True Salmon or Salmon Substitute?

Demonstration Experiment on Video

True Salmon or Salmon Substitute?

Objective: Test for Food Dyes

Peter Keusch

German version

Supermarket products:
true salmon
salmon substitute

Chemicals:
ethanol
petrol ether

Glass wares:
gas washing bottle 100 mL (without head), fitted with a stopper
4 beakers 100 mL
2 snap-cap vials 20 mL
2 petri dishes d = 9 cm

Hazards and safety precautions:

Ethanol is highly flammable.

Petroleum ether is volatile and very flammable. Petroleum ether presents a high fire risk. The toxicity of petroleum ether varies according to its composition. Many of the components are of quite low toxicity, but some formulations may contain chemicals that are suspected carcinogens. Avoid ingestion and inhalation.

Safety goggles and protective gloves required. The experiment should be performed under a portable fume cupboard giving all-round visibility!

Experimental procedure:

A piece of salmon flesh is given in one ot two gas washing bottles. The other gas washing bottle contains a piece of salmon substitute. 65 mL of ethanol are added to each sample. The bottles are stoppered and shaken. Then the two samples are mixed with 20 mL of dist. water and 65 mL of petrol ether. The stoppered bottles are shaken again.

Result:

Video clip (Download RealPlayer .rm file)

Ethanol containing the salmon substitute turns pink. The solvent containing the true salmon remains colorless.

Discussion and background:

· The colour of salmon substitute, usually made from coalfish and pollock ("sea salmon"), is due to food dyes. The added dyes such as Cochenille Red or the azo dyes Yellow Orange S or Ponceau 4R (1) are dissolved in ethanol. The two azo dyes are thought to be non-toxic. They are used instead of the anthrachinone dye Cochenille Red that is obtained from the dried bodies of the female cochenille lice.

· Astaxanthin and Canthaxanthin (2) gives wild salmon the characteristic pinkish colour. Fish in the wild accumulate the natural carotenoids up the food chain (cyanobacteria, algae, tiny shrimp-like plankton, krill). Canthaxanthin is not encountered in wild Atlantic salmon but represents a minor carotenoid in Pacific salmon. The two pigments are bound exclusively to the myofibrillar protein actin. Astaxanthin is accumulated more readily in the muscle than Canthaxanthin. The ability to bind pigment evolves in the muscle; juvenile salmon actin binds much less pigment than actin from mature salmon.
The flesh of farmed salmon is naturally grey. The consumers believe color is an indicator of salmon quality, they will pay more for red salmon. In order to meet market needs the feed of farmed salmon is complemented with Astaxanthin or Canthaxanthin or both.
There is evidence, that high intakes of Canthaxanthin leads to yellow, crystalline deposits of the pigment in the inner layer of the retina.

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