You’ve smelt it so many times before and it seems so familiar, but what is it? In this Everyday Olfactics we delve into those familiar smells to unearth their sometimes less familiar origins.
You wake up one morning, open your window and the smell of fresh rain hitting the tarmac tells you it’s a bad day to hang the washing out. The next morning you open your window, there’s no rain but the smell of manure tells you it’s probably another bad day for hanging the washing out. What aromatic molecules are to blame for our ever growing pile of dirty laundry? Let’s take a look at the common smells that fill the air around us- the good, the bad and the gross!
SURVIVAL OF THE GRASSIEST
The first sunny day of spring and lawn mowers nationwide are getting liberated from their sheds. Up and down the streets you can hear them humming and you can smell possibly one of the greenest aromas known to man, that very smell is in fact Green Leaf Volatiles. GLVs are released upon tissue damage to most leafy green plants, so that smell we are so fond of is in fact thousands of blades of grass scrambling to survive by releasing stress signals (or releasing several hydrocarbons if you’re feeling less dramatic)…
“When the grass is mechanically damaged, by a lawnmower or otherwise, it triggers enzymes in the grass to start breaking down fats and phospholipids that are present. This leads to the formation of linolenic and linoleic acids, which are oxidised and subsequently broken down by another enzyme. The breakdown splits the molecule into fragments which tend to contain either 12 or 6 carbon atoms. It is these fragments that lead to the ‘cut grass’ smell” 1
Of the Hydrocarbons released is Z-(3)-Hexenal which due to being somewhat unstable quickly rearranges to become (E)-2-Hexenal, or Leaf Aldehyde. It is these molecules most associated with the cut grass smell. It is thought that grass and other plants release these GLVs when they are threatened for several reasons, partly to trigger healing of the cut wound, to protect against bacteria and to signal to other plants, upon which a gene can be awoken and defence mechanisms activated in anticipation for the threat approaching. This is known as plant-plant interaction, while plant-insect interaction is the emission of GLVs to warn insect of an incoming threat. Furthermore a study by Northwestern University indicates different GLVs are released depending on the threat, as in a study they found that parasitic wasps are more attracted to plants emitting herbivore related GLVs than to plants that are emitting GLVs due to mechanical damage2. So next time you cut your grass spare a thought for our green friends, bask in the sophistication of our biological world and consider that the grass is in fact grassing on you.
The summer months mean muck spreading throughout the British countryside, which means the unwelcome aroma of sewage sludge (or the slightly more discreet terminology, biosolids) filling the air. The odour molecules responsible for this unbearable smell are largely ammonia and Sulphur containing compounds. But why are they so unbearable? There’s no denying that these molecules trigger a sense of disgust in us humans, which Charles Darwin hypothesised to be one of the six basic human emotions. It is this evolutionary disgust that reduces our risk of being exposed to deadly diseases found in faeces. Furthermore it seems to be the exact combination of faecal odours that disgusts us; whilst Indole, Methyl Sulfide and Skatole all help shape the faecal odour profile, Indole plays an important role in the aroma of Jasmine Absolute, Methyl sulphide is also present in Cabbage and Skatole contributes to the aroma of Orange Blossom absolute. Hydrogen sulfide however is kept largely at bay by nature and man (other that a few labs here and there), and it is this molecule that is often held most responsible for the overall aroma of manure. For now we can remain hopeful that muck spreading will be faded out 3, giving way to more sophisticated (and less smelly) methods such as directly injecting soil with the necessary biosolids.
For a creative take on the aromas that fill cities around the world, have a look at goodcitylife.org. A project that works to capture information via social media to compile urban smellcapes into ‘smelly maps’. https://goodcitylife.org/smellymaps/project.php