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Catnip for humans – Nature’s fragrances that smell of health and happiness

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In my previous post, I looked at the nasty smells that are associated with illness and death, and most often found in hospitals and medical facilities. That is the unfortunate reality of the situation, and though I mentioned quite a long list of smelly substances, I did not list all of them. But wait! There’s more! The point of the article is that good, pleasant odours are used to mask unpleasant odours, and these smells include those that come from substances meant to reduce or control decay. These may not, themselves, smell pleasant, but what they control probably smells worse. That point was humorously made in British author Evelyn Waugh’s 1965 novel, The Loved One, in which he satirizes death practices in England and America. It is a short novel about a subject that not many authors dared to tackle at the time – especially not with black humour and the finer details of the odours associated with mortuaries:

“The pickled oak, the chintz, the spongy carpet and the Georgian staircase all ended sharply on the second floor. Above that lay a quarter where no layman penetrated. It was approached by elevator, an open functional cage eight feet square. On this top floor everything was tile and porcelain, linoleum and chromium. Here there were the embalming-rooms with their rows of inclined china slabs, their taps and tubes and pressure pumps, their deep gutters and the heavy smell of formaldehyde. Beyond lay the cosmetic rooms with their smell of shampoo and hot hair and acetone and lavender.”

Luckily, while nature presents us with these unpleasant realities, it also offers the joy of the very best and loveliest, natural smells your nose could encounter. These are like beautiful gifts that have a feel-good element built in. How could you not feel happy when you smell the earth after rain, newly cut grass, fragrant roses, the sea and the sea-wind, and wild flowering trees and shrubs? (Even if you have allergies and your nose is blocked and you can only smell them a little bit.) Getting outside is a good idea, because all of these evocative and heady scents are free and likely to envelop you as you stroll around. Certain indoors odours are also perennially popular, particularly the smells fresh laundry dried in the sun, and cookies and bread fresh from the oven, so much so that bakeries make sure to send the smells drifting out of their shops to tempt customers to come inside, following their noses as it were.

Why do certain things smell as nice as they do?

Below are analyses of what constitutes these aromas, and why we are in love with them, like cats with catnip.

  • earth after rain
  • fresh-turned earth
  • newly cut grass
  • the sea and the sea-wind
  • flowering trees and shrubs
  • bread fresh from the oven
  • cookies and fresh bread

The smell of rain

What makes the smell of rain so great? There is a scientific explanation:

“One of these odors, called ‘petrichor’, lingers when rain falls after a prolonged dry spell. Petrichor — the term was coined in 1964 by two Australian scientists studying the smells of wet weather — is derived from a pair of chemical reactions. Some plants secrete oils during dry periods, and when it rains, these oils are released into the air. The second reaction that creates petrichor occurs when chemicals produced by soil-dwelling bacteria known as actinomycetes are released. These aromatic compounds combine to create the pleasant petrichor scent when rain hits the ground. Another scent associated with rain is ozone. During a thunderstorm, lightning can split oxygen and nitrogen molecules in the atmosphere, and they in turn can recombine into nitric oxide. This substance interacts with other chemicals in the atmosphere to form ozone, which has a sharp smell faintly reminiscent of chlorine.” (by Elizabeth Palermo on LiveScience)

For people who live in a place with an arid or semi-arid climate, the smell of rain is particularly appealing and quite distinctive. Toto’s famous song, Africa, became famous despite its decidedly hokey lyrics and video, but the line “I bless the rains down in Africa” perfectly caught people’s love of the smell of rain on dry earth.

The smell of earth

Some people, like me, get a kick from digging in the garden and planting stuff. There is a great deal of satisfaction in having your hands in good, rich soil. One of the reasons for this is that soil or earth smells really good to some people. The smell of earth is called “geosmin”(chemical formula: C12H22O). Its name is derived from the Ancient Greek γεω- geō- “earth” and ὀσμή osmḗ “smell”. The word was coined in 1965 by the American biochemist Nancy N. Gerber (1929–1985) and the French-American biologist Hubert A. Lechevalier (1926–2015). Geosmin is a volatile substance that has a distinct earthy or musty odor, which most people can easily smell and identify with earth or soil. Geosmin is also responsible for the earthy taste of beetroots and a contributor to the strong scent (petrichor) that occurs in the air when rain falls after a dry spell of weather or when soil is disturbed.

