What Gives Summer That Distinct Smell?
As summer approaches us of the southern hemisphere we look forward to the sunny afternoons, the days at the beach and the smell of freshly mown grass - but what is that smell and why is it so relaxing?
When grass is cut, fats and phospholipids are broken down into long chain fatty acids, eg. linoelic acid, these fatty acids are then oxidised and chopped up by enzymes to form an end product that is either a six or twelve carbon chain. The six carbon chain molecule is the one responsible for the smell of grass so we shall focus on it. The molecules have a carbonyl group on the end and a double bond three carbons in from the end, and thus using some high school chemistry we can determine that the name of the organic molecule is cis-3-hexanal. Our noses are fantastic tools for discovering molecules and are particularly good at detecting this one; humans can detect cis-3-hexanal at 0.25 parts per billion. Cis-3-hexanal is very unstable and quickly mutates from a cis to trans - a molecule which is known as trans-2-hexanal. This molecule is known as the ‘leaf aldehyde’ and in part of what makes up commercial odours as ‘green odour’.
Cis-3-hexanal plays a role in many other aspects of life other than the smell of freshly mown grass - is it also important in the smell of strawberries, in the recovery of plants when they are damaged by pests and surprisingly in medicine. Studies have shown that molecules very similar to cis-3-hexanal can have a positive healing effect on the psychological damage caused by stress by activating blood flow in the primary olfactory cortex. So, maybe next time you’re having a rough day, go mow the lawn and spend some time sun bathing on your freshly trimmed grass.