Compound Interest – New favorite website. Not only does it provide a good amount of smell-chemistry, but it looks good. (Are inforgraphics still cool, or was that so 2010's?)
Fyi, the site won the Association of British Science Writers’ Dr Katharine Giles Science blog award in 2018, and the graphics have been featured on about a hundred other websites, most of which you've probably heard of.
Why do I care? Because there is a whole section on Aroma Chemistry, with two dozen posts. I've covered just about all of them on my own blog already, but these give an explanation way more grounded in the chemical sciences. Also way more informative graphics.
So today I'm running a marathon through all of them, highlighting summary snippets and adding relevant links for those trying to go deeper into the great olfactory mystery that is the invisible world around us.
nitrogen-containing compounds found exclusively in bacon:
"meaty" compounds found in bacon:
As you’ll see with basically all of the items listed here, there is rarely ever one molecule that makes up the smell of a thing. Instead it’s a bunch all smashed together. And because we can’t discriminate between more than 4 things at once anyway, we will tend to perceive them as unitary.
Being that this is bit anti-climactic, let me finish!
I looked not to the scientific literature, but the chemical distributor who provides the flavor and fragrance industry with its chemical components. Sigma Aldrich has a whole catalog where you can buy flavors, and there is one compound in particular that shows up under the description "bacon."
2-Methoxy-4-methylphenol aka 4-methyl guaiacol
Now this doesn't mean that it smells like bacon either, only that if you wanted to create the impression of Bacon, you would include this compound. On that note, the same compound can be used to create all of the following flavors: smoky, medicinal, cheese, coffee, vanilla, jasmine, sweet, clove, carnation, mesquite.
Check out the extensive descriptions for Guaiacol over at the Good Scents Co.
And this handy chart from an older post.
M.L. Timón et al. 2004
Ultimately, the smell of cut grass is a defense response. You see, when we cut grass, the grass thinks it's being attacked (probably because it is being attacked) and it launches a chemical attack back. Unfortunately, we like it.
The chemistry story is this – upon being cut, fats and phospholipids in the grass break down into linolenic acids and linoleic acids, which are then oxidized and broken down further by enzymes into some of these chemicals:
(E)-2-hexenal aka Lead aldehyde
I've covered this topic pretty in-depth (keywords Petrichor and Geosmin), but Compound Interest adds an important distinction by separating the smell of rain into pre- and post-rain.
The pre-rain smell is funny because it's less of a smell than a multisensory experience that everyone knows as "it's about to rain" (or where I live, "it's about to snow" is even more common, and comes sometimes a whole day in advance).
It’s the inclusion of ozone in the smell of rain (chlorine, burnt wires) that has more to do with the pre-rain event than the rain itself, and the ozone comes down to us from extreme disturbances in the upper atmosphere that rip apart and rejoin oxygen molecules from high above, then smash them back down to the surface.
As a general rule of thumb, the post-rain smell – geosmin, "earthy," smells like boiled beets. If you're not sure what "earthy" smells like, boil some beets and take a whiff.
I J Bear & R G Thomas. 1966
I J Bear & R G Thomas. 1965
Despite the fact that there’s a trillion smells out there, I guess we all write about the same stuff; I have a few posts on this. But once again, The Compound Chem version adds a whole new layer.
For starters, we've got New Books. These smells come from treatment to the paper itself, adhesives that hold the pages together, and the ink. These chemicals might not smell themselves, but react with other things to create an odor.
And then there's the smell of Old Books. These smells come from the breakdown of paper, and are in addition to those listed above. The main thing to know about this is cellulose and lingin. Books are made from paper, paper from trees, and trees from cellulose. Lingin is a part of cellulose and it smells. It's the main thing that makes an old book smell like it does.
Granted, as with all smells, every book is specific; it's made with its own materials, has been exposed to its own environment for its own duration, and so it will smell distinct. But lingin is a big part of that smell. And I cannot talk about the smell of old books (or lignin) with talking about the smell of old people, because they share lingin. That's right, we turn into plants as we get older, because a lifetime of eating plants will do that to you.
We also smell like nonenal, aka “the smell of old people.” Check out this story on smelling like grandpa.
The smell comes from the offgassing of carpets, upholstery, plastics, molding, adhesives and leather and vinyl treatments used in the car, and change from car to car. Chemical types include aromatic compounds toluene, xylenes, styrene, and trimethylbenzenes, and some alkanes.
He's saying that the smell decreases at 20% per week. Sounds like it doesn't stay new for long. But those numbers are all temperature and ventilation dependent.
