Material guide: citral — the actual lemon molecule, and IFRA's model student

If you want a knife-cut lemon in a formula, this is the material to reach for. Lemon oil will not give you that edge.

What matters

• Citral is the molecule that actually means "lemon": sharp, fresh, peel-and-acid. When you want brightness, this is it.
• It is a collective name for two molecules: geranial + neral, cis/trans twins of one structure. The lemony edge that grows in ageing geraniol? These two, exactly.
• Brightness has a price. Citral is a regular on fragrance-allergen lists, and the IFRA quantitative risk assessment paper uses it as the worked example. Check the limits for anything on skin; smell-only use is unaffected.

The record

CAS	5392-40-5
Formula	C₁₀H₁₆O
MW	152.24
Odour	sharp, lemon, sweet, fresh, juicy, lemon peel, tart, green
Strength	medium
Substantivity	12 h (neat, on a strip)
Specific gravity	0.885–0.891 (25 °C)
Shelf life	24 months or longer (cool, dark, sealed)
Registers	GRAS, JECFA, CoE, FEMA GRAS, IFRA

Plain reading: CAS is the material's ID number. 12 hours of substantivity puts it in linalool's class: a top-note temperament, quick to arrive and quick to leave. "Sharp" leads the descriptor list, and once you smell it neat you see why.

What it is

Lemon oil is mostly limonene, a fainter, more orange-peel molecule. The thing that makes you shout "lemon!" is the much smaller citral fraction. So when perfumers want more lemon, they usually add citral, not more lemon oil.

And citral itself is a collective name: geranial and neral, cis/trans isomers of one structure (plainly: the same skeleton with one section pointing the other way). The geraniol guide told the first half of this story; the aldehydes that geraniol grows as it oxidises are exactly these twins. The two stories join here.

On the natural side, the classic citral-rich oils are lemongrass (88 suppliers) and litsea cubeba (71). When a brief calls for "natural citral character", those are the usual catalogue answers.

The price of brightness: measured sensitisation

In 2020 Hagvall and colleagues (the same first author as the geraniol oxidation work) published a multicentre patch test study in Contact Dermatitis: 1,476 dermatitis patients with suspected allergic contact dermatitis were tested with citral and its constituents.

Tested	Positive rate
Citral	2.9%
Geranial	3.4%
Neral	1.9%
Citral or geranial (combined)	4.2%

Name the population (the storage article's standing rule): these are dermatology patients with suspected contact dermatitis, not the general public — the rates do not generalise directly. The direction is clear nonetheless: citral is a common real-world fragrance allergen.

The same study carries the finding that ties two guides together:

Among patients positive to citral or its components, 61% also reacted to oxidised geraniol, while testing with unoxidised geraniol was of little value.

The geraniol guide said an old bottle grows citral; here is the clinical version. More than six in ten of those allergic to citral (or its components) also reacted to oxidised geraniol. For them, your oxidised bottle is very likely the same problem. Two bottles, one set of sensitising molecules.

IFRA's model student

The IFRA article cited Api and Vey's 2008 paper: RIFM and IFRA adopted dermal sensitisation Quantitative Risk Assessment (QRA) as the core strategy for preventing sensitisation, setting limits across 11 product categories. The step-by-step worked example in that paper, the one deriving an acceptable exposure level, is citral.

So citral stands in the rulebook as the textbook case of concentration limits by product category: different ceilings for face products, body lotion and candles. What IFRA caps is the concentration in the finished product, and the material itself stays in the catalogue.

Practice stays the usual three-way split: smell-only (strips, diffusers), use freely; on skin, check the current IFRA Standards for your product category; selling, follow the rules where you sell.

Smelling it

• Start at 10%. Medium strength on paper, sharp in temperament; neat, it stabs.
• It is the brightness knob. One to two percent visibly lifts a citrus accord. If you stack it with bergamot, remember their furocoumarin and sensitiser ledgers are separate accounts.
• A top note's fate: 12 hours, bright and gone. Bridging into the heart is the evaporation curve article's homework.
• It oxidises too, like every aldehyde (the storage article's nitrogen list is full of its relatives). Decant, keep it dark, date the opening. My first bottle went stale before I got through a quarter of it; that lesson is why I buy small now.

The catalogue family

Item	What it is
citral (135 suppliers)	the material itself (geranial + neral)
lemongrass oil (88)	the classic citral-rich natural
litsea cubeba fruit oil (71)	the other citral-rich oil — a cleaner lemon-candy tone
citral dimethyl / diethyl acetal (27 / 25)	acetal forms — stable in soap and alkaline bases (the same trick as vanillin's acetal)

Buying it

1. 135 suppliers, and it is cheap. The real homework is freshness: with aldehydes, buy small bottles and recent batches.
2. For skin formulae, check the limit before you design (the IFRA article: a limit is a design constraint, something you look up before sketching).
3. Manage it together with geraniol. One allergen family across two bottles, so keep retains and opening dates as one system.

→ Citral in the database: full properties, supplier list, suggested pairings. → Lemongrass | Litsea cubeba | Search by odour: try "lemon sharp" for its neighbours.

References

L. Hagvall et al., Contact allergy to citral and its constituents geranial and neral, coupled with reactions to the prehapten and prohapten geraniol, Contact Dermatitis, 82(1), 31–38 (2020). PMID 31566752

A. M. Api & M. Vey, Implementation of the dermal sensitization Quantitative Risk Assessment (QRA) for fragrance ingredients, Regulatory Toxicology and Pharmacology, 52(1), 53–61 (2008). PMID 18635300