Geosmin is produced by, in part, particular algae and bacteria, and communities in which water supplies depend on surface water can periodically experience episodes of unpleasant-tasting water when a sharp drop in the population of these bacteria releases geosmin into the local water supply. The human nose is extremely sensitive to geosmin and is able to detect it at concentrations as low as 5 parts per trillion. Since geosmin is present under normal conditions, humans instinctively associate this very subtle, almost primal scent with good earth and verdant plants.

Newly cut grass

Everyone who has ever cut a lawn knows that it releases a delicious smell: pepperminty, sweet, sharp – in short, “green”. It’s worth the schlepp of pushing a lawnmower around. Chemically speaking, that classic lawn smell is an airborne mix of carbon-based compounds called green leaf volatiles, or GLVs. The green smell is a mixture of oxygenated hydrocarbons that include methanol, ethanol, acetaldehyde and acetone. Plants often release these molecules when damaged by insects, infections or mechanical forces — like a lawnmower. Green plants in other words have a distinct smell, same as fragrant flowers, serving either to attract pollenitors or eaters of those plants. GLVs are small enough to take to the air and float into our nostrils. In some cases, they can be detected more than a mile from the plant where they originated.

Humans don’t typically eat grass or the insects on it, though you might lie on your back and chew on a blade of grass while looking at the sky, but the GLVs that grass releases aren’t that different from those of plants we do find tasty – just about all fresh vegetables have some GLV bouquet to them. There isn’t anything specific to grass that makes it smell nicer to us than another plant. But we are more likely to mow it, crushing a lot of plant tissues at once and releasing a concentrated cloud of GLVs. With nice, green, manicured lawns part of the image of an ideal home, along with white picket fences and an apple tree in the yard, there’s a lot of opportunity to enjoy the clouds of GLVs of a fresh mow.

Sea and surf

Many people love the smell of the sea and the sea breezes – fresh, salty, a bit green, a touch of sulphur. Some say it smells of yeast, others of pine, some say tinned corn (?). What you are smelling from the water is likely to come from the things in the sea – the seaweed, plants, fish, and other critters. Many scent molecules combine for that specific marine fragrance that people associate with freshness, freedom and holidays on the beach.  The dominant three are:

  1. dimethyl sulfide (produced by decaying sea plants),
  2. dictyopterenes (produced by amorous seaweed), and
  3. bromophenols (produced in the stomachs of marine life).

One of the most common and best-understood components of seashore aroma is dimethyl sulfide, or DMS.

“This stinky sulfur compound puts the funk in everything from nori, to truffles, to beer. It’s also abundant in farts. In the ocean, DMS is produced in large part by bacteria [in the ocean floor] that eat dying phytoplankton—microscopic organisms that use light from the sun to make their own energy. For a clearer idea of what DMS smells like, visit a salt marsh. The flooded soils of salt marshes release a whole bouquet of sulfur smells, and one is DMS. Barring that, your local artisan-cheese counter is one of the best places to get a nice hit of DMS. Microbes such as the bacterium Brevibacterium and the fungus Geotrichum that grow on ripening cheeses can produce high levels of DMS as a consequence of decomposing cheese proteins.” (Benjamin Wolfe, Why Does The Sea Smell Like The Sea?, in Popular Science, August 19, 2014)

So the lovely sea smell is actually to a large degree the pong from rotting sea life. Scientists have even identified the specific element in DMS which smells so nautical – Dimethylsulfoniopropionate (what a mouthful!), DMSP for short. After sequencing  its genes and comparing the genetic structure to other known bacteria, they were able to identify the gene involved in the mechanism that converts the plants’ decay products, DMSP, into DMS. The extent of this decay process can influence cloud formation over the open ocean, and “…DMS plays an important biogeochemical role in the global sea-to-land transfer of sulfur, and it has a potential effect on remote oceanic weather patterns due to its oxidation in the atmosphere, which generates cloud-condensing nuclei leading to reflection of solar radiation.”  (DMSP: Its Sources, Role in the Marine Food Web, and Biological Degradation to DMS, Duane C. Yoch, 2002, US National Library of Medicine, National Institutes of Health, 2002)

Another component of the sea smell is the dictyopterene volatile compound, which is produced in large amounts by seaweed eggs while the seaweed algae is reproducing. The dictyopterene acts as a lure to the eggs, so you could say you are smelling seaweed pheromones. What does it smell like? Like seaweed of course!

