Material guide: hydroxycitronellal — barely exists in nature, and lily of the valley depends on it

Ranking materials by how often they appear in real formulas, in the previous article, meant running that ranking back against our own starter list. The first gap that fell out was this one: rank 9, present in 16.9% of the public formulas at a median dose of 8%, and we had never written about it.

Filling it in.

The short version

• It is the backbone of the muguet accord. Lily of the valley cannot be distilled, so every lily of the valley on the market is constructed, and this molecule is the most common brick in that construction.
• The database records it as not found in nature, apart from a trace in Greek clary sage oil. A molecule that imitates a flower, and barely grows on one.
• 218 hours on a blotter. That is a long number. This is not a floral top note; it stays to the end.
• It is one of the 26 fragrance substances the EU requires on cosmetic labels. In a 2016 Thai study of 312 consecutive patients it was the third most common individual allergen at 3.8%.

The basics

CAS	107-75-5
Chemical name	3,7-dimethyl-7-hydroxyoctanal
Formula	C₁₀H₂₀O₂
Molecular weight	172.27
Type	Synthetic single molecule, semisynthesised from citronellal
Odour	Floral, lily, sweet, green, waxy, tropical, melon, citrus
Odour family	floral
Strength	Medium (evaluate at 10% or below)
Longevity	218 hours (blotter, neat)
Appearance	Colourless to pale yellow clear oily liquid
Boiling point	241 °C @ 760 mmHg
Flash point	123.89 °C
Specific gravity	0.917–0.921 @ 25 °C
Refractive index	1.447–1.450 @ 20 °C
Water solubility	Slight; 3,042 mg/L @ 25 °C (est)
Other solubility	Ethanol (50% alcohol is enough), essential oils, fixed oils, benzyl alcohol, glycerin
Suggested product levels	0.5–5% across fine fragrance, soap, shampoo, detergent and candles (database record)
Storage	24 months or longer; cool, dry, sealed, away from heat and light
Regulatory	FEMA 2583, JECFA, CoE, FLAVIS 05.012
Suppliers	53
Corpus rank	9th (161 of 954 formulas, 16.9%, median dose 8%)

In plain terms. The hydroxy is one extra −OH hanging off the molecule, and that single part is what makes it stickier, longer-lasting and more soluble in water-containing products than the citronellal it came from. A flash point of 124 °C is high, so shipping and storage are easier than for citrus materials. 3,042 mg/L in water counts as good solubility for a fragrance material, which is why soap and shampoo formulas reach for it.

A floral molecule that doesn't grow on flowers

Lily of the valley is a mute flower: it smells clearly, and neither distillation nor solvent extraction yields a usable oil from it. Wanting muguet means assembling it from other molecules.

Hydroxycitronellal has been the longest-serving brick in that assembly. It entered the industry around 1905, earlier than most common synthetics. It is made by adding water across the double bond of citronellal — the aldehyde of citronellol, which occurs naturally in citronella and lemon eucalyptus — giving an aldehyde with a hydroxyl at the far end.

That leaves it in an odd position. The feedstock is natural, the product barely exists in nature, and the flower it imitates happens to be the one least willing to hand over an oil.

On its own it reads as a sweet floral with green and waxy edges and melon-citrus behind. Alone it does not particularly say muguet. Put it beside linalool and citronellol and the shape stands up.

What 218 hours means

The database records blotter longevity at 218 hours, over nine days.

Material	Blotter hours
Linalool	12
Dihydromyrcenol	16
Benzyl alcohol	35
Cashmeran	48
Hydroxycitronellal	218
Vanillin	400

Read that number carefully. The field measures neat material on paper, not diluted material on skin, and the two do not convert. What it does tell you is that this material will not confine itself to the opening. You will meet it again in the heart and in the drydown, so an overdose presses floral sweetness across the whole wear.

The screening mixes miss it

Vejanurug and colleagues, publishing in Contact Dermatitis in 2016, patch tested 312 consecutive patients with both a baseline series and the 26 individual fragrance substances the EU requires to be declared.

Eighty-four patients (26.9%) reacted to at least one of the 26 or to a standard screening marker. Of those 84, 15 (17.8%) were negative on the screening markers and would have been missed without testing the individual substances.

The three most common individual allergens were cinnamyl alcohol (11.2%), cinnamal (9%) and hydroxycitronellal (3.8%).

Worth unpacking. The "fragrance mix I / II" preparations that dermatology clinics use are screening reagents that blend several common allergens, so one test catches most cases. This study is about the word "most" — one positive patient in six does not show up on the mixes, because the individual concentration is diluted down, or because the substance they react to is not in the mix at all.

