Benzoin is a balsam, which in the chemist’s sense means a resin whose character comes from benzoic and cinnamic acids and their esters. That single fact explains most of what benzoin does: why it is sweet, why it fixes other smells, why it dissolves in alcohol, why it preserves, and why some people react to it.
The big picture
Roughly speaking, benzoin resin is:
- 60–80 % esters of benzoic and cinnamic acid with coniferyl alcohol and p-coumaryl alcohol. These are large, heavy, nearly odourless molecules. They are the body of the resin and the reason it is a solid that softens with warmth rather than an oil.
- 10–25 % free acids: benzoic acid in Siam, cinnamic acid (with some benzoic) in Sumatra.
- A few percent of small aromatic molecules: vanillin, benzyl benzoate, benzyl cinnamate, cinnamyl cinnamate, benzaldehyde, styrene in traces.
- Traces of triterpenoids (siaresinolic and sumaresinolic acids, named for the two origins), waxes and moisture.
The molecules that matter to the nose
Vanillin. One to three percent by weight, and the reason every description of benzoin says “vanilla”. It is the same molecule that gives vanilla pods their smell; benzoin is one of the few resins that contains it naturally in meaningful amounts.
Benzoic acid. Crystalline, faintly sweet and balsamic, with a slightly sharp, “clean” edge. It sublimes when the resin is heated, which is why the smoke of benzoin has a lightly acrid, medicinal top. In pharmacy it is a preservative and mild antifungal.
Cinnamic acid. The acid of cinnamon and storax. Warmer, honeyed, spicier than benzoic acid. Its presence in Sumatra benzoin is the single biggest reason it smells warmer and rougher than Siam.
Benzyl benzoate. Faint, sweet, balsamic, very persistent. A first-class fixative and a solvent for other resins. It is also a labelled cosmetic allergen in the EU and one of the main reasons benzoin is restricted on skin.
Cinnamyl cinnamate (“styracin”) and benzyl cinnamate. Heavy, sweet-balsamic esters, more abundant in Sumatra grades. They add the storax-like warmth and long drydown.
Benzaldehyde. A trace component, but it explains the faint bitter-almond note some people find in Siam benzoin.
Why it fixes
A fixative slows the evaporation of lighter materials. Benzoin does this in two ways. Physically, the heavy esters form a soft film on skin or paper that traps volatile molecules. Chemically, the acids and phenolic groups interact with many top-note molecules and reduce their vapour pressure. The result is that a citrus or floral note in a benzoin base lasts noticeably longer, and its sharp edges are rounded.
Why it dissolves in alcohol and not in water
The esters are large and non-polar. They will not dissolve in water but dissolve freely in ethanol and in solvents such as benzyl benzoate. Pour a benzoin tincture into water and the esters come out of solution as a fine white cloud. That milky liquid was sold for centuries as a cosmetic, lac virginale.
Why it preserves
Benzoic acid inhibits yeasts and moulds at low concentrations, particularly in acidic conditions. Sodium benzoate, its salt, is still one of the most common food preservatives. Historically benzoin was added to fats, ointments and even snuff to slow spoilage.
Why it can sensitize
Benzyl benzoate, benzyl cinnamate, cinnamic acid and benzoic acid are all recognized skin sensitizers or irritants in susceptible people. Compound benzoin tincture is a classic cause of allergic contact dermatitis. This does not make benzoin dangerous in incense or in properly formulated perfume, but it is why IFRA sets limits. See Benzoin safety and allergies.
Siam and Sumatra in one line
Siam benzoin is a benzoate resin; Sumatra benzoin is a cinnamate resin. Everything else, colour, scent, price and use, follows from that. See Siam vs Sumatra benzoin.
Frequently asked questions
Does benzoin contain vanillin?
Yes, typically around one to three percent. It is why benzoin smells vanilla-like even though it is unrelated to the vanilla orchid. The rest of the sweetness comes from benzoic and cinnamic esters.
What is the main component of benzoin resin?
In Siam benzoin, coniferyl benzoate, an ester of benzoic acid, makes up most of the resin. In Sumatra benzoin, coniferyl and cinnamyl cinnamates dominate. Free benzoic and cinnamic acids, benzyl benzoate and vanillin are the main odorous minor components.
Is benzoin resin soluble in water?
No. Like most resins it is practically insoluble in water and dissolves in ethanol, which is why it is prepared as a tincture. Adding a tincture to water gives a milky suspension, the historical 'virgin's milk'.
Sources & further reading
- benzoin
- chemistry
- benzoic acid
- vanillin