Bromophenols, a third component, are found in the stomach and flesh of wild seafood (not so much farmed seafood), including fish, mollusks, oysters, clams, shrimp, and crabs. What this means is that, in essence, you are smelling the food in the seafood.

“The success of aquaculture products has been hampered by problems relating to the quality of their flavor. Since many consumers can clearly distinguish the difference between the flavor of cultivated and wild harvest seafoods. Knowledge of the factors and chemical substances that determine flavor can contribute significantly to the improvement and expansion of aquaculture and to the preservation, storage, control and improved quality of seafoods. […] Bromophenols, which have been found in marine fishes, crustaceans and mollusks, are strongly associated with pleasant (marine- or ocean-like) or unpleasant (plastic-, medicinal-, disinfectant-, iodoform or iodine- like) flavors, alone or in different combinations and concentrations. Marine food gourmets describe the [pleasant] flavor of some fish species as mildly candy-like and others as marine or oceanic-like, which is characteristic of the presence of bromophenols in different concentrations.” (Bromophenol concentrations in fish from Salvador, BA, Brazil, by Aline S. Oliveira, Vilma M. Silva et al,  Instituto de Biologia, Universidade Federal da Bahia, UFBA, Rua Barão de Geremoabo s/n Campus Universitário de Ondina, 40170-290 Salvador, BA, Brasil, October 7, 2008)

In low concentrations, these bromophenols are described as smelling sea-like, fishlike, and crab-like – going around in a circle with the descriptions really. In high concentrations, as when the seafood is old or has gone off, they smell harshly chemical, like iodine.

These three, and many more substances, contribute to the much loved smell of the sea. It shows, as is often the case, that what we make of smells have as much to do with our memories associated with them, as the actual origins and characteristics of the substances of which they are composed.

Flowering shrubs and trees

Linden trees

This time of the year (spring and early summer) in the Northern Hemisphere, linden trees are in bloom.  The linden tree is a romantic thing, with fragrant furry yellow flowers and heart-shaped bright green leaves. And when you walk along an avenue of linden trees, you understand why people sing about them, because you are suddenly enveloped in a cloud of earthy, sweet fragrance which goes straight to your head. Its leaves and blossoms are great for use in perfume, as a medicinal herb, to make a tea with and to attract bees for a very fragrant honey.

A linden tree in an avenue in our neighbourhood, showing the honeydew on the leaves and the blossoms. (Photo by MF O’Brien, 2019)

The linden is one of the species of the Tilia, a genus of about 30 species of trees or bushes native throughout most of the temperate Northern Hemisphere. The genus is generally called “lime” or “linden” in Britain, and “linden”, “lime”, or “basswood” in North America. The name, linden, is related to the old English “lithe” and German “lind” which means  “lenient, yielding”, but – despite the similarity in name and fragrance – the tree is NOT related to the citrus fruit tree called “lime”.

Tilia trees are subject to the attack of many insects. You can see the effect of these when you walk past the trees in spring, stepping over smears of something sticky on the ground below – and on the cars parked under the trees. In particular, aphids are attracted by the rich supply of sap, and are in turn often “farmed” by ants for the production of the sap that the ants collect, and the result can often be a dripping of excess sap onto the lower branches and leaves, and anything else below. Cars left under the trees can quickly become coated with a film of this dripping syrup (called ”honeydew” – what a nice name considering what it is).

Elderberry shrubs

 

(Above) Sambucus canadensis (probably) showing the inflorescence, or flowering phase, early in the Spring. These are found everywhere in British Columbia.

