Triethyl citrate (TEC) is one of the few ingredients in natural cosmetics whose mechanism of action is described at the level of enzymatic kinetics rather than marketing abstractions. It is not about masking odor with fragrance, nor is it a mechanical blockage of sweat glands like aluminum-based antiperspirants. TEC is a prodrug: it is not active on its own, but upon contact with esterases in the stratum corneum of the skin, it releases citric acid, locally lowering the pH and creating an environment in which odor-causing bacteria cannot metabolize the fatty acids in sweat. Let’s break down this mechanism in full detail — from molecular structure to finished formula solutions.
What is triethyl citrate: chemistry and nomenclature
Triethyl citrate (INCI: Triethyl Citrate) is a triester of citric acid and ethanol. Essentially, it is a citric acid molecule where all three carboxyl groups are "closed" by ethyl ester bonds. CAS number: 77-93-0. Molecular formula: C₁₂H₂₀O₇. Molecular weight: 276.28 g/mol.
In its pure form, TEC is a colorless, oily liquid with no distinct odor, highly soluble in ethanol, diethyl ether, and many cosmetic solvents. Its solubility in water is limited (~6.5 g/100 ml at 25°C), which determines its behavior at the product-skin interface and influences its bioavailability.
| Parameter | Value |
|---|---|
| INCI | Triethyl Citrate |
| CAS | 77-93-0 |
| Molecular weight | 276.28 g/mol |
| Boiling point | 294°C at 760 mmHg |
| Density (20°C) | 1.136 g/cm³ |
| Water solubility | ~6.5 g/100 ml (25°C) |
| Refractive index | 1.439–1.441 |
| Safety status | GRAS (FDA), permitted in the EU without restrictions |
Key concept: TEC is a prodrug. It does not possess antimicrobial activity on its own. The biological effect occurs only after enzymatic hydrolysis by stratum corneum esterases. This fundamentally distinguishes it from classic bactericides like triclosan.
Mechanism of action: from esterases to citric acid
Sweat odor is not the smell of sweat itself. Apocrine glands secrete a sterile fluid without a distinct odor. The characteristic "sweat smell" appears when bacteria of the genera Corynebacterium and Staphylococcus break down long-chain fatty acids from the apocrine secretion. It is the products of this bacterial metabolism — short-chain volatile acids (3-methyl-2-hexenoic acid, 3-hydroxy-3-methylhexanoic acid) and thioalcohols — that create the unpleasant odor.
TEC interferes with this chain not at the bacterial level (it does not kill them), but at the level of the habitat. Here is how it works:
- Application and distribution. TEC in a deodorant is applied to the skin surface of the underarm area. Due to its moderate lipophilicity (logP ≈ 0.33), it distributes throughout the lipid matrix of the stratum corneum.
- Enzymatic hydrolysis. Esterases of the stratum corneum (carboxylesterases, lipases) cleave the ester bonds of TEC. Reaction: Triethyl Citrate + 3 H₂O → Citric Acid + 3 Ethanol. The process occurs gradually, ensuring a prolonged effect for 6–12 hours.
- Local pH reduction. The released citric acid lowers the skin surface pH from a physiological 5.5–6.5 to 4.0–5.0. This is critical: the optimal pH for Corynebacterium growth is 6.0–7.5, and at pH < 5.0, their metabolic activity drops by 70–90%.
- Suppression of odorogenesis. In an acidic environment, bacteria do not die, but they lose the ability to effectively break down apocrine sweat substrates. No metabolism → no volatile products → no odor. Simultaneously, the released ethanol provides a short-term antiseptic effect.
Important to understand: TEC is not a bactericide or an antiperspirant. It does not kill bacteria and does not block perspiration. Its mechanism is the creation of an environment with a pH at which odor-forming metabolism is suppressed. This is a fundamentally gentler and more physiological approach that does not disrupt the skin microbiome as a whole.
Dosages and formulation parameters
The dosage of TEC depends on the product type, the desired duration of the effect, and the presence of synergists in the formula. Below are recommendations based on published data and practical formulation experience.
| Product type | TEC dosage, % | Duration of action | Note |
|---|---|---|---|
| Stick (solid) | 3.0–5.0 | 8–12 hours | High occlusion → slow release. Optimal: 4% |
| Deodorant cream | 2.0–4.0 | 6–10 hours | Emulsion improves skin contact. 3% recommended |
| Roll-on / spray | 1.5–3.0 | 4–8 hours | Liquid forms evaporate faster. Fixing agents needed |
| Powder / dry deodorant | 2.0–3.0 | 6–10 hours | Applied to absorbent (talc, starch). Activation upon sweating |
Base formula: coconut oil 25%, beeswax 15%, shea butter 15%, tapioca starch 12%, TEC 4%, zinc oxide 5%, tocopherol 0.5%, fragrance 1%. Pour at 65–70°C, let set until fully solidified.
