posted by Dave Arnold
We were looking for a good fish anesthetic. We figured that knocking fish out prior to killing could improve taste and texture even more than just using our fancy Japanese Ike Jime techniques alone.
Our assumption is that anesthesia reduces stress in fish. Stressed fish make inferior meat. Stress reduces the energy reserves in living muscles. After death, muscles use up their remaining energy and, as a result, go into rigor mortis. The lower energy reserves of stressed musclesÂ lead to a faster, harder rigor than you get in rested muscles. Hard rigor reduces meat quality by reducing flesh firmness and increasing water loss â€“the forces of rigor literally tear the delicate muscle fibersÂ apart.
We originally tried two different anesthetics: Carbon Dioxide and Nitrous Oxide (aka laughing gas, the dentistâ€™s favorite). Both knocked out the fish, but neither produced good results. We thinkÂ the fish became agitated before they lost consciousness, negating any possible benefit. You can read the blow-by-blow here.
We have now found a better alternative: Aqui-S. Aqui-S is a fish anesthetic produced in New Zealand. Its active component is isoeugenol –one of the main compounds in clove oil. Clove oil, you may remember, was the tooth numbing stuff given to Dustin Hoffman by Lawrence Olivierâ€™s Nazi doctor character in the Marathon Man. Isoeugenol has some fantastic properties: it is natural, works in low doses, knocks fish out without killing them, is easy to administer, isnâ€™t toxic to us (it is an approved food additive), and, best of all, doesnâ€™t agitate the fish. It has one bad property: it isnâ€™t approved in the United States.
We canâ€™t buy Aqui-S, but we can buy isoeugenol, so we did.
Aqui-S is 50% isoeugenol. The other 50% is stuff to help it disperse in water â€“it doesnâ€™t want to dissolve on its own. That dispersing aid is proprietary, so we donâ€™t know what it is. Instead we mixed our isoeugenol with 95% ethyl alcohol, which works fine. We tested both carp and striped bass with 17 mg of isoeugenol per liter of water. It worked beautifully on both types of fish. After a couple of minutes, the fish lost their sense of equilibrium and started floating sideways. After 10 to 12 minutes they seemed completely knocked out but were still breathing. Perfect.
We only had one carp, so we couldnâ€™t taste test – but the texture of stripers knocked out with isoeugenol and then killed with spinal cord destruction (Ike Jime) was better than stripers with Ike Jime alone. We were so excited by the results that we called the Aqui-S Company and pleaded with them to send us some of their product â€œjust to test.â€ Nils and I plan on using it in our Star Chefâ€™s demo next Tuesday (Yes, we will do Ike Jime).
Why isnâ€™t isoeugenol approved in the US?
Oil of clove has been used an anesthetic and as a food and perfume additive for a long-long time. It contains both eugenol and isoeugenol. Recently, the National Toxicology Program of the NIH has called eugenol (not isoeugenol) an â€œequivocal carcinogenâ€ (meaning maybe a carcinogen, maybe not). The exhaustive study is here. The evidence seems rather weak. Furthermore, the same group didn’t find isoeugenol to be a carcinogen (see here). Also, the amount of isoeugenol we would consume from eating a knocked out fish is low (see here). Nevertheless, the FDA has been hesitant to approve it. Their current opinion is here.
As a final note, here is a paper on the ins and outs of anesthetizing fish.