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?