Material guide: gamma-undecalactone (aldehyde C-14) — it is not an aldehyde and it has no fourteen carbons

Its database entry reads like this:

gamma-undecalactone (aldehyde C-14 (so-called))

That so-called in the brackets is where this article starts.

The short version

• It is peach and apricot, with cream, coconut and a fatty edge. 119 suppliers, easy to buy.
• The common name "aldehyde C-14" is wrong twice: it is a lactone rather than an aldehyde, and its formula is C₁₁H₂₀O₂, eleven carbons rather than fourteen.
• 338 hours on a blotter, with a median dose of only 1.40%, because it makes itself felt.
• Oral rat LD50 is 18,500 mg/kg, unusually generous for this series.

The basics

CAS	104-67-6
Chemical name	Undecano-1,4-lactone
Common name	Aldehyde C-14 (a misnomer, see below)
Formula	C₁₁H₂₀O₂
Molecular weight	184.28
Type	Naturally occurring, also synthesised
Odour	Fruity, peach, creamy, fatty, lactonic, apricot, ketonic, coconut
Odour family	fruity
Strength	Medium (evaluate at 10% or below)
Longevity	338 hours (blotter, neat)
Appearance	Colourless to pale yellow clear oily liquid
Boiling point	297 °C @ 760 mmHg; 164–166 °C @ 13 mmHg
Flash point	93.33 °C
Specific gravity	0.941–0.947 @ 25 °C
Refractive index	1.449–1.454 @ 20 °C
logP	3.30 (est)
Natural occurrence	Peach, apricot, plum, passion fruit, apple juice, butter, milk products, trace in osmanthus absolute
Storage	24 months or longer; cool, dry, sealed, away from heat and light
Regulatory	FEMA 3091, JECFA, CoE, FLAVIS
Suppliers	119
Corpus rank	28th (85 of 953 formulas, 8.9%, median dose 1.40%)

The name is wrong twice

From the late nineteenth century onward, the industry used "aldehyde C-n" numbering for a series of aliphatic aldehydes: decanal was aldehyde C-10, lauric aldehyde C-12. The convention made sense, because those were aldehydes and the carbon counts matched.

This one is neither.

It is a five-membered lactone, formed when a carboxylic acid and a hydroxyl on the same molecule close into a ring. Chemically it has nothing to do with aldehydes. And it carries eleven carbons, not fourteen.

Accounts of how it acquired the name differ; the common one is that it was marketed into the existing "aldehyde" commercial series. Whatever the reason, the name is comprehensively wrong in chemical terms, and it survives to this day — which is why the database can only append "so-called".

Material names mislead is about the same problem, and this is its most extreme case: what the name gets wrong is not the spelling, it is the class.

Practically: use CAS 104-67-6 or the systematic name with suppliers. When a formula sheet says "aldehyde C-14", know that it means this, and not some fourteen-carbon aldehyde.

The core of the fruity cluster

In the pairing analysis, it holds the highest-lift pairing in the fruity ester cluster:

Pair	Lift	Formulas
Gamma-undecalactone + ethyl 2-methyl butyrate	7.69	24
Gamma-undecalactone + Verdox	5.26	23
Gamma-undecalactone + hexyl acetate	5.19	25

That 7.69 is the highest lift in the whole corpus. The three esters supply fresh, crisp, just-bitten-into; this lactone supplies the flesh and the ripeness. When a fruit accord will not come together, this is usually the missing half.

It and gamma-decalactone are neighbours, one carbon apart. The decalactone leans peach and coconut; the undecalactone is fuller and more apricot. Smelling the two side by side is worth the trouble.

Ring size decides the activity

Yoshida and colleagues, in Current Issues in Molecular Biology in 2026, measured the antifungal activity of medium-chain fatty acids with eight to eleven carbons, their lactones, and one partially fluorinated fatty acid, against Candida albicans.

The fatty acids showed a clear trend: longer chain, stronger activity. Octanoic and nonanoic acid both gave a minimum inhibitory concentration (MIC) of 225 µg/mL and a minimum fungicidal concentration (MFC) of 450 µg/mL; decanoic and undecanoic acid dropped to 112.5 µg/mL MIC and 225 µg/mL MFC.