If you walk outdoors in Western Canada this time of the year, chances are good you will encounter a sudden whiff of honey fragrance coming from ubiquitous shrubs with masses of pale yellow flowers. This is the Sambucus canadensis, also called the American black elderberry, Canada elderberry, or common elderberry. Ripe elderflowers have a unique, sweet fragrance reminiscent of honey spiced with a little clove and ginger. It’s no wonder that bees – and humans – find them irresistible. When the flowering is at its peak, the smell is subtle but pleasant. It is a species of elderberry native to a large area of North America. It is a deciduous shrub growing to 3 m or more tall. In summer, it bears large (20–30 cm diameter) corymbs of white flowers above the foliage.

The elderberry fruit is a dark purple to black berry, produced in drooping clusters in the fall. It is closely related to the European Sambucus nigra – and some authors use the name Sambucus nigra subsp. canadensis. The genus name comes from the Greek word “sambuce”, an ancient wind instrument, in reference to the removal of pith from the twigs of this and other species to make whistles. There are many useful species of Sambucus that grow in this region, including the Blue Elderberry, Red Elderberry and Common Snowberry, and they all look similar.

Fresh bread

 

“Here’s a fun thing to try: stand outside a bakery on an early summer morning, and watch how people react to the smell of baking bread wafting out the door as they walk by. Their heads turn, their noses lift, their eyes close . . . It’s only a matter of time until someone says, “Oh my God—that smells good!” What is it about the aroma of bread in the oven that is so irresistible? Yes, for many people, the odors evoke powerful, pleasant memories of childhood. But even people who grew up on plastic-wrapped, essentially aroma-free Wonder Bread break into contented smiles when they enter a bakery while the ovens are going. The reason has as much to do with chemistry as it does with psychology.

We can get some clues as to where the aromas originate by considering wheat products that don’t smell quite as good. Wheat pasta, for example, has essentially no odor when boiled, and not much even when baked—that heartwarming aroma from a baked lasagna comes mainly from the sauce, cheese, and meat, not the noodles. Most unleavened crackers don’t do much for the nose, either.

But chemically leavened baked goods such as biscuits and muffins (made with baking soda and baking powder rather than yeast) can smell very tempting once they start to brown. The color change is a sure tip-off that the so-called “Maillard reactions” [/maɪˈjɑːr/ my-YAR; French: [majaʁ]] are happening. These reactions, in which sugars combine with amino acids to form tasty golden and umber complexes, throw off lots of volatile aromatic compounds that float through the kitchen air and into your nostrils.” The Maillard reaction gives browned food – anything from grilled steaks to toasted marshmallows – its distinctive flavour and fragrance.

“Recipes for biscuits and muffins almost always call for added sugar of some kind: the lactose in buttermilk, the fructose in fruit, the dextrose in corn, or even crystals of sucrose sprinkled into the mix. Added sugars help kick-start Maillard reactions. Another, even better way to generate pleasant aromatic compounds such as ethyl esters (ethyl acetate, hexanoate, and octanoate) is to leaven the flour with yeast. As a by-product of the microbes’ metabolic processes, the yeast cells produce chemicals that break down during baking into delicious-smelling aromatics. The longer the fermentation, the more pronounced the yeast flavors become since the microbes have more time to produce these compounds.” (Why Does Baking Bread Smell So Good? in Modernist Cuisine, August 22, 2018)

(See? That’s why sourdough bread smells heavenly. Natural yeast!)

Cookies!

Dorothy Winters: “It’s cookies, he smells like cookies, and the smell gets stronger when he’s in heat.” (From the film “Michael”)

In the 1996 film Michael, starring John Travolta, Travolta plays the angel “Michael”, who appears on earth and shows human characteristics. He loves dancing, fighting bulls, romance, pies, and tourist sites. My favourite part of the film is that Michael is irresistible to women because he smells of cookies. Fresh-baked, warm cookies have one of the most enticing and attractive scents in the world; caramelly, sugarally, spicy, festive, good enough to eat. The smell of their sweet ingredients are enhanced by the heat from the baking process. The heat causes the Maillard reaction mentioned above. The release of vapour (mostly water) carries the scent of the cookies into the air. The sugars in the dough are caramelizing and that breakdown by heat creates fragrant chemicals that make that lovely smell, something that home realtors are known to use to positively influence buyers. Truthfully, I have tried to bake cookies while having potential buyers in my house but all that happened was that I ate all the cookies.


Next post: These smells recreated in perfumes


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