Welcome to the wide world of smells – the ‘osmocosm’. That’s the word coined by Harold McGee (famed author of On Food and Cooking) as he explores the cosmos of scents in his new book, Nose Dive. And yes, molecules are the stars of this story! There’s more than enough information to make a chemist’s mouth water, or should it be, nose twitch?
Nose Dive is more of a compendium. Like McGee’s aforementioned famous book, you don’t have to read it linearly (although I’m doing so) and you’re welcome to dip in here and there, or take a deep dive! There are five parts to the six-hundred-page tome. Part 1 begins with the simplest smells, the backstory being the Big Bang beginning of our universe. Part 2 discusses animal and animalistic bodily odors. I’m smack in the middle of Part 3, which comprises slightly over a third of the tome, on plants. Part 4 covers other smells of the lithosphere and the hydrosphere. Part 5, titled “chosen smells” will discuss cooked and cured/fermented foods.
As an origin-of-life researcher, I am familiar with many of the molecules mentioned in Part 1. But I hadn’t thought about their smell until prompted by McGee. Four small molecules he highlights that humans can smell are H2S (“eggy, sulfurous”), SO2 (“irritating, sulfurous”), NH3 (associated with “household cleaning products, overripe cheeses and salamis, underripe animal manures, and urine”), and O3 (named ozone “by a German chemist from the Greek root for smell” and associated with “lightning strikes, power-line arcing or prolonged use of high-voltage laser printers”).
I find it hard to describe smells because I don’t quite have the vocabulary for it. As a computational chemist who spends little time in a ‘wet lab’, this is clearly not my forte. (I have an aroma wheel pasted to an office filing cabinet to help me improve.) And it turns out that how one describes smells can be very personal and subjective depending on one’s prior experience. McGee has a great story about tribespeople who enjoy the taste of Amazonian ants, as told to him by Brazilian chef Alex Atala. Upon tasting the ant samples, McGee thought they tasted like lemongrass and ginger, tastes associated with Asian food. Atala went back to the cook who had made the ant broth presenting both lemongrass and ginger (that she had never encountered before), and she thought they tasted like ants! McGee refers back to this story throughout his book because it illustrates how we experience smells.
The really smelly stuff are the thiols, molecules that contain carbon-sulfur bonds. CH3SH is associated with rotting cabbage, and humans are very sensitive to its scent. One of my organic chemistry colleagues works with thiols in his lab. I make it a point never to go into his lab. Some smells can stick with you. I think it’s both funny and marvelous that H2S, COS, and organosulfur compounds likely played an important role in the chemistry of the origin of life. (For more info, look up Christian De Duve’s ‘thioester world’ hypotheses.) The beginning of life was a big stink!
My current interest in the organic molecules that may have contributed to the evolution of a proto-metabolism involve alcohols, aldehydes, ketones, and carboxylic acids. These all contain carbon, hydrogen, and oxygen; and in particular tend to be of the more ‘oxidized’ variety (although not in the very highest oxidation states). These small molecules often range from sharp vinegarish scents to cheesy and rancid notes, and pungent or ethereal wafts. Interestingly, formic acid comes from formica meaning ant! The larger versions of these molecules begin to introduce descriptors such as “fresh, green-apple, citrusy, waxy, soapy” and the smell of goat. Trivia I learned: the six-, eight- and ten-carbon simple carboxylic acids were originally known as caproic, caprylic, and capric acid respectively. The root word is capermeaning goat!
In Part 2, one chapter is devoted to the smells of the human body. What we call ‘bad breath’ comes from volatiles emitted from “microbial metabolism of mouth proteins”. This includes the stinky CH3SH but also a number of nitrogen-containing compounds that have their characteristic stenches. These are described as fishy, spermous, and putrid. When I teach chemistry for non-science majors I often use putrescine and cadaverine as examples of ‘rotting’ smells. (No, I don’t bring them into the classroom, but I do draw their chemical structures.) But I hadn’t used spermine and spermidine as examples to show their close relation to their putrid cousins because I hadn’t looked up the structure until McGee mentioned it. The smells of life and death are intertwined.
Part 3 has been eye-opening. I’m amazed by the range and diversity of volatile compounds emitted by plants. There are many reasons of course. Plants, being relatively immobile compared to their animal counterparts, utilize chemical signals to attract pollinators and ward off pests that try to eat them, among other things. Having spent most of my time studying the more ‘oxidized’ compounds in metabolism, it was eye-opening to see mostly low-oxidation state ‘reduced’ compounds being emitted by plants. There are fruity esters (and lactones) that I discuss in my class; I also talk about isoprene and its relationship to rubber (both natural and synthetic) but I hadn’t considered the range of terpenoids and benzoins. Wow, there are a lot of them, they have very interesting structures, and such a diverse range of scents! It’s amazing what we food-cooking humans have done by utilizing many of these.
I had not thought of smells as having a visible component or affecting climate, and McGee illustrates this with some nice factoids. I’ll quote him here. “Both isoprene and terpenoids have another property that makes their initially invisible tides visible, as the haze that gives the world’s various Blue and Smoky mountains their names. As they react with other chemicals in the atmosphere, their by-products form clusters, some of which attract water molecules and develop into water droplets or ice crystals. These suspended particles, or aerosols, both absorb and scatter light – which is why they’re visible as haze – and thus deflect some of the sun’s energy from reaching the leaves… these isoprene and terpenoid aerosols encourage the formation of clouds… and so have significant effects on local climate and possibly global climate as well.”
In reading about flowers, I didn’t know that humans have been breeding the scent out of commercial flowers. Here’s McGee again: “Our love for flowers and our ability to produce them at will have conspired to drain them of both significance and smell. The modern global flower industry is the product of growing wealth and city markets for cut flowers, the professionalization of gardening and plant breeding, commercial flower shows, and plant-collecting expeditions. Its business is driven by the competitive breeding of visually striking new varieties that have a long vase life. This program has often meant a steep decline in floral scents: in part because volatile and pigment molecules share biochemical resources, so more color means less scent, in part because some scent volatiles are also plant hormones that shorten vase life. And in today’s largely deodorized world, buyers often prefer scentless flowers for their unobtrusiveness.”
There’s so much more that can be said about Nose Dive, and I’m not quite halfway through the tome. Yet I already have enough ideas if I ever teach a ‘Chemistry of Smell’ course. My department has recently discussed expanding the options and themes of our non-majors chemistry courses. In the past I’ve injected different themes when I teach the generically-named ‘Chemistry and Society’ course but these are often in dabs and smears and sprinkles. When I had an Infographic project several years ago, a significant number of the students chose to do things related to the cosmetics and perfumery industry. Certainly there’s interest in this sort of chemistry! I think many of them would enjoy a Nose Dive chemistry course.
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