Why are some smells so hard to get rid of?

Question

What may seem to be a silly question could actually have roots in some core chemistry principles. The other day, I was cooking fish when I noticed that I'd forgotten to use a fork, and touched the fish with my bare hands. When I tried to get rid of the fishy smell, it just wouldn't go away.

Why are some smells so hard to get rid of?

My Hypothesis:

I think that the phrase "hard to get rid of" really means "resistant to washing," because that's what we're actually thinking of when we say you can't get rid of a smell. And when we say "wash" we really mean "attempt to dissolve in water using soap emulsification".

I'd conclude that "a smell being hard to get rid of" is caused by chemicals that are not soluble in water, and for some reason, possibly because the molecules are too large to fit in a stable emulsive bubble, refuse to be emulsified into aqueous suspension by soaps. But that's just my theory.

Answer 1

Food flavors and odors are comprised of complex mixtures of hundreds of chemicals, which act as stimuli to the human sensory organs, for example, human nose. The odor of food may be attributed to the odor of a single chemical or an integrated response by nose to a mixture of chemicals. This reference to a single chemical or a mixture of chemicals allows trained analysts to develop a common terminology in describing odor, one of which called odor thresholds. Threshold is defined as the degree of stimulation of a nerve or nerve center that just produces a response. If the concentration of the odorous chemical (usually given in parts per million or ppm) is below this point, it will be judged not odorous. Hence, accepted definition of threshold is the minimum identifiable odor (MIO) or recognition threshold. MIO of some chemicals is very high while some is extremely low. The latter chemicals are the ones you said, "hard to get rid of." For example, the oder of dimethylformamide is described as 'fishy' and pungent, but you won't smell it if its concentration in air (or on your skin) is below $\pu{100 ppm}$. On the other hand, tar-like, pungent smell of p-cresol can be detected if its concentration in air (or on your skin) is greater than $\pu{0.001 ppm}$.

Now considering the fish smell, it was described short and sweet in following website from American Society for Nutrition: Read

Accordingly, it says:

Water in the open ocean is about 3% salt by weight, but the optimal levels of dissolved minerals inside an animal cell is less than 1%. In order to maintain fluid balance, ocean creatures must fill their cells with amino acids and amines to counter the saltiness of seawater. Ocean fish tend to rely on trimethylamine oxide (TMAO) for this purpose.

The problem is that when fish are killed, bacteria and fish enzymes convert TMAO into trimethylamine (TMA), which gives off the characteristic “fishy” odor.

Now, it is nice to know MIO of trimethylamine (TMA) is $\pu{0.00021 ppm}$ or $\pu{0.21 ppb}$ (parts per billion). That's why it is so "hard to get rid of" fishy smell whenever you touch or cook dead fish. But the good news is TMA is a organic base, which would react with any acid to form water soluble ammonium salts. Citric acid from lemon (or lime) or acetic acid from vinegar would do the job.