For someone composing, the practical consequence is that a customer saying "I'm not allergic to fragrance" is not reassurance. They may simply never have been tested for the right thing.

The authors note the study was run in Thailand, where cinnamyl alcohol and cinnamal stand out particularly, so the ranking should not be carried straight to another region.

An unexpected second job

The same molecule turned up in a completely different experiment in 2021. Andrade and colleagues, in BioMed Research International, gave Swiss mice intraperitoneal hydroxycitronellal at 12.5, 25 and 50 mg/kg and ran rota rod, elevated plus maze and open field tests.

The rota rod showed no impairment of motor activity, indicating no myorelaxant or sedative effect at those doses. In the elevated plus maze, all three doses significantly raised the proportion of entries into the open arms (34.8%, 33.8%, 38.6%) and the time spent there (49.9%, 56.1%, 57.0%). Combining in vivo and in silico work, the authors attribute the anxiolytic-like effect to GABA-A receptor involvement.

This is an animal study using intraperitoneal injection at doses nowhere near what smelling a perfume delivers, so it does not support claiming that this material relieves anxiety. It is here because it illustrates something else: fragrance molecules frequently hold a second job inside the body, and that job has nothing to do with what they smell like. Eugenol opening TRPV3 is the same kind of case.

When you smell it

• Start at 10%, then walk away. The first ten minutes tell you little. Come back to the blotter the next day to see what it actually is.
• Put it beside citronellal or citronellol. Same carbon skeleton, one hydroxyl apart: one is bright rose-citrus, the other is sweet clinging lily. It is the cheapest possible demonstration of one small part changing everything.
• Build a muguet once. A little of it with linalool and citronellol, plus a touch of benzyl acetate, and the green-white shape appears. Alone it doesn't read; assembled it does.
• Watch the waxiness. Push the level up and the waxy body behind the sweetness starts covering everything else, and the formula goes dull.

Buying and storage

With 53 suppliers this is an easy material to source. Name variants are few — hydroxycitronellal, and the systematic 3,7-dimethyl-7-hydroxyoctanal. What to watch is the acetal derivatives: hydroxycitronellal dimethyl acetal (CAS 141-92-4) and diethyl acetal (CAS 7779-94-4) are different materials with different CAS numbers, different smells and different specifications. Match 107-75-5 before you order.

It is an aldehyde and oxidises slowly in storage. Seal it, keep it dark and cool, decant into small bottles once opened, and date them. The 124 °C flash point means no special fire precautions in storage.

The database lists product usage at 0.5% to 5%. Note that this sits on a different base from the corpus median of 8%: the first is concentration in the finished product, the second is the share inside a fragrance concentrate. A shampoo dosed at 1% fragrance, using a concentrate that is 8% hydroxycitronellal, ends up at 0.08% in the bottle. Both numbers are right; they measure different things.

Regulatory and safety

Three GHS statements all matter here: H315 causes skin irritation, H317 may cause an allergic skin reaction, H319 causes serious eye irritation. Acute toxicity numbers are generous — oral rat LD50 above 5,000 mg/kg, dermal rabbit LD50 above 2,000 mg/kg — but acute toxicity and sensitisation are separate questions.

It is one of the 26 fragrance substances the EU requires to be named on a cosmetic ingredient list, which means using it obliges you to declare it rather than hiding it under "parfum". RIFM published a full safety assessment in 2022 (Food and Chemical Toxicology 163 Suppl 1: 112983).

For actual use limits, check the current IFRA Standards, which set different values per product category and get revised. A flavour listing (FEMA 2583, JECFA) and skin safety are separate questions; neither implies the other.

→ Hydroxycitronellal in the database: full physical data, 53 suppliers and suggested blends. → Search by odour: try "lily floral sweet".

References

P. Vejanurug et al., Fragrance allergy could be missed without patch testing with 26 individual fragrance allergens, Contact Dermatitis, 74(4), 230–235 (2016). PMID 26948414. doi:10.1111/cod.12522

J. C. Andrade et al., Involvement of GABAA Receptors in the Anxiolytic-Like Effect of Hydroxycitronellal, BioMed Research International, 2021, 9929805 (2021). PMID 34222487. doi:10.1155/2021/9929805

A. M. Api et al., RIFM fragrance ingredient safety assessment, hydroxycitronellal, CAS Registry Number 107-75-5, Food and Chemical Toxicology, 163 Suppl 1, 112983 (2022). PMID 35381319. doi:10.1016/j.fct.2022.112983