To many members of my family, eating raw fish is as distasteful as the idea of being served, well, bugs. I can easily discriminate between the two. Eating raw fish in the form of sushi and sashimi is yummy; eating bugs — not so intrinsically appetizing. But this morning I ran across a new paper that might give pause to raw-fish aficionados: It shows that uncooked fish can host detectable concentrations of potentially toxic chemicals, pollutants that can disappear with cooking.

Liana Del Gobbo of the University of Toronto and her colleagues focused on perfluorinated chemicals, a family of nonstick compounds that included PFOS (perfluorooctanesulfonic acid) and PFOA (perfluoroctanoic acid). Although fish and shellfish accumulate these compounds — some of which, like PFOS and PFOA, are suspected carcinogens — there had been some question about whether cooking would alter the initial concentrations existing in fresh-caught animals. Turns out it does, Gobbo’s group reports in the Journal of Agricultural and Food Chemistry.

The researchers purchased fish from markets in three Canadian cities. Assays for 17 perfluorinated chemicals most frequently turned up PFOS, typically in concentrations of 0.21 to 1.68 nanograms per gram (or parts per billion). Altogether, perfluorinated-pollutant totals ranged from 0.21 to 9.20 ng/g, which the authors note is about what has been reported in other market-basket surveys.

Boiling reduced perfluorinated contamination by 79 percent on average; frying by 54 percent. But baking proved best. No perfluorinated pollutants were detectable in the fish that had been cooked in the oven for 15 minutes at 163 °C (325 °F). Interestingly, the researchers didn’t test what happens when people broil these foods (which is how I sometimes serve them).

The finfish sampled included silver pomfret, milkfish, grouper, red snapper, catfish, monkfish, mackerel, yellow croaker, gray mullet, and whiting. Not being much of a consumer of such bony fish, I was more interested in the shellfish and other marine animals, many of which show up on sushi menus: cuttlefish, octopus, sea squirt, skate, squid, conch, cherrystone clams, and scallops.

Perfluorinated tainting was highest in raw samples from carnivorous species, such as octopus (9.20 ng/g), skate (6.14 ng/g), yellow croaker (3.26 ng/g ww), and monkfish (2.69 ng/g). Species (such as scallops, silver pomfret, clams, and conch) that typically fed on plankton or other species low in the food web also tended to exhibit low concentrations of perfluorinated pollution. One notable exception: sea squirts (2.89 ng/g).

So where did the disappearing perfluorinated pollutants go during cooking? Good question. The paper offers no good answers.

The authors didn’t, for instance, show that when values decreased after boiling it was due to the compounds migrating into the water. Or that frying released the pollutants into the air or fat at the bottom of a pan. In fact, they didn’t even look at such issues.

Because they didn’t, the researchers acknowledge that it’s possible cooking merely “rendered the compounds more difficult to extract” or diluted them to a value below the limit of detection. In other words, they say, observed decreases in tainting after cooking may not correlate with reduced toxicity.

Nor is there any expectation that cooking destroyed these pollutants, which Gobbo and her colleagues observe “are relatively stable.” That’s an understatement. A few years ago I had one chemist who studied these pollutants tell me that you could boil PFOS or PFOA in acid for 1,000 years and they wouldn’t break down. In fact, he said environmental scientists should expect them to effectively last forever.

That’s not very reassuring.

Then again, Gobbo’s team argues that the concentrations measured in the fish they sampled were generally low. So “reducing consumption of fish . . . is not warranted on the basis of PFC exposure concerns at the reported levels of contamination, even for high fish consuming populations.” That conclusion might be more suspicious if the authors had been funded by the sport-fishing association or some such group. In fact, the scientists cite their grant money as coming from the “Center for Urban Health Initiatives, through a Canadian Institute of Health Research grant, Toronto Public Health, and Health Canada.”

Oh, and their paper: Due to appear in print soon, it was posted on the journal’s website yesterday morning.

Janet Raloff

Janet Raloff is the editor of Science News for Students, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer.

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