W/O emulsion: shea butter 20%, coconut oil 15%, starch 10%, TEC 3%, caprylyl glycol 0.5%, magnesium hydroxide 3%. pH 5.0–5.5. Add TEC at temperatures below 40°C.
Hydroalcoholic base: water 60%, ethanol 20%, propanediol 5%, TEC 2.5%, allantoin 0.3%, polysorbate 20 — 1%. Solubilize TEC via surfactant. Preservative based on pH.
Compatibility and limitations
TEC is a chemically stable ester, but its biological activation depends on conditions. An alkaline environment accelerates chemical (non-enzymatic) hydrolysis, but can simultaneously inactivate acid-dependent preservatives and disrupt emulsion stability. An acidic environment slows down hydrolysis but better preserves the stability of the finished product.
| Ingredient | Compatibility | Comment |
|---|---|---|
| Sodium Bicarbonate (soda) | Limited (< 5%) | pH > 8 accelerates non-enzymatic hydrolysis → premature destruction of TEC in the product, not on the skin |
| Magnesium Hydroxide | Good (3–5%) | Gentler than soda, pH 8–9. Synergy: magnesia neutralizes sweat acids, TEC lowers pH |
| Zinc Oxide | Good | Additional antibacterial effect. Does not affect TEC hydrolysis |
| Phenoxyethanol | Excellent | pH-independent preservative. Ideal partner for TEC-containing formulas |
| Essential oils (tea tree, lavender) | Good | Additional antiseptic action. Do not conflict with the TEC mechanism |
| AHA acids (glycolic, lactic) | Caution | Double acid impact can cause irritation. Reduce dosage of both components |
Critical incompatibility: Sodium Bicarbonate in concentrations > 10% creates a pH > 9.0, at which TEC hydrolyzes in the jar, not on the skin. You will get a product where all the citric acid has already been released — and neutralized by the soda. Deodorant effectiveness → zero. If you use soda — no more than 3–5%, and be sure to check the pH of the final product.
Practical formulations
Below are four tested base formulas with TEC. Each can be adapted for specific tasks by adding synergists (Farnesol, Zinc Ricinoleate) or functional additives (absorbents, fragrances).
Shea butter 22%, candelilla wax 8%, coconut oil (fractionated) 20%, tapioca starch 15%, zinc oxide 5%, TEC 4%, caprylyl glycol 0.5%, vitamin E 0.5%, fragrance 1%. Surface pH: 5.5. Pour at 68°C.
Shea butter 18%, coconut oil 15%, Mg(OH)₂ 4%, starch 10%, TEC 3%, beeswax 5%, allantoin 0.3%, phenoxyethanol 0.8%, fragrance 0.8%. pH 7.5–8.0. Synergy of magnesia and TEC.
Water 65%, propanediol 8%, TEC 2.5%, polysorbate 20 — 1.5%, xanthan gum 0.3%, allantoin 0.2%, Sodium Benzoate 0.3%, Potassium Sorbate 0.2%. pH 4.5–5.0.
Tapioca starch 40%, kaolin 20%, zinc oxide 10%, Sodium Bicarbonate 5%, TEC 3% (on absorbent), arrowroot 10%, lavender essential oil 0.5%. Apply with dry hands or a sponge.
Quality control and stability
TEC is a thermostable ester (boiling point 294°C), but that does not mean quality control can be overlooked. Enzymatic hydrolysis can begin in the product itself if lipases from plant raw materials or microbial contamination enter the formula.
- pH of the finished product — measure at release and after 1, 3, and 6 months. A pH drop > 0.5 units signals premature hydrolysis of TEC in the product.
- Addition temperature — add TEC when the emulsion temperature is below 40°C. At higher temperatures, the risk of non-enzymatic hydrolysis and loss of activity increases.
- Microbiological control — TEC is not a preservative on its own. An adequate preservation system is mandatory, especially for aqueous formulas (see our article on preservation).
- Organoleptics — the appearance of an acidic smell in closed packaging = a sign of hydrolysis. The product lost activity before application to the skin.
- Packaging compatibility — TEC is compatible with PET, PP, and glass. For sticks, a PP tube with a twist mechanism is recommended.
- Accelerated stability tests — 40°C / 75% RH for 3 months (ICH Q1A). Monitor: pH, viscosity, separation, odor, microbiology.
Advice from Oksana: Triethyl citrate is a tool, not a magic bullet. It is magnificent in a properly designed formula and useless in an incorrect one. Before adding TEC, ensure you understand the pH of your system, component compatibility, and storage conditions. Invest in a pH meter once — and it will pay for itself with hundreds of stable batches.
Triethyl citrate is a rare example of an ingredient that is simultaneously simple in chemistry and elegant in mechanism. Citric acid folded into three ester bonds, activated by the enzymes of your own skin — this is biomimetic approach in its purest form. For a cosmetic chemist, understanding this mechanism opens up opportunities to create truly effective natural deodorants — without aluminum, without aggressive chemistry, and with an evidence base.
Read also: Preservation in cosmetics • Azelaic acid
