The Science Behind Why Cow's Milk Sometimes Tastes 'Fishy'

Odds are, most of us have experienced the stomach-churning, sewage-like smell of expired milk on more than a few occasions — much to our sensory dismay. But what are you to do if you open a fresh carton of milk and find that it reeks of a fishmonger's dumpster? Your first thought might be to question the milk's expiration date. Could it be that the producer or distributor somehow screwed up, printing the wrong date?

In reality, that's probably not the case, and besides, that fishy smell isn't the same as the stench of rotten milk. It's a unique type of scent all to itself. The good news is that you're very unlikely to find milk that smells this way, unless you live in a country that uses particular cow-breeds for milking. While this is rare in most places, it's not unheard of, and the science behind the phenomenon is quite fascinating. Interestingly, this effect isn't even exclusive to cow's milk, as it turns out human beings can experience the same odorous malady.

When milk takes on a fishy smell and flavor, it means the cow it came from has a condition similar to what's known in humans as Trimethylaminuria, colloquially called fish-odor syndrome. More specifically, it's the scent of a chemical called Trimethylamine, or TMA, which has an odor ranging from mildly fishy to powerfully ammonia-esque depending on how highly concentrated it is. TMA is produced in a person's intestines as they break down certain foods, but it usually doesn't cause any problems. That's because a special liver enzyme called flavin-containing mono-oxygenase (FMO) converts TMA into an odorless compound that the body flushes out with urine.

The problem is, that process doesn't happen in certain individuals and apparently some cows, according to a study published in Genome Research. Fish-odor syndrome is actually a genetic condition caused by a mutation of the gene FMO3, which controls the production of various enzymes. When the FMO3 gene doesn't function in the expected manner, the body doesn't break down TMA. Then, instead of escaping the body in an odorless state, the chemical makes bodily fluids more pungent.

Something like fish-odor syndrome is especially common in Swedish Red and White cattle. There have been reports of fishy-smelling milk in other parts of the world besides Sweden; though, they are much rarer, or at least, not as well documented. This genetic knowledge means that farmers could screen cows and bulls for this condition before milking and breeding them, reducing the odds of TMA in future milk.

Humans have FMO3 genes subject to the same mutation that makes cow's milk smell fishy, and that mutation is typically passed down from parents to children. However, offspring must have two sets of mutated FMO3 genes for this condition to manifest. That means both parents need to have at least one copy of the mutated gene to pass along in order for their kid to develop Trimethylaminuria. The individual will then excrete TMA through bodily fluids including saliva, sweat, and urine. Presently, there is no known cure for fish-odor syndrome, but a change in diet can counteract its symptoms.

Thus, the concern about TMA in cow's milk is that it could be passed along to the person drinking it. In 1985, Pediatric Dermatology presented the case of a 10-year-old boy who experienced pronounced fish-odor syndrome for two years. Doctors noted that his diet included a particularly large quantity of milk, so they had him cut it from his diet. The problem resolved itself almost right away, indicating that eliminating foods high in TMA and related compounds (like the odorless byproduct trimethylamine oxide and a nutrient called choline) could help to assuage any fishy odors the body may be emitting. This may entail cutting cow's milk from one's diet or perhaps even replacing all animal milks with plant-based milks.