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When milk smells like rotten fish
  
By
Edward R. Winstead


Milk that smells like rotten fish isn't usually sold in grocery stores, but it sometimes ends up on shelves anyway—at least in Sweden. For a particular breed of Swedish cattle, about two cows in a hundred produce the foul-smelling milk, which can contaminate the pool of milk collected on a farm.


Swedish Red and White Cattle

The practice of pooling milk makes it impossible to know which cow caused the stink. Now, Swedish scientists have a solution to the problem. They have pinpointed a genetic mutation in cows that causes the fishy odor, making it possible for breeders to screen cattle for the mutation.

The researchers discovered the mutation by studying a human disorder known as fish-odor syndrome. Individuals with this devastating syndrome give off the odor of fish in sweat and saliva. References to the syndrome have appeared in literature throughout history. Caliban, a character in Shakespeare's The Tempest, is among the literary characters who smell like fish.

Though the syndrome is extremely rare—only about a hundred or so cases have been reported—it has been well studied. "The gene was identified a number of years ago, and there have been many papers on it," says Leif Andersson, of the Swedish University of Agricultural Sciences in Uppsala, who led the new study of cows.

In humans, the strange smell is due to an overabundance of a chemical compound known as TMA (trimethylamine). In fact, TMA in fish is what causes the unpleasant odor. "When you smell rotten fish, you're smelling the TMA," says Andersson.

The cause of fish-odor syndrome in humans is a mutated gene that would normally break down TMA. The gene is called FMO3.

The story is similar for cows. Animals that produce the odorous milk do not break down TMA normally. The Swedish team reports in Genome Research that a mutated FMO3 gene in cows is the source of the problem.

The intensity and the type of fishy odor in humans and cows seem to vary from one individual to the next. Diet and other genes probably influence the severity of the syndrome and the quality of the odor.

Most reports of the tainted milk have involved the Swedish Red and White dairy breed. Although the researchers have heard reports of fishy-smelling milk in other countries, they expect the problem to be rare worldwide.

The syndrome is recessive in humans and cows, so individuals must inherit two copies of the mutant gene to develop the smell. This is good news for cattle breeders, because they could eliminate the syndrome by screening bulls for the mutation.

"You'd still have cows that carried the mutation, but you wouldn't have the problem," says Andersson.

Read passages about Caliban from The Tempest

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Lunden, A. et al. A nonsense mutation in the FMO3 gene underlies fishy off-flavor in cow's milk. Genome Res 12, 1885-1888 (December 2002).
 

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