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:
2,5-dimethylpyrazine
2,3-dimethylpyrazine
2-ethyl-5-methylpyrazine
2-ethyl-3,5-dimethylpyrazine
"meaty" compounds found in bacon:
2-pentylfuran
3,4-dimethylpyridine
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.
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:
(Z)-3-hexenal
(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.
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.
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:
Cadaverine
Putrescine
Skatole
Indole
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.
Christmas Trees, aka Pine Trees, aka Coniferous Trees are
pretty straightforward, smellwise:
Terpenes
Alpha Pinene
Beta Pinene
Esters
Bornyl Acetate
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.)
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.
In General:
rose oxide
beta damascenone
beta-ionone
geraniol
nerol
(-)-citronellol
farnesol
linalool
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
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.
Fatty Acids
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
hydrogen sulfide
methanethiol
Nitrogens like Rotten Fish
indole
skatole
trimethhylamine
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.
maltol
isomaltol
diacetyl (roasted, toasted buttery popcorn flavoring)
2-acetyl-1-pyrroline
(^this one has a low odor threshold, so it becomes a big
part of the overall aroma)
(E)-2-nonenal
(E,Z)-2,6-nonadienal
(^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:
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.
Links:
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.”
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.
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!)
Notes:
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.
