Citric Acid
Citric Acid · INCI Citric Acid
Citric acid is one of the most widely used ingredients in food, pharmacy and cosmetics, and it belongs to the alpha hydroxy acid family. That family membership is where most of the confusion starts. In a cream held near the acidity of skin, it sets the pH and binds trace metals.
An alpha hydroxy acid by classification, used here to set the pH.
What it is
Citric acid is a small organic acid with three acid groups on it, and those three groups are most of why it is useful. The commercial material is not squeezed out of fruit. It is made by microbial fermentation, classically with the mould Aspergillus niger growing on a sugar substrate, a route industrialised somewhere around 1916 to 1919 and largely unchanged in principle since. What arrives is a colourless, odourless crystalline powder that dissolves readily in water.
Three acid groups means three points at which the molecule gives up or takes back a proton, and that is what makes it a buffer. It can bring a formula to a chosen acidity and then hold its ground when something tries to push it back. Put it with sodium citrate, which is its own salt, and the pair forms a citrate buffer, more stubborn than either alone and gentler than the acid on its own. It also sequesters trace metal ions, iron and copper among them, which is a second and quite separate use.
What citric acid does depends almost entirely on pH. In its free acid form at low pH, like other alpha hydroxy acids, it loosens the bonds between surface skin cells and speeds their shedding. As the pH climbs towards and past its own dissociation points, more of it exists as citrate salt, and that action falls away. The exfoliating benefit and the stinging share the same lever. Our formulas sit at pH 5 to 5.5, close to the pH of skin, and at that acidity citric acid works as a pH ingredient and a chelator. The research that supports the exfoliating and structural claims was produced at 20%, or at a pH the papers never recorded.
Two things keep each bottle sound. The first is the preservative, a ready-made blend of sodium benzoate and gluconolactone. Sodium benzoate does the preserving, and gluconolactone helps it work. The second is the pH. Sodium citrate buffers the cream at pH 5 to 5.5, close to the skin's own. At the end of making, the pH is checked and fine-tuned with a little sodium hydroxide or citric acid.
What it does in our formulas
Citric acid sets the pH of the formula and binds trace metals in it.
It is the acid half of a citrate buffer
Citric acid carries three acid groups and gives up or takes back a proton at three different points along the pH scale. That lets it bring a formula to a chosen acidity and then resist being pushed off it, which is what makes it a buffer. Paired with its own salt, sodium citrate, it makes a citrate buffer, which holds a product at pH 5 to 5.5 far more stubbornly than either half would alone, and takes the bite off the acid while it does. The independent expert panel that reviews cosmetic ingredients lists pH adjuster as one of its two primary reported cosmetic functions.
Well characterisedIt binds trace metals
Iron and copper get into any formula, from the water and from the raw materials, and metal ions speed the oxidation of oils. Citric acid sequesters them, and chelation is the second function that same panel lists for it. That helps protect the oxidation-prone materials in a formula, and it supports organic-acid preservation, which only works in acid conditions.
Well characterisedSkin's own surface is mildly acidic, and the research suggests that acidity supports the enzymes that build the barrier's lipids, restrains the enzymes that break the barrier down, and tips the surface balance towards the microbes that prefer acid. Citric acid helps hold the cream near the pH of skin, so the cream sits within the acidity those processes favour. The direct human evidence is thinner than the mechanism. The study closest to our pH used cotton textiles treated with citric acid, worn against the skin for two weeks, and it lowered surface pH and improved barrier measures. That was a treated fabric rather than a cream.
What the research does not show
Citric acid is an alpha hydroxy acid, and that single fact carries an enormous amount of marketing behind it. Much of the science underneath is real. It was produced under conditions that have little to do with a leave-on product at the acidity of skin: a different concentration, a different pH, or a different material altogether.
Marketed as A collagen-building acid
Marketed as Inhibits tyrosinase
Marketed as An antimicrobial active
Change the pH and raise the concentration and citric acid does become an exfoliating acid, with the sting and the caution about sunlight that come with it.
On the label and in the range
Citric acid does not appear on a Biotaderm ingredients list, because it is not added as a formula ingredient. It is used during manufacture, in very small quantities, to set the pH.
Its salt, sodium citrate, is a formula ingredient, and it sits high in every list at a published percentage. Reaching an acidity takes almost nothing. Holding it steady across a shelf life takes a real amount, and most of that comes from the salt.
Is it suitable for me?
The independent expert panel that reviews cosmetic ingredients assessed citric acid and its salts and found them safe as used, and repeated-insult patch testing at 4% produced no irritation and no sensitisation. True contact allergy to citric acid is uncommon. It suits a vegan routine, and UK and EU cosmetic law places no concentration restriction on it. You will sometimes see 4% quoted as its legal ceiling. That figure is the highest leave-on level the panel found in use when it surveyed the market, and no law sets it as a cap. The same panel noted that citric acid is chemically and biologically distinct from the alpha hydroxy acids that do carry recommended limits. Tolerance depends on concentration and pH together. Severe facial burns have been reported from a 10% peel, and one comparative study found citric acid more irritating than lactic acid on sensitive skin. At buffer levels in a product held at pH 5 to 5.5 that risk is low.
As with anything new, if your skin reacts easily it is worth patch testing first.
The research behind this page
We review the published literature. We do not conduct the studies below.
- Fiume MM, Heldreth BA, Bergfeld WF, et al. Safety Assessment of Citric Acid, Inorganic Citrate Salts, and Alkyl Citrate Esters as Used in Cosmetics. Int J Toxicol 2014;33(2 Suppl):16S-46S. →
- Tang S-C, Yang J-H. Dual Effects of Alpha-Hydroxy Acids on the Skin. Molecules 2018;23(4):863. →
- Schmid-Wendtner MH, Korting HC. The pH of the skin surface and its impact on the barrier function. Skin Pharmacol Physiol 2006;19(6):296-302. →
- Jaeger T, Rothmaier M, Zander H, Ring J, Gutermuth J, Anliker MD. Acid-coated Textiles (pH 5.5-6.5), a New Therapeutic Strategy for Atopic Eczema? Acta Derm Venereol 2015;95(6):659-663. (background; not evidence for this product) →
- Bernstein EF, Underhill CB, Lakkakorpi J, Ditre CM, Uitto J, Yu RJ, Scott EV. Citric acid increases viable epidermal thickness and glycosaminoglycan content of sun-damaged skin. Dermatol Surg 1997;23(8):689-694. →
- Zhou S, Sakamoto K. Citric acid promoted melanin synthesis in B16F10 mouse melanoma cells, but inhibited it in human epidermal melanocytes and HMV-II melanoma cells via the GSK3-beta/beta-catenin signalling pathway. PLOS ONE 2020;15(12):e0243565. →