Simon Cotton looks at the molecular structure and chemical properties of holy smoke
Incense is in most peoples’ minds associated with the gifts of the Magi; gold, frankincense and myrrh. Gold, like money, does not grow on trees, but incense most certainly does. It is associated with trees of the Boswellia (Burseraceae) genus, found particularly in India, Arabia and the north-eastern Africa. There are around 25 different Boswellia trees – the best known are probably B. papyrifera associated with Ethiopia; B. sacra with Oman and Yemen; and B. serrata with India. B.sacra is the most valuable. The 5th c. Greek chronicler Herodotus said that flying snakes known as drakontes lived under frankincense trees to guard them, but the snakes could be driven away by burning resin of the Liquidambar tree. When cuts are made in the bark of these trees a milky liquid emerges which solidifies to afford frankincense, usually a pale yellow in colour.
In addition to its applications in worship and perfumes, frankincense has long had medicinal uses in both East and West, which are first mentioned in the Ebers papyrus of c. 1500 BC. It has been used in therapies to treat a wide range of complaints, including allergic asthma; inflammatory bowel diseases (e.g. Crohn’s disease); rheumatoid arthritis and osteoarthritis; brain tumours and oedema; scrofula; and syphilis. Apart from it being an anti-inflammatory, individual compounds have been associated with anti-neoplastic activity preventing tumour growth. In the Babylonian Talmud, boswellia resin is stated to have been administered in wine to condemned prisoners to numb the senses before execution, leading to the suggestion that it was present in the drink offered to Jesus at His crucifixion.
It is only recently that the individual molecules present in frankincense have been identified; there are well over 300 of them, with varying amounts of each in the different types of frankincense, and some of them will be discussed in this article.
A number of different molecules contribute to the smell of incense, but the two boswellic acids are not amongst them, as they are too involatile (-boswellic acid has a similar structure). These make up around 30% of the resin of Boswellia serrata.
They are medically important, not least due to their anti-inflammatory properties. Archaeologists are interested in boswellic acids, using them to discover whether resins they unearth are frankincense. They use a technique known as mass spectrometry (aka MS). This relies on vaporising molecules, knocking an electron off to form a positive ion, then sending these ions down a kind of molecular shooting-gallery, during which the molecular ions pass through a magnetic field. This causes the ions to follow different paths, sorting them in order of their masses (lighter ions are deflected most). The bottom line is that MS can be used to find the masses of the molecules present and thereby identify them. The presence of boswellic acids confirms frankincense. This technique has been used to identify frankincense in a mid-2nd century AD Roman period burial in a round barrow at Mersea Island, Essex, showing the import of an exotic resin into Britain from the farthest south-east extremity of Roman influence. Going much further back in time, frankincense was found in an ointment vase in a scent casket from the tomb of 12th dynasty princess Sat-mer-Hout (ca.1897–1844 B.C), the sister of the Egyptian pharaoh Amenemhat I.
Most of the molecules present in frankincense are much smaller than the boswellic acids, so that they are quite volatile, and may contribute to its smell.
Limonene is usually present in significant amounts. Limonene is a compound also found in the peels of a lot of citrus fruits, making up some 60-80% of the essential oil from lemon peel. When pure, it hardly has any smell. It is an insect repellent, which is maybe why nature creates it, and commercially it is used to make all sorts of substances from floor waxes to perfumes.
Octyl acetate is an ester which is the most abundant component of the oil obtained from Boswellia papyrifera. Esters are an important family of organic compounds, produced by many fruits, contributing a major part of their smells – think bananas, pears, pineapples and apples. By themselves, octyl acetate molecules smell like oranges.
Rotundone is a molecule that was very recently detected in Boswellia sacra. It has only been identified quite recently and crops up in Syrah grapes and wines made from them, notably in some parts of Australia. It gives these wines a ‘peppery’ flavour and aroma. It has also been identified in some other places, including herbs like rosemary, basil and thyme.
Incensole and incensole acetate are widely found in Boswellia, the acetate being more abundant, and also more important. Interest grew in incensole acetate in 2008 when it was found that the smoke from burning frankincense relieved depression and anxiety in mice; incensole acetate was identified as the compound responsible for this. Incensole acetate interacts with a receptor known as TRPV3, short for Transient Receptor Potential cation channel, subfamily V, member 3.
TRP receptors are a group of receptors found in cells that respond to sensations such as taste, temperature and pain. Of those sensitive to temperature, the best known type is TRPV1, responding to temperatures above about 43°C; it is also sensitive to capsaicin, found in hot chilli peppers. TRPA1 is sensitive to temperature, and to allyl thiocyanate, in mustard. This is why curries and mustard seem ‘hot’. In contrast, the TRPM8 receptor responds to cool temperatures (below about 25 C) – and also to menthol – which is why menthol feels ‘cool’. The TRPV3 receptor is also sensitive to warmth (ca. 33-39°C), as well as being activated by certain chemicals, including incensole acetate. The TRPV3 receptor is found in the skin, where activating it gives a warm sensation, and is also found in the brain. We may think that this is a reason for us getting a warm glow after being at High Mass. Incensole acetate activates the amygdala and septum, as well as the motor cortex. Incensole acetate has also been linked with neuroprotective and anti-inflammatory effects, so scientists wonder if it may be useful in treating brain injuries.
Incensole acetate isn’t the most recent discovery about incense. In 2016 a group of researchers in France and Italy reported that they had found two very similar molecules that were responsible for the characteristic ‘endnote’ of frankincense, an ‘old church’ smell. They are the closely related (+)-trans- and (+)-cis-2-octylcyclopropyl-1-carboxylic acids.
These two molecules have very intense smells, but are only present in tiny amount in frankincense. The researchers distilled 3 kg of frankincense (under reduced pressure, because so many of the molecules are very large and have high boiling points). The key odorant was present in a fraction that made up only 0.2% of the oil. The researchers carried out further separations on this fraction and obtained a 1 milligram sample that contained the molecules responsible, less than one-in-a-millionth of the original sample. They then did spectroscopic measurements on them, and worked out what their structures were. These molecules had never been made before, so the scientists devised a way of making them ‘from scratch’ to confirm their identities.
The Catechism of the Catholic Church teaches: ‘In human life, signs and symbols occupy an important place. As a being at once body and spirit, man expresses and perceives spiritual realities through physical signs and symbols. As a social being, man needs signs and symbols to communicate with others, through language, gestures, and actions. The same holds true for his relationship with God.’ (CCC 1146)
Dr Simon Cotton is honorary Senior Lecturer in Chemistry at the University of Birmingham and a former churchwarden of St Giles, Norwich and St Jude, Peterborough. He is a member of the Ordinariate of Our Lady of Walsingham.