I'm stealing this idea for a blog post theme from a friend. I think it will make a good umbrella under which I can occasionally post about odd and intriguing things that I learn about.
Today I want to write a bit about four different mechanisms for making poultry meat smell fishy. This is certainly not a desired experimental outcome--chicken tenders that smell like fish? That's gross. But this very outcome has been reported by some researchers conducting broiler and layer feeding trials. How this can happen was a matter of some debate during my thesis committee meeting last week. It wasn't heated in the sense of being antagonistic but it certainly was intense. Since it seemed to be a topic of interest, I thought it would be time well spent to read some papers and try to figure out why this happens. What I found out is kind of interesting so I thought I'd write about it here too.
We first have to start with the larger issue: why do fish smell fishy. Living fish and fresh fish don't smell fishy. The fishy odor develops in marine products after they have been harvested. The smell is mainly caused by the formation of a molecule called a trimethylamine (although several dozen other kinds of molecules are also involved in creating the fishy odor).
See, animals that live in the ocean have a bit of a problem. They live in a saline solution but their cells also contain saline solutions. The concentration of sodium and potassium ions inside and outside of cells is delicately balanced. If those ions are out of balance, cells can't make energy and things generally go downhill from there. A very large number of biochemical processes depend on this balance being kept just so. So back to fish. They have developed some special biochemical mechanisms to keep the sodium and potassium ion concentrations in the correct balance. One of these depends on a molecule called trimethyl oxide. When a fish is killed, these molecules are slowly and inexorably converted to the insanely stinky trimethylamines.
You might take fish oil supplements. I don't take them myself but I give them to the dogs. I learned that the fish oil in those capsules has been subjected to an amazingly complicated purification process which includes a deodorization step to remove the trimethylamines.
So the first way we can make poultry meat smell fishy is to feed chickens diets that contain fish meal or fish oil that has not been purified to levels acceptable for human-grade consumption. Chickens consume the trimethylamines and their eggs and meat will smell fishy.
It also turns out that some chickens and some people have a genetic mutation in their DNA so that they don't make working copies of a particular enzyme that breaks down trimethylamine. We need this enzyme because our gut bacteria naturally make trimethylamines as part of their life cycles. If we don't have the mutation, no problem. The enzyme breaks down the molecules in our gut. But if you or a chicken has this mutation, then you can still smell fishy even if you never consume any fish product of any kind because the trimethylamine that is made by your gut bacteria is never broken down. So the second way for (some) poultry meat to smell fishy is to have strains of chickens with the mutated gene. People with the mutated gene have a disease called trimethylaminuria and they smell fishy too.
However, our gut bacteria have plenty of tricks up their tiny sleeves. They can make trimethlyamines out of choline. Choline is an essential nutrient which means that we must consume it in our diets; it is essential for chickens too. Diets deficient in choline can cause disease. However, if we feed chickens diets that contain large amounts of choline, we overwhelm the ability of their systems to break down all the resulting trimethylamines...and their meat and eggs can smell fishy. This third mechanism is not well quantified (how much choline is "too much", does it depend on age or type of chicken, does it depend on other dietary elements) but we know it happens.
And finally, the fourth mechanism for making poultry meat smell fishy has nothing at all to do with fish or bacteria. When you are trying to create a consumer product that is "enriched in omega-3 fatty acids" you are trying to increase the concentration of long-chain polyunsaturated omega-3 fatty acids in the parts of the chicken that we eat. I know, that's a mouthful. We generally abbreviate that mess to LC n-3 PUFA. Long-chain means the carbon backbone of the fat molecule contains between 18 and 22 carbons linked together. Polyunsaturated means that some of the carbons are linked with special molecular bonds called double bonds. You should already know that diets high in saturated fatty acids, fats with no double bonds at all, are strongly correlated to increased risk of heart attack and stroke; to be healthy we are supposed to consume fewer saturated fats and more unsaturated fats. Some of the omega-3 fatty acids can have up to six double bonds. We are trying to put more of these kinds of fatty acids into chicken meat and eggs. But there is a problem.
When a fatty acid has lots of double bonds, it is more susceptible to peroxidation than the unhealthy fatty acid that lacks all those double bonds. Peroxidation refers to a specific process by which a chain reaction breaks the unsaturated fats apart at their double bonds. Not only does lipid peroxidation form free radicals (very, very bad things to have floating around your cells) but the fatty acid fragments are converted into other molecules such as aldehydes and ketones. And most of these are quite stinky.
When this happens, the meat tissues can change color, will taste bad (rancid), and will develop an off-flavor that is often described as "fishy" even though it isn't caused by the same molecules as fishy odors in fish. This is not an immediate process in general. Normal, every-day chicken meat will undergo peroxidation given enough time but it will happen sooner in "enriched" chicken meat.
So that's the fourth mechanism to make poultry meat taste fishy: enrich that meat with long-chain polyunsaturated fats and fail to protect the fats from peroxidation by lax handling, storage, and/or lack of antioxidants in the meat or diet.
It turns out this animal nutrition business is rather complicated. And there is no way to avoid the related issue of human nutrition and health--we and chickens are what we eat.
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