Compound Interest: What Causes The Smell of Fresh-Cut Grass?

The second in the ‘Aroma Chemistry’ series, this graphic examines the smell of fresh-cut grass. This is oft-mentioned when discussions of favourite smells come up, so what are the chemical compounds behind it?

Grass emits volatile organic compounds normally, even without being cut. Research has shown that the amount of the compounds emitted can vary depending on light intensity and temperature. A wide range of compounds are given off, and both the intensity, and the identity of these compounds, is impacted when the plant is damaged. The emissions increase markedly when grass is cut, and it is the production of compounds containing six carbons, and oxygen, that causes the fresh-cut grass smell.

Firstly, let’s consider how these compounds are formed. 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.

The key aroma compound produced by this process is (Z)-3-hexenal. The odour threshold for this compound (the amount of it that needs to be present in order for the human nose to be able to detect it) is a very low 0.25 parts per billion, which means there doesn’t need to be a lot for its smell to be noticeable. As a compound, it’s quite unstable, and relatively quickly will rearrange to (E)-2-hexenal. This compound is known as leaf aldehyde, and has a higher odour threshold. Along with leaf alcohol ((Z)-3-hexen-1-ol), it is produced industrially on a large scale for use in the perfume and food technology industries.

There are a number of suggestions as to the purpose of the grass emitting these compounds when damaged. They are active against a range of bacteria, so one role they perform may be to protect the plant from bacteria and allow the cut ends to heal. Additionally, in plants, they are also released when insects damage the plant – they may act as a ‘signal’ of sorts to other plants, priming them to ‘switch on’ defensive mechanisms, or release compounds that attract the predators of such pests.

As an interesting aside, this study from 2002 suggests that the compounds emitted by cut grass, as well as lending it a pleasant odour, may also contribute towards smog and air pollution.