Fresh fish has a characteristic, but not necessarily unpleasant smell. However, after a few hours out of the water, it develops a strong particularly unpleasant smell. This phenomenon is not due to the fact that the fish is dead but has its roots in specific biochemical processes by bacteria who proliferate on the skin and bowels of the fish after his death. In fact, from these reactions, volatile compounds are formed, and one in particular, the trimetilammina (Tma, a molecule also present in humans) has been associated with the characteristic smell of fish. Other molecules also come into play, such as the putrescineand oxidation reactions of hepatic fatty acids that go to spraying, contributing to the formation of unpleasant odors for our nose, but as confirmed also by studies published byAmerican Chemical Society, it is the tma la main responsible of the unpleasant smell of fish. Not everything is lost, because with ice and a few drops of lemon, it is possible neutralize This stinks. Unfortunately, however, in some cases, we too humans We can literally stink of fish, due to an accumulation of trimethylamine that is expelled with sweat.
The role of bacteria and the importance of conservation
Microorganisms play a fundamental role in the formation of the smell of fish. After the death of the animal, the bacteria present on its surface and in the bowels begin to proliferate and degrade its tissues, producing the volatile compounds responsible for the stench, such as aldehydes, sulphides and ketones. Among the compounds produced, the most important is the trimetilammina (TMA), deriving from the rapid degradation of the oxide oxide (Tmao), a molecule normally present in fish and other living beings, including us humans! In fish, the main responsible for the transformation of Tmao into Tma, thanks to the enzyme TMAO Redittasi, are bacteria of the Shewanella genre, in particular SHEWANELLA MASSILIA.
The TMA, unlike its precursor, is a molecule volatilethat is, it passes very easily in the air spreading a strong and pungent smell, typical of the avaried fish, which can also have notes similar to ammonia. In fact, ammonia and trimethylamine have similar chemical structures: they are both amineformed by an atom of nitrogen central with three ties. The ammonia (NH3) is the simplest form, with three hydrogen atoms, while the trimetilamine has three methyl groups (-ch3) instead of hydrogens.
Given the structural affinity, these two compounds, once they arrive at our nose, is likely to interact with the same receptors as the olfactory bulb (or in any case belonging to the same family). According to recent research, which require further confirmations, the TMA is likely to bind to the so -called receptors Taars (Trace Amine-Associated Receptor): these recognize and bind specifically with the amines and have been discovered in the olfactory tissue of many vertebrates, including the human being by the researchers of the Smbe (Society of Molecular Biology & Evolution).
The “antidotes” to neutralize the stench: like chemistry it helps us
To counteract bacterial proliferation, in fish shops The fish is maintained in contact with the ice: low temperatures slow down the growth of bacteria and enzymatic activity, prolonging the freshness of the fish e delay The formation of unpleasant odors.
But there are also other strategies. Tma, from a chemical point of view, is one basis. It is no coincidence that one of the most effective methods to reduce the stench of fish is precisely the use of acid: bases and acids react together through reactions of neutralization. In our case, the reaction between the TMA and an acid substance leads to the formation of a salt non -volatilewhich therefore hardly passes to the aeriform state, that is, it does not spread in the air and cannot reach our nose. If after cooking the fish we have foul -smelling fingers we can rub them with vinegar or lemon, which contain respectively acetic acid And citric aciduseful for eliminating or at least decreasing the stench. Also for this reason, many recipes involve the addition of lemon to the fish: not only for the flavor, but also for its ability to mitigate the smells.
Because even humans can stink fish
The TMA is also located inside other organisms, including thehuman being. Here, it is converted into Tmao (that we remember being the odor) from an enzyme called Flavina-Monossigenasi 3 (Fmo3). A defect of this enzyme can cause the onset of a rare metabolic disease known as trimetilaminuria (Tmau), or also Fish smell syndrome: in subjects affected by this particular disorder, the TMA is not metabolized and accumulatesfirst in the blood and then be excreted through sweat causing a strong and unpleasant body smell. In some cases the condition is constant, in others it occurs only in periods of stress, of hormonal changes or after the consumption of foods rich in Precursors of the TMA such as eggs, some legumes and, guess a little … the fish!
