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&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 :)

  • My cousin is busy right now. Could you explain (or link to something that explains) how this works? – Dr. belisarius Aug 13 '14 at 20:43
  • As I understand it, the DAPI reacts with the polyphosphate by binding to it and in the process of binding to it, it causes the wavelength that the DAPI fluoresces at to change from the low 400nm (blue) to the low to mid 500nm (green). I'm not 100% certain on whether there is a need for a catalyst to cause the excitation, or if just dabbing it topically would work (which to my understanding should be enough as the DAPI). Then again, my cousin too is busy. – jsanc623 Aug 13 '14 at 21:04
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    Thanks a lot for the explanation. Not sure if I'll try it, but at least I could menace my monger with it. – Dr. belisarius Aug 14 '14 at 2:01
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    @belisarius Why does "menace my monger" sound vaguely dirty?? – Jolenealaska Aug 14 '14 at 5:25
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    @Jolenealaska If it sounds vague to you, it's because you never saw him working – Dr. belisarius Aug 14 '14 at 15:23

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