Not a scientist, but, interested in these things.

I would say it’s because spicy foods have highly volatile aroma compounds. Tannin and acids aren’t as volatile. Any aroma we perceive is the result of volatile molecules, solubalized in our mucus, binding to receptor sites in our olfactory epithelium - in other words, olfaction is a chemical sense. The tannic mouthfeel of a dry red wine is (if I’m not mistaken) a result of nonvolatile acids acting mechanically on the tongue, so olfaction, a chemical sense, doesn’t come into play in that mechanical sensation of acidity. Also consider that the tannins in a red wine are in a liquid solution. Fine, dry citric acid powder will irritate the nose if you breathe in the particles, just like fine dry cayenne pepper will.

Not sure what you mean with cooling. Something like camphor is highly volatile, and gives a cooling aromatic sensation. Think eucalyptus, fisherman’s friend, vick’s vapo rub, things like that. Do those smell cooling to you?

Metals don’t have volatile aromatic compounds in them at low temperatures, but, they definitely smell like something when they’re red hot. Again, the idea here is that metals are heavy and nonvolatile, so they’re invisible to our noses at room temperature, unlike say, a freshly sliced jalapeno pepper. Out of curiosity, what tastes metallic to you?

I can smell mint and metallic. No clue what’s going on with you. 🤷‍♂️

Well, taste and smell are all tangled together anyway.

There’s no way to be certain, but there are several factors in what smells we can and can’t detect, when the level of a substance is high enough that we can detect it at all.

Genetics is the big one. Not all people have the exact same thresholds for scent detection, nor the ability to smell everything.

Then, even if your genes are ripe to be able to detect something, it doesn’t mean that the little sensors inside your airway are distributed in a way to detect it reliably, at any given saturation.

But, the specific smells may still go unrecognized even when detected because the brain still has to interpret the signals.

Chances are you do smell those things, but they’re just not being interpreted at the same level of importance as other smells in the environment.

When you’ve got chemicals like tannins, they’re already less “aromatic” to begin with. Not only do they not have a strong presence, they (for a reason I’ve never found an explanation for) don’t seem to travel as far from their source.

Example:

Wine. If you take a really dry red wine, and hold a glass of it at arms’ length, chances are that you’ll get some smells from it. But it’ll be that boozy smell, along with the general wine smell unless you’ve got a very sensitive nose. But even “super smellers”, the kind of people that get paid big bucks in m the perfume industry, are likely to still miss the tannins.

Bend your elbow a little bit, you get some of those fruity and floral notes. A little more and you get stuff like oak and similar earthy scents. It isn’t until it’s at your nose that most people are going to pick up the actual tannins, and they’ll still be a scent that’s hidden among the rest, so that you have to really sniff for it for most people.

It happens with things like leaf litter too. You’ll get the general smell next to the woods, but won’t get those nose tickling bits until you’re walking through it, and it still won’t have that same musty, petrichorian tang that you’d get if you were laying down among the leaves. But! Part of that smell is actually tannins. They leech out of fallen leaves into the soil, and are on the surface of the leaves after it rains.

So it could just be that you can’t smell therm under normal circumstances, but might if they were isolated and concentrated. You can kinda test that using witch hazel products that are in an alcohol base. Plain alcohol has one smell, witch hazel has its own, and in that particular situation is almost exclusively the tannins.

Really strong tea has tannic notes. If you ever run peat moss as a filtering agent in aquariums, you can smell it.

But the actual tannins are mingled with other things, so your brain has to learn how to pick it out of the crowd. To do that, you have to experience a wide range of things that are very tannic, then let your brain figure out what the common scent is between them. Again, assuming your olfactory neurons are picking it up in the first place, which they should, if you’re smelling anything at all. And you are, because if you’re tasting things that are tannic, then you’re smelling them already, but it may be such a teeny tiny degree that you can’t pick it out at all.

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Now, it’s surprising you can’t smell minty things. You should be able to if you can smell anything at all. The compounds in mint family herbs are pretty volatile and aromatic. They have a long reach and our noses are good at detecting them. I suspect that the very fact you’re associating it with taste as a pseudo taste means that you are picking up those chemicals because mint has very little taste if you hold your nose. Next to none; it’s just slightly sweet and a little “grassy” for me.

So I’d suggest trying that. Get something minty, try it with your nose pinched shut, and without. See if there’s a difference in the taste. If there is, it’s because you’re smelling it when it isn’t pinched.

Now, metallic smells are kinda weird. It’s very rare for any metal to be up and about in the air in any quantity to smell at all. When it is the case, you’re breathing in metal vapors, and need to run.

What we detect as a metallic smell is the reaction of metals with other substances. Like the smell of pennies. If you perfectly clean pennies, and don’t touch them or let anything other than clean cotton or paper come in contact, there will be no smell, or only the faintest of smells.

You can then grab them with your grubby monkey mitts, and experience the smell of pennies blooming from your hands as the oils and acids from your skin react with the surface. This is also the case with pretty much every metal. The ones that are most inert just don’t have a smell, no matter how close you get. Lead and gold essentially do not have a smell at all, in their pure forms.

Supposedly you still aren’t smelling the metal itself at all, as the scent is more due to the oils changing than the metal, but I ain’t a chemist and can’t be 100% certain of the reactions involved, only that it can be tested and shown that clean, pure metals don’t have a scent because there’s nothing “evaporating” from them. And that when the same metals are in contact with acids and oils that develop a smell pretty much instantly if they’re not inert.

So, if you do any of those self tests, and you still don’t smell anything, chances are that you’re one of those rare folks that have something different about their nose, and just don’t smell things as much. There’s actually a YouTube channel host, Nile red/blue, that does some really stinky things, and he has way less reaction to them than the person running the camera in some videos.

Then you’ve got assholes like me that are just sensitive enough to start sneezing at the other end of a mall from a candle shop, but still not sensitive enough to make big bucks at a perfume company.

There’s outliers in the olfactory curve. You may well be towards the less sensitive end, or your brain may filter out smells for some reason, or whatever. But most people just haven’t learned how to pick scents apart into their components. It is something learned, not automatic. Some of it we learn as a survival thing, like with spicy chemicals.

Others that aren’t a survival factor, there’s little need for our brains to pay attention to every component of smells to find related commonalities. Tannic acid is only dangerous in concentration, and when ingested, and the astringency of it is enough of a warning, so the smell is essentially unimportant.

See? A lot of factors into why you don’t smell things. Could be any of them, or combinations.

https://en.m.wikipedia.org/wiki/PTC_tasting