Paprika Extract), Sweetener (Sucralose)
Nutritional yeast (which is what I assume the 'dried yeast' is) is often used in vegan recipes to simulate a cheesy flavor, so it's probably not that. You can imagine what sugar, glucose, salt, citric acid, tomato extract, and dried onion taste like. Potassium Chloride is commonly used as a salt replacement to lower the salt content of food without losing that salty flavor.
I suspect it's that first ingredient, 'flavoring'. You're probably talking about a product like this: http://www.alibaba.com/product-detail/prawn-Flavor-O5557-used-in-puffed_1699947706.html?s=p
prawn Flavor O5557 used in puffed foods
- Professional and mature R&D team, Application team.
- Manufacturer of flavor
Origin: Jining city, Shandong Province, China.
The operative word in prawn cocktail crisps is cocktail. They're essentially Marie Rose sauce flavour, there's barely any prawn notes. Marie Rose sauce is usually tomato ketchup, mayo, a little Worcestershire Sauce, and cayenne. Should be easy to replicate at home.
Prawn cocktail is in fact made of orange zest, salmon and chicken stock.