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Science Society explores fragrance and how biochemists and chemists can manipulate our sense of smell

On Friday 9 February, the Science Society welcomed Tim Harrison from Bristol University to give a talk on Fragrance. Read the report of the lecture from our Head of Science, Dr Mike Kearney, below:

Physical changes like subliming carbon dioxide and boiling liquid nitrogen are not chemistry. To understand fragrance you need to know about gases, vapours and mixtures. You need to know about smelly chemistry. We all carry several fragrances due to the various soaps, gels, toothpaste, deodorant, and clothes with fragrance from washing powder we use everyday. Even bleach will have a fragrance, posing some difficult chemical challenges.  

Perfume is an alcoholic mixture with essential oils. Essential as in the word essence. The concentration of essential oils determines what they are called. 4% of oils is called eau de cologne, while perfume is 22%. Ethanol is a good solvent and has hardly any smell of its own. Very few people are allergic to it on their skin. It is volatile so it vaporises quickly. It is also flammable when a vapour.

The alcohol is denatured by adding denatonium benzoate, otherwise known as bitrex. This makes the alcohol unfit and drink.  It is also no longer subject to taxes and it stops toddlers drinking perfumes.

The sense of smell is chemistry as molecules interact with detectors in the olfactory epithelium. But the mechanism for how this works is not fully understood. Is it shapes of molecules or vibrations? Is it quantum tunnelling? Compounds must be volatile to be detected. Solids that don’t release molecules will be not be smelt. Some gases just have no fragrance.

Most of what we perceive as flavour is actually odour. The tongue can sense only a few flavours (sweet, bitter etc) and the rest of flavours are actually odours. Losing your sense of smell removes most of the pleasure from eating.

A perfume is sprayed onto skin. The strength of intermolecular bonding dictates which molecules evaporate first. Small molecules evaporate first. The smell changes as time passes and other molecules evaporate.  

Fragrances exist in families. Ambers, cypress, citrus and floral. Each family has a variety of fragrances described in terms of different flowers and materials.

A selection of fragrances were passed around for the audience to try.  Ambergris (whale vomit) is used in high end perfumes and is very expensive. It’s smell divided the audience with a small percentage finding it unpleasant. The most expensive component of Chanel Number 5 used to be obtained from the anal gland of a civet cat. It is now artificially manufactured. Many in the audience didn’t like it even before they knew where it came from. It is curious to think about how this was discovered.  

A fragrance will contain a mixture of many compounds. Essential oils are extracted from plants by processes such as steam distillation, solvent extraction, expression and enflureage. The processes have historically required a lot of labour for a low yield.  Sometimes the oils are impossible to extract or are very expensive. 5 to 6 tons of roses are needed to obtain one kilogram of essential oil. About 20 tonnes of rose oil is produced annually worldwide. Chemistry can make the same amount of synthetic rose oil in a week and at a much reduced cost.  

Chemical reactions go on in bottles changing the fragrance over time.  Avoiding light can slow or stop reactions.  Keep the perfume cool as well but do not use the food fridge as various foods will absorb any fragrance. Headspace is the air gap above the level of the liquid in the bottle. Oxygen in the air will cause oxidation affecting the fragrance.  Fill up the empty space with glass beads to reduce the air.  Buy perfumes from shops with a good turnover!

Tim finished by identifying a range of molecules and the characteristic smells they produce. An ester that is the smell of banana, another that is the smell of pineapples. He revealed the molecules that were behind the smelt including cloves, citrus and geranol.

When choosing a fragrance, take time to let the fragrance develop, avoid putting any perfume on (or eat spicy foods) beforehand.  Menstruation or pregnancy will change your sensor smell and perfume preference.