Chemicals & Petrochemicals

The Fragrance of Frankincense

Pairing advanced science with primal senses

Christmas is a time for music, family and traditions. If those traditions include spending time in a church or selecting gifts at a fragrance counter, you’ll likely be in the presence of frankincense. It’s burned in religious ceremonies and is added to perfumes. It’s a spice of such historic importance that it is said the three Wise Men presented it to baby Jesus in celebration of his birth. Its history? Epic, mysterious and exotic. The secret to its powerful odor is a little better understood now that researchers at the Institut de Chimie de Nice in France, a country synonymous with perfume, have identified two components that give frankincense its signature scent.

Perfumes are constructed of notes — top, middle and base. The subtle components of scent can be difficult for most people to put into words, but odor experts use the descriptors citrus, woody, spicy, floral, weedy and, for frankincense, the word earthy. Even in this almost ethereal realm of aromas, there is a place for science.

Frankincense, known scientifically as olibanum, comes from an essential oil in the sap of Boswellia trees. Relying on aroma extract dilution analysis and gas chromatography-olfactometry, along with the expert noses of skilled panelists, researchers identified 26 “odor zones” in the essential oil. The team homed in on one of those odor zones because of its powerful retention index and its scent, the one often noticed in old churches.

To study the key odorant, researchers distilled the essential oil under reduced pressure. After liquid–liquid extraction and flash chromatography, the researchers located the odorant in the acidic part of the distillation residue alongside previously identified acids of frankincense. A comparison of GC-MS and GC-O profiles showed two unknown compounds in the odor zone. After additional separations, researchers called in four experts in detecting smells to sniff out the compounds using smelling strips doused in the liquid. Using NMR and other methods, researchers identified the components as (1S,2S)- (+)-trans- and (1S,2R) -(+)-cis-2-octylcyclopropyl-1-carboxylic acids.

In all, it took 3 kilos of essential oil to collect 1 mg of these key (+)-cis- and (+)-trans-olibanic acids. Because the power odorants exist only in trace amounts, they had long-eluded scientific identification. Three hundred molecules combine to create frankincense, but (+)-cis- and (+)-trans-olibanic acids are the most critical in evoking the telltale church scent of frankincense. Although they are present in only minuscule amounts, these olibanic acids punch above their weight to power an aroma that can fill a cathedral and define a perfume.

If you find yourself in a church or fragrance store this season, know that the frankincense scent you’re noticing is thousands of years old. It was carried by the Magi to the Christ child, and brought by the Queen of Sheba to King Solomon. It originated in a tree on the shore of the Red Sea, and, not long ago, in a lab on the coast of France, part of its power was identified and even replicated scientifically. Still, the mystery of much of its structure, like the scent itself, lingers.

Celine Cerutti-Delasalle, Mohamed Mehiri, Cecilia Cagliero, Patrizia Rubiolo, Carlo Bicchi, Uwe J. Meierhenrich, and Nicolas Baldovini

The (+)-cis- and (+)-trans-Olibanic Acids: Key Odorants of Frankincense, Angew. Chem. Int. Ed. 2016, 55, 13719 –13723 DOI: 10.1002/anie.201605242

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