Btw, Ford is trying to get rid of "new car smell" for its Chinese customers who don't like it:
Aside from the hydrogen sulfide that's created when seaweed decomposes, it's really the algae in the ocean that make the unique "smell of the sea." These algae contain a molecule called dimethylsulfoniopropiante (DMSP), which has something to do with fluid volume in their cells, which might be why they're specific to the salty ocean. That chemical gets broken down by bacteria to produce dimethylsulfide (DMS).
Dimethylsulfide is also the smell of rotten cabbage, and is pretty obnoxious at higher concentrations. Which reminds me; I think he forgot to add trimethylamine, the smell of rotten fish.
Click here for a link to an Urban Odor Lexicon, which contains lots of other descriptors for dimethyl sulfide.
And click here for a post about Kramer's idea for a cologne that smells like the beach (I Could Have Been a Fragrance Millionaire, Jerry).
Here the topic is not seaweed decay, or algae decay, but human decay. We smell different from the ocean. But it shouldn't be a surprise, this one, because the major smell that we associated with rotten flesh is aptly named:
Skatole and Indole are used in perfumery, because they smell like flowers at low doses.
And for the record, methyl anthranilate is a chemical used in perfume, but can break down into cadaverine on the wrong kind of skin (Looking at you Tom Ford Neroli Portofino).
And again, just like our plant brothers and sisters, there's a bunch of sulfur-this and sulfur-that involved in our post-mortem metabolism (methyl sulfides, and hydrogen sulfide).
Last thing, can't ignore coffin liquor. Yup, exactly what it sounds like.
Sensory Scientist Avery Gilbert's I Smell Dead People installments.
Christmas Trees, aka Pine Trees, aka Coniferous Trees are pretty straightforward, smellwise:
Terpenes are related to turpentine, and they come from the resin inside the trees. Tree blood, if you will. But if you're interested in going any further, why don't you just search up cannabis terpenes instead, because you'll probably get more hits that way! (See: Myrcene, Limonene, Camphene, etc.)
Sensory Scientist Avery Gilbert's Cannabis Odor Vocab
On another note:
Orange juice breaks down into turpentine over time. True.
Also, pineapple juice, which contains ethyl butyrate, becomes butyric acid (rotten butter mmm).
Any verbal summary of this post won't do it justice; you have to see the infograph.
Carnation: eugenol, beta-caryophyllene, benzoic acid derivatives, methyl salicylate
Violets: ionones (which deactivate their own olfactory receptors, so we can't smell it for long)
Lily: beta-ocimene, linalool, eucalyptol
Hyacinth: ocimenol, cinnamyl alcohol, ethyl 2-methoxybenzoate
Chrysanthemums: chrysanthenone, chrysanthenyl acetate, alpha-pinene, eucalyptol, camphor, borneol, beta-caryophyllene
Lilac: beta-ocimene, lilac aldehyde, lilac alcohol, rose oxide, benzyl methyl ether
Volatile compounds from flowers – H Surburg et al
They are exaggerating when they say that coffee is one of the most complex flavors ever. Granted, there's been a decent amount of research on it, and as a reproducing, living thing, it has evolved as a major staple within our interglobal, multicultural trade networks since just about forever. This gives it a complex genome.
Furthermore, because it's roasted, it's chemical complexity benefits from the Maillard reaction, which transforms its natural chemicals into a bunch of different delicious and intoxicating volatiles, all of which make up the steep list of characters.
Most exciting here, is that in this post, for the first and only time, we see mention of a most-important concept in smells of things: "odor activity value," which is a ration of the concentration of the compound, and the compound’s odour threshold.
In so many, just about all of the things you'll read about "What makes xyz smell like xyz," you'll see lists of chemicals. But just because a chemical smells, and it's in the thing, that doesn't mean it's responsible for the most representative molecule of that thing's odor-identity. Some things we can smell at 1 part per trillion, and some at 1 part per billion. So comparing overall concentrations with the odor threshold is critical to communicating what something smells like.
Also, there's a pretty easy answer to this one. 2-furfurylthiol, or furfuryl mercaptan, is a good representation of smell of roasted coffee.
Butyric acid and Isovaleric acid are the one-two punch of human waste.
These are the smells of rotten butter, vomit, and Parmesan cheese.
But together, the two are called "barnyard."
Sulfurs like Rotten Eggs
Nitrogens like Rotten Fish
Since we're on the subject, your urine shouldn't smell like fish, and if it does, you might need to go to the doctor.
