By Simon Cotton, University of Birmingham

There are two types of perfume in the world: the fast turnover celebrity perfumes, designed to hit the market and make profits before a star’s capital wanes; and the timeless classics, with their expensive ingredients and loyal followings.

While the names endure, many of the ingredients and quantities have changed over time.

As The Financial Times recently reported, ingredients and the amounts perfumers are allowed to use are increasingly being restricted by the International Fragrance Association, which regulates the industry.

One reason is the classification of some key ingredients, such as oakmoss – found in perfumes including Guerlain’s Mitsouko – is that they are allergenic. Other restrictions are now being applied to jasmine and ylang ylang.

Health, it seems, trumps pleasure. During the past half-century, concerns have grown over allergic reactions to molecules found in perfumes. And organic materials are just as much in the frame as synthetics. Each synthetic ingredient is comprised of just one molecule, but each natural material contain a lot of different molecules, each a potential allergen.

So jasmine has come into the cross-hairs, as has oakmoss, a lichen. It is believed that two molecules found in oakmoss, atranol and chloroatranol, cause at least 20% of fragrance contact allergens.

Many perfumers are concerned that there is no real substitute to supply the inimitable smell of oakmoss, and are looking to the European Commission to set levels below which these key molecules can still be used rather than be removed entirely.

Lucrative trade

Perfumed materials have been around for thousands of years. The ancient civilizations of the Near and Middle East knew all about the fantastic odors when frankincense and sandalwood were burned, and these aromatic properties underpinned a lucrative trade.

Trade brought them to other lands, such as Pharaonic Egypt (think, Cleopatra). Arabs, 1,000 years ago, perfected the art of using steam distillation of plants as a way of obtaining essential oils, with a side benefit of producing delicately scented aqueous solutions such as rose water, popular in Arabia down to the present day.

By the late Middle Ages, perfumes began to resemble the types we would recognize today, based on essential oils in alcoholic solution.

People in the Middle Ages did not necessarily apply perfumes to the body – Henry V carried scented materials like musk and ambergris that were believed to ward off the plague and bad odors.

Henry of Navarre, arguably France’s most popular monarch, reportedly smelled so strongly of perfume that his second wife, Marie de Medicis, fainted when she met him for the first time. After the French Revolution in 1799, the name Eau de Cologne became synonymous with Napoleon Bonaparte, who ordered a fair amount of the stuff.

Planet organic

Until the second part of the 19th century, perfumes were totally based on nature-sourced ingredients, largely obtained by pressing or distilling plant products. As organic chemists started to understand the structure of molecules and how to synthesise them, so new fragrances became possible.

Synthetics gave assured supplies of more familiar molecules while some were new, unknown in nature. Houbigant’s Fougere Royale (1882) made use of synthetic coumarin, originally isolated in tonka beans in 1820, while Guerlain’s Jicky (1889) used synthetic vanillin, made possible by the discovery of what is now known as the Reimer-Tiemann reaction in 1874.

You can still buy Jicky today. It is often thought of as the first modern perfume made using vertical structuring – the three “notes” we often think of comprising a perfume.

Taking notes

The “top note” is the initial impact of a fragrance due to the lightest molecules, which predominates for the first few minutes, leading to the “middle note”, comprising the main fragrance and personality of the perfume, which can last for several hours.

Eventually after all other molecules have escaped, this is replaced by the “bottom note” (base note or drydown), the residual smell, which remains with you until the perfume has totally evaporated.

These molecules – the heavy ones – also help to reduce the volatility of the others, so they are often known as fixatives. In Jicky, the initial impact is provided by top notes of lavender, lemon and bergamot, followed by a similarly cold and rosy middle, supported by a warm base of amber, musk and vanilla (“un ballet olfactif”, as it was once described).

Cheaper synthetics

Almost at the same time that Jicky was launched, a great piece of serendipity occurred. In 1888, a German chemist named Albert Baur nitrated 3-tert-butyltoluene, hoping that his product would be an even better explosive than TNT.

It wasn’t, but it did have an outstanding musk smell and became known as “musk Baur”. Six years later, Baur went further, making musk ketone, reckoned to be the nearest in smell to natural musk. Until then, the source of musky smelling molecules for the “base note” of perfumes was the musk deer.

Though muscone, the primary chemical compound behind the musk smell, can be obtained without killing the animal, in practice the result was one dead deer. Baur’s discovery provided a cheaper synthetic and humane alternative. Nitromusks proved absolutely indispensable to perfumers for nearly a century, until it was found that they could act as photosensitizers, which could lead to skin rashes. No wholly satisfactory replacement has been found.

Just after 1900 another great class of odour chemicals reached the perfumer’s palette. Georges Darzens hit upon the synthesis which bears his name, the Darzens condensation reaction, which proved to be an excellent way of making aldehydes, notably methyl nonyl acetaldehyde, which gives perfumes a fresh scent.

This was used in Houbigant’s Quelques Fleurs (1912) and just after World War I it was among the aldehydes famously used by Ernest Beaux to supply a dazzling brightness to No. 5 for Coco Chanel.

Chemists have created many other affordable odorant molecules in the service of the perfumer, such as ethyl vanillin (Shalimar), gamma-undecalactone (Mitsuoko), methyl dihydrojasmonate (Eau Sauvage; Chanel No 19), ethylmaltol (Angel) and Karanal (Gucci pour Homme).

Aromatic blends

Nearly all perfumes today use a blend of natural-sourced and synthetic molecules - more likely to be available and usually much cheaper too. Even though the cost of the ingredients in a perfume is much less than the 5% of the shop price(packaging and advertising cost far more), perfumers are regularly under pressure from bean-counting accountants to find cheaper ingredients, and that is no easy task.

Chemistry has helped perfumers find new ingredients and cheaper ones, but we also know much more than we did about allergens and what chemicals cause reactions. In more recent history, the nitromusks were among the first to be outlawed. Perfumers now have to find ways of perfecting smell without causing unwelcome reactions.

Next time you catch a whiff of an amazing scent, reflect on the skill of the people – the chemists and perfumers – who bring this sensation to you and how the science of perfume has become even trickier.

Simon Cotton, Senior Lecturer in Chemistry at University of Birmingham, does not work for, consult to, own shares in or receive funding from any company or organization that would benefit from this article, and has no relevant affiliations.

This article was originally published on The Conversation. Read the original article.