But this is specifically not smell, or even about the brain: it’s about what chemicals the tastebuds are physically capable of detecting. In particular the swee

But this is specifically not smell, or even about the brain: it’s about what chemicals the tastebuds are physically capable of detecting. In particular the sweet taste of heavy water seems to be purely related to the sweet taste buds (though there may be secondary flavors that come from the smell). Within humans there is considerable variance among bitter tastebuds. Roughly speaking, the salt/sour/sweet tastebuds are comparatively simple ion detectors: salt = sodium, sour = hydrogen (from the acid), and sweet = glucose. And yes, sweet is more complicated, because it’s trying to detect a more complicated form of aquatic solute. But the idea is that these tastes are trying to find a specific and well-defined chemical property. Bitterness is different. Bitterness is basically a poison detector, which means that to be effective it needs to register a wide variety of potential toxins. So akalais/bases tend to be bitter because naturally-occurring bases tend to be toxic. But there are all sorts of complex organic molecules that may be toxic (or are chemically very similar to toxins), which some people can taste and others cannot. Cilantro is probably the most famous example: people who hate cilantro are tasting a (harmless) toxin that other people can’t detect. But this taste is a true taste from the tastebuds, not a smell.