All about the microbes. They live in dog hair. Water "liberates" their excreta from the hair to the air.
Not for nothing, but I have to go here: Back in the day before Google was woke, circa 2014 thereabouts, you used to be able to get predictive search options for the following phrases: "Why do white people smell like..." and the top hits were all "wet dog."
This has more to do with the difference between "white hair" (microscopically it's twirled like an old phone-cord, but macroscopically it's either straight or bigger curls) and "black hair" (microscopically straight lines, macroscopically made of tiny twirls).
The "white hair" is just like dog hair.
You can't see these predictive results anymore, because google changed that part of the algorithm, and you can probably understand why.
I like the way they break down the aromas as coming from three phases in the breadmaking process. There's the base ingredients of the wheat, the enzymatic products of the yeast that eat that wheat and make carbon dioxide along with their own odoriferous fermentation byproducts, and finally the Maillard reaction of the baking process, which make all the "baked" parts of the overall odor complex.
diacetyl (roasted, toasted buttery popcorn flavoring)
(^this one has a low odor threshold, so it becomes a big part of the overall aroma)
(^although these scents can be described as "cucumber" depending on the concentration, these are more widely known as the "smell of old people.")
These folks say they can make a believable "baguette" aroma with only 14 compounds:
Key odorants of baguettes prepared in two different ways (£), G Zehentbauer & W Grosch
Speaking of diacetyls and buttery flavor, some animals have it in their urine:
First things first. I know this guy is British because he calls a garbage can a bin.
Next, garbage, and all bad smells, are broken down into two categories – Sulfur and Nitrogen.
The nitrogen powerhouse is the trifecta Cadaverine, Putrescine and Trimethylamine.
rotten flesh, semen, rotten fish
The sulfurs are hydrogen sulfide, dimethyl sulfide, and methanethiol.
rotten eggs, rotten cabbage, rotten cabbage
And in the "Other" category are Ammonia, Acetaldehyde, and Acetic Acid.
ammonia/medicinal, green apple/nail polish remover, vinegar
Finally, as with all bad smells, time and heat are the enemy.
The Cloaca Maxima was Ancient Rome’s first major sewer, built in 600 BCE and still works today.
Alain Courbin’s Foul and the Fragrant is a book about how the smell of poorly designed sewage systems of 18th century Paris led to the French Revolution.
“The grid layout of New York City, for example, encourages large-scale collective odor experiences as it was designed in a way to facilitate airflow using prevailing westerly winds to dissipate the disease-carrying miasmas of the late 18th century.”
Smell Maps, Kate McLean
Urban Odor Dictionary, Jane Curren
Neutralene is an odor masking product emitted from a vaporization pole, and it either pairs with malodors thereby camouflaging them, or it slows down the fugitive offenders by attracting and combining with the offending molecules, thereby making them heavier, so they fall to the ground before leaving the vicinity of the landfill.
Daniele Quercia, Luca Maria Aiello, Rossano Schifanella. 2016.
Link to your personal tour of the Olfactory Museum that is the New Jersey Turnpike.
This one is actually featured in Chemical and Engineering News.
Air fresheners either mask bad smells with good smells, they break them down with organic acids, or they literally encase them in bigger molecules, so your nose never makes contact with the bad smell inside.
As a professional air quality advisor, I will conclude this piece with the following advice -- if you have an odor problem, you need to find the source of the odor and remove it. All of the above are temporary solutions that may actually create their own problems due to adverse reactions or accumulation of molecules in our respiratory system that may not be good for us at certain concentrations.
Oh, and the best air freshener of all? Open a window.
(Or increase the percentage of fresh air intake in your HVAC system, which will cost more money depending on how hot or cold it is outside, but will provide way better air quality for your occupants and maybe even increase their productivity!)
This WELL Certification regimen, which looks to ensure an overall healthy work environment, which I will assume to mean psychological health in addition to physical health, lists Olfactory Comfort as one of its standards.
Sometimes an air freshener is not what you need. Carbon dioxide is the biggest problem with "stale air," and according to this below, it's actually a pollutant that gives you brain fog.
Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderate CO2 Concentrations on Human Decision-Making Performance.
Usha Satish, Mark J. Mendell, Krishnamurthy Shekhar, Toshifumi Hotchi, Douglas Sullivan, Siegfried Streufert, and William J. Fisk. Environmental Health Perspectives. Vol. 120, No. 12. 1 December 2012.
That's it for the Compound Chem aroma expose, check out the site with all your chemistry questions.