Citrus, rose and lavender – nature-identical fragrances from the chemical laboratory
Small difference, big effect: How BASF’s raw material citral becomes an enchanting bouquet of fragrances
In today’s world dominated by visual and acoustic sensations, the sense of smell is often regarded as one of the “lower” senses. And yet it plays a much greater role in our lives than is generally assumed: without it, we could neither enjoy the aromas of an appetizing meal or wine or recognize tainted food. The saying “You really get up my nose” also indicates how the sense of smell exerts a subliminal influence in all our lives: it can make the difference between sympathy and antipathy and cause long-forgotten memories to resurface.
While people can basically recognize only four different types of taste – sweet, sour, salty and bitter – the human olfactory mucosa actually possesses about 350 different types of olfactory receptors. The “lock and key” principle, however, dictates that only very specific aroma molecules can dock onto these receptors and stimulate an aroma sensation in the brain. Complex aromas simultaneously activate a large number of receptors, greatly multiplying the number of perceivable scents.
A well-trained nose can therefore distinguish between more than 10,000 nuances of fragrance.
People surround themselves with pleasant fragrances to flatter their sense of smell. BASF is one of the world market leaders in the field of synthetic fragrances. The core product is citral, an aroma chemical which has been manufactured since 2004 in Ludwigshafen in a new plant with an annual capacity of 40,000 metric tons.
The molecule consisting of ten carbon atoms (C atoms) is also responsible for the aroma of citrus in nature, for example in citrus fruits and lemongrass. Synthetic citral from BASF, however, not only provides the fragrance of citrus. By making minor modifications to the molecular structure, other fragrances important for the aroma industry can be created, such as linalool which smells of lavender and geraniol responsible for the typical smell of roses. “These flowery fragrances are very similar in their molecular structure. All three have an identical backbone consisting of ten carbon atoms and, as a special feature, one oxygen atom,” explains Dr. Klaus Ebel, Research Manager at BASF. “The critical difference is the exact position and type of binding of the oxygen. With citral and geraniol, it is located at the end of the carbon chain. In citral, however, it is attached by a double bond and in geraniol by a single bond. With linalool, on the other hand, the oxygen atom is simply bound to another carbon atom located further inside the chain”.
A small difference with a big effect: just like changing even a single indentation on a key, the three molecules then dock onto different olfactory receptors of the nose where they induce the corresponding aromatic sensations. Since chemically speaking these aroma chemicals are exactly equivalent to their natural counterparts, even the most sensitive nose will be unable to detect the difference. BASF’s nature-identical aromas, moreover, offer two decisive advantages: firstly their consistently high purity which is almost impossible to achieve with a natural extract. Secondly, synthesis is often the only way of producing the required amounts of fragrance at an acceptable price. For example, to obtain from lemongrass the 40,000 metric tons of citral produced annually in Ludwigshafen, it would be necessary to cultivate an area of about 40,000 hectares – roughly the size of the Mediterranean island Majorca.
The technical resources required for the synthesis of citral and its related compounds are nevertheless quite considerable. “In the citral plant in Ludwigshafen inaugurated in 2004, reaction temperatures of up to 300 centigrade and extreme pressures of up to 300 bar are maintained in some parts of the system. To induce the necessary chemical reactions and minimize the formation of undesired by-products, we had to develop our own special catalysts with silver and other precious metals as active components,” says Dr. Martin Schmidt-Radde, responsible for aroma chemicals production at BASF.
Even the “marriage” of the starting materials for citral production can only be achieved under high pressure: firstly, the relatively simple molecules isobutene (consisting of four carbon atoms) and formaldehyde (one C atom) have to be joined together to produce the intermediates prenol and prenal (each with five C atoms). One prenol and one prenal molecule are then finally “married” to produce citral with its ten C atoms from which its fragrant cousins geraniol and linalool are created in further synthesis steps. About one third of the citral produced in Ludwigshafen, however, is used to create substances that at first appear to have little to do with sophisticated perfumes: vitamins, or more precisely vitamins A and E as well as carotenoids that are used as vitamin precursors and antioxidants by the metabolism. These two vitamins have a backbone of 20 and 29 carbon atoms, and carotenoids of even 40 carbon atoms respectively. As a result, they are of low volatility and not perceptible to the sense of smell.
Particularly high requirements are placed on the citral based fragrance components, because even the tiniest aroma-relevant impurity can impair or even destroy the desired, typical aromatic impression. Special distillation processes developed at BASF therefore make up the final technical production step. Consistent quality is thereby assured. Now only one hurdle remains before delivery to the customers, namely Alfred ten Haaf and his experts from the sensory evaluation team of the Competence Center Analytics at BASF: “The end product can be as perfect as can be in terms of analytical composition, but if it doesn’t have the right aroma, that’s the end of the line as far as release is concerned. The customers for BASF’s aroma chemicals expect absolutely consistent aromatic quality. Only on this basis can the fragrant composition of a perfume – consisting of a large number of fragrance components – guarantee the expected perfume experience. Otherwise, a fragrant disappointment is inevitable,” explains the expert. Together with his team, the trained perfume specialist and technologist for cosmetics and detergents puts his nose into every batch of fragrance ingredients before it is sent off to the customer. At the end of the day, no gas chromatograph in the world can approach the sophistication of the human nose.
To acquire a pleasant odor, creams, shampoos and detergents need only very tiny amounts of fine aroma compounds such as citral. On the global scale, however, this requirement adds up to huge amounts: since 2004, BASF’s existing plant in Ludwigshafen has been producing around 40,000 metric tons of citral per year – four times the former capacity. This makes BASF the world’s largest producer of citral and its chemical congeners. The main customers are cosmetic and detergent manufacturers (fragrances) and manufacturers of food supplements and animal feedstuffs (vitamins).
Concurrently with the citral family, BASF is also developing other fragrances: one current example is rose oxide which is part of rose fragrance and is also used as an “aroma intensifier” for other perfumes. Since pleasantly smelling products are now taken for granted in today’s world, fragrances are a slowly but steadily growing market unaffected by economic cycles – because people will always be needing to bathe and wash.
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The wide world of fragrances
For centuries, perfumers had to rely exclusively on natural fragrance components, some of which were extremely rare. The age of synthetic fragrances only began with the manufacture of vanillin (1874) and musk (1888). In the 1930s, BASF entered the aroma production business with phenylethyl alcohol, a nature-identical component of rose oil with the fragrance of rosewood and rose leaves. Since the 1960s, citral has formed the basis for the production of vitamins and aroma chemicals in Ludwigshafen.
Of the approximately 3,000 synthetically producible fragrances known worldwide, BASF has about 100 in its portfolio – those that are economically viable to produce. Because in many cases a fragrance is only needed in such small quantities that its synthesis – although possible – is not worthwhile and it makes more sense to use the natural substance instead.
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