How can I avoid getting swindled? How can I detect polyphosphates in fish fillets? Fish fillets are treated with polyphosphates for several reasons, but mainly to increase their weight (and hence the profit).

According to this FAO paper:


The first and universal effect of all polyphosphate treatment is to
increase the weight of the fish by retaining water
Application of polyphosphate to ...(high quality fish) ... is generally
only of slight value
The dull surface of poor quality fillets from stale fish can also be
given a gloss by treatment with polyphosphate.
Excessive treatment of small products such as shellfish or thin
fillets can even result in undesirable flavour changes and sloppy
texture.


So ... is there a way to know if a fillet has been treated with polyphosphate without buying it first and lending it to your chemist cousin?
Put the fish under a blacklight and dab a bit of 4',6-diamidino-2-phenylindole stain on it. If it changes glow from blue to green, its been treated with polyphosphate.

You can purchase DAPI stain here: http://www.sigmaaldrich.com/catalog/product/sigma/d9542?lang=en&amp;region=US

I would suggest cutting a piece of the fish that you will not be eating, dabbing the stain on that piece and tossing it in the bin after.

Or lend it to your chemist cousin :)