On episode 445 of Cooking Issues Radio, Dave Arnold, Harold McGee, Robert Datta, and I used our motley backgrounds (food and cooking tech expert, world’s foremost popular authority on the science of cooking and smell, Harvard sensory neuroscientist, and flavor chemistry wonk) to delve into a growing but still nascently-understood side effect of the COVID-19 pandemic: people who get COVID, experience a loss of their sense of smell (anosmia), and then, when their sense of smell comes back, experience some formerly-tasty smells and flavors (which is mostly smell) as horrible, like rotting garbage. Some of the common triggers are garlic and onions, nuts, browned or grilled meat, coffee beans, and even flowers.
Flavor and smell molecules can have almost any atoms in them, but pretty much all of the, have a backbone of carbon, and the majority are all or mostly carbon and hydrogen. Some other common atoms are oxygen, nitrogen, sulfur, and halides like chlorine and bromine.
Garlic and onions, famously, get their characteristic aroma from molecules that have sulfur atoms in them. In our convo, I got excited because browned meats have a lot of nitrogen-containing flavor molecules in them called pyrazines. So does coffee. But what about raw nuts and flowers? Well, nuts ALSO have some pyrazines in them, even before they’re roasted, and flowers often have either aminoacetophenone (which smells like a mix of orange blossom and grape soda) or indoles (which smell like different mixes of jasmine, rotting flowers, and sex) in their aroma bouquets. Both aminoacetophenone and indoles ALSO have nitrogen atoms in them.
The cool thing is that humans tend to be unusually sensitive to sulfur and nitrogen-containing molecules. Many of them, we can detect in the parts-per-trillion range. In grad school (in the viticulture and enology. department at UC Davis) I did a little work that involved handling pyrazines, and several other researchers were working on pyrazines and sulfur volatiles more intensely. And besides knowing olfactory sensitivity from the literature, I unfortunately know from experience that if you even open a vial of these pure compounds fore any length of time, the miniscule amount that evaporates out will make you smell like concentrated green bell pepper (if you open methoxyisobutyl pyrazine) or some kind of bizzare, rotten fruit (some sulfur molecules that otherwise smell amazing in tiny amounts in wine). I focus mostly on the sense of smell as part of eating, but it plays a really important role in self-protection, as well. The smell of a fire can wake you up in your sleep, for example. Maybe our general innate sensitivity to these is going haywire, a kind of panicked grab at self-protection, as our smell rewires itself from COVID anosmia?
Harold had seen that a team from the UK had just come out with a preprint about COVID-19 Parosmia and trigger smells—specifically, flavor molecules in coffee—and you can find it here, and FYI: a “preprint” means it is brand-new research that has NOT been peer-reviewed yet. They found that 9/10 of the most frequent “trigger smell molecules” they identified were either (nitrogen-containing) pyrazines or types of sulfur volatiles called thiols and disulfides.
It’s a really unfortunate side effect of the pandemic—losing or having your sense of smell altered not only makes it really boring, or unpleasant, to eat, it can also trigger or worsen depression (probably since smell is really tightly integrated to emotion and memory). It’s going to be fascinating seeing how this area of research develops, what happens to parosmics over time, and what gets thought up as treatment. One thing recommended now is “smell training”, which you can find more info on over here at Abscent.