Composition and perception of different aromatic molecules

An Aroma is a mixture of compounds that can be detected by the olfactory receptors in the nose. These compounds are usually volatile, meaning they can easily evaporate and become airborne, allowing them to be detected by our nose (read more about this in the previous post #1).  

Aroma compounds are typically composed of a combination of carbon, hydrogen, and oxygen atoms, and they may also contain other elements such as nitrogen and sulphur. 

It goes without saying that everything that possesses a scent or a smell is made up of different aroma compounds. What we experience as an aroma, scent, or smell (for example, the ripe flavour of peaches, the aroma of freshly baked croissants, the scent of a flower, or even the smell of garbage), is actually a composition of different aromatic molecules that make up the scent we perceive. 

Aromatic molecules can be classified into several different types based on their chemical structure and properties, some common types are terpenes, esters, aldehydes, ketones and lactones (we’ll describe those in another post). 

For example, a rose's aroma is produced by a complex mixture of different aromatic compounds. Each of these compounds has its own unique aroma (floral, citrusy, fruity, spicy, etc.), and when they are combined together in the right concentrations, they create the overall aroma of the rose.  

Some of the compounds that contribute to the characteristic aroma of roses are: 

Geraniol is a terpene alcohol that is responsible for the rose-like aroma, and it is also found in other flowers and fruits (such as geranium, lavender, lemon grass, plum, peaches, etc,). 

Citronellol is a terpene alcohol with a strong lemony character, which is also the main aromatic component of lemongrass. 

Phenylethyl alcohol is an alcohol that gives a rose-like aroma and it’s also found in some fruits such as apricot and raspberry, and in a variety of flowers like ylang-ylang, hyacinth, orange blossom, carnation, etc. 

Eugenol is a phenylpropanoid that is present in cloves and has indeed a pleasant, spicy, clove-like scent. 

Rose Oxide, also known as oxide alcohol, is a chemical compound with a strong rose-like aroma. 

Linalool is a terpene alcohol that is known for its floral and lavender-like aroma (it is indeed one of the main aromatic compounds in Lavender), and it is also found in other herbs like mint, basil and coriander. 

Limonene is a terpene that is known for its fresh, citrus-like aroma. It is present in the essential oil of all citrus fruits, and it is also found in several flowers, such as jasmine, chamomile, and of course, roses. 

The type and concentration of aromatic molecules in roses can vary depending on several factors such as the variety of rose, the growing conditions, the method of harvesting, as well as the processing and extracting methods of the aroma. However, this is an average concentration of the aromatic compounds mentioned above: 

Damask Rose composition: 

Geraniol (rose-like aroma): 2-5%

Citronellol (rose-like and lemony aroma): 2-7%

Phenylethyl alcohol (rose-like aroma): 2-5%

Eugenol (clove-like aroma): 1-2%

Linalool (floral and lavender-like aroma): 1-3%

Limonene (fresh, citrus-like aroma): 1-2%

Citral (lemony aroma): 1-2%

Nerol (floral aroma): 1-2%

Rose Oxide (rose-like aroma): 0.1-0.4% 

What is interesting to note here, is the concentration of each aromatic compound, and the effect it has on the final perception of the rose aroma. For example, rose oxide, despite being one of the main compounds responsible for the characteristic aroma of roses, is present in small concentrations, around 0.1-0.4% (and normally never over 1%).

Consider that the full list of aromatic compounds in roses is likely to contain hundreds of different molecules, and the ones listed above are only the main ones responsible for the scent of Damask Rose. Out of all the other compounds, some of them may not have a pleasant smell (if taken in isolation), and some others may possess no scent at all. However, they all contribute to the final overall aroma, and removing even just one of the compounds can greatly change the aroma of the rose, removing complexity and nuances. 

How does this affect us? 

Let’s see what factors affect our ability to detect, identify and recognize aromas. For example, let’s take the molecule hexyl acetate, commonly found in white wine. 

Hexyl acetate has a fruity aroma and can be described as having notes of apples, pears and tropical fruit. 

While drinking a glass of white wine boasting this particular molecule, different people may perceive the flavour of the compound differently: one person may perceive a stronger apple-like aroma, another person a pear-like aroma, and a third person may identify it as pineapple, or even banana. The aromatic molecule is always the same, but our perception of it changes. 

This variation in perception can be caused by several factors. One explanation is the difference in the number and sensitivity of olfactory receptors between individuals. Some people have more receptors for certain odours, making them more intense for them, while others may have fewer receptors, making them less intense. 

Most importantly, personal experience and cultural background play a role in how individuals perceive aromas. Consider the scents that we are familiar with from our upbringing, as well as the variety of cuisines and flavours of different cultures – those can have a significant impact on how a person experiences different aromas, especially new ones. 

When we drink our glass of white wine (or whatever else) our brain quickly compares the detected smell to stored memories, information and representation of different scents. If the representation of the current scent matches a stored representation, the brain can quickly identify it. If the brain cannot identify the scent, it will try to march it with the closest perception it can find based on the person’s cultural background and personal experiences. This explains people commonly mistake simple scents for similar ones.

It may also happen that the brain can identify the scent but cannot assign it a name, this is known as ‘olfactory agnosia’. 

In conclusion, an aroma is a complex mixture of compounds that our olfactory receptors detect. The composition and concentration of these compounds determine the overall aroma, and factors like personal experience, cultural background and genetics, can influence our ability to identify and recognize them. 

We will explore aromatic compounds, our ability to detect and identify them in another post, and how understanding them can help us improve our tasting knowledge, as well as the ability to combine and create flavours in beverages.