Gamma-undecalactone gave an MIC of 112.5 µg/mL and an MFC of 225 µg/mL — the same as undecanoic acid. Closing that carboxyl into a five-membered lactone ring cost it nothing.

The control is the interesting part: delta-undecalactone, with the same eleven carbons but a six-membered ring, showed no antifungal activity, and neither did the partially fluorinated acid. Multiple regression found polarizability significantly related to MIC and MFC (R² = 0.78, p < 0.05). Scanning electron microscopy showed that at MIC the Candida still formed hyphae or biofilms, with dead cells appearing only at or above MFC. Gamma-undecalactone itself showed no haemolytic activity.

This is in vitro microbiology, not a basis for cosmetic preservation and certainly not a health claim. It is here for the structural comparison: gamma and delta differ by one atom in the ring, and the biological activity goes from present to absent. A useful reminder for anyone composing that the Greek letter in front of a name is not decoration.

Its neighbour is being biomanufactured

Carballido and colleagues, in Bioresource Technology in 2026, assessed the industrial feasibility of producing gamma-decalactone by de novo fermentation using the fungus Ashbya gossypii, combining technical, economic and environmental analyses.

To be clear: that paper covers gamma-decalactone (C10), not the gamma-undecalactone (C11) of this article. It is the neighbour. It is here because the process logic carries across, and because it is the latest entry in the biomanufactured materials thread.

Their simulations reach a final purity close to 96%, through downstream processing using adsorption resins and ethanol recovery after extraction. The techno-economic assessment found performance strongly affected by the cost of myo-inositol and a low fermentation titer, though competitiveness may be possible under favourable market and process-improvement scenarios. Life cycle analysis identified mineral salts, steam and resin as the main contributors to environmental impact.

Worth noting what this does to the word "natural". A fermentation route can often be labelled natural in many markets, while conventional extraction carries far higher cost and supply risk. The natural-versus-synthetic line blurs again here.

When you smell it

• Start at 1%. The corpus median dose of 1.40% is not caution; it genuinely asserts itself.
• Put it beside gamma-decalactone. One carbon of difference moves peach toward apricot. It is the cheapest demonstration of chain length changing an odour.
• Watch the cream and coconut. Push the level and the fruit steps back, leaving dairy and coconut fattiness, and the formula turns cloying.
• It lasts. 338 hours means you meet it again in the drydown. Excellent for a fruity base, disappointing as a top note.

Buying and storage

With 119 suppliers it is cheap and easy to find. Three naming points: gamma and delta are different materials; undecalactone and decalactone are one carbon apart; "aldehyde C-14" is a misnomer. CAS 104-67-6 is the safest way to communicate.

The 93 °C flash point removes fire concerns from storage. Shelf life is 24 months or longer, sealed and dark.

Regulatory and safety

Five GHS statements: H315 causes skin irritation, H319 causes serious eye irritation, H335 may cause respiratory irritation, H402 harmful to aquatic life, H412 harmful to aquatic life with long lasting effects. The last two mean waste does not go down the sink.

Acute toxicity is remarkably low: oral rat LD50 18,500 mg/kg, among the most generous figures in this series. Dermal LD50 not determined.

Its flavour listings are comprehensive (FEMA 3091, JECFA, CoE, FLAVIS), consistent with its natural presence in peach, apricot and dairy. For use limits, check current IFRA Standards — here is how.

→ Gamma-undecalactone in the database: full physical data, 119 suppliers and suggested blends. → Search by odour: try "peach apricot creamy".

References

M. Yoshida et al., Antifungal Activity of Four Medium-Chain Fatty Acids and γ-Undecalactone Against Candida albicans, Current Issues in Molecular Biology, 48(2), 150 (2026). PMID 41751414. doi:10.3390/cimb48020150

A. Carballido et al., De novo gamma-decalactone bioproduction from Ashbya gossypii: Process design, techno-economic evaluation and environmental assessment, Bioresource Technology, 458, 135029 (2026). PMID 42219126. doi:10.1016/j.biortech.2026.135029