Lactic acid: formula, pH and uses

Lactic acid is a small molecule with the formula C₃H₆O₃, obtained through fermentation and used in cosmetics as an alpha-hydroxy acid. Despite its name, it's not exclusively found in milk: the lactic acid in modern formulations almost always comes from plant sugars. Its behavior on the skin doesn't depend on its nature but on three parameters: concentration, pH, and the carrier.

A molecule with two functions, and what that changes

Lactic acid has two chemical groups on three carbon atoms: a carboxyl group, which gives it its acidity, and a hydroxyl group, which gives it its affinity for water. Its molar mass is approximately 90 g/mol, making it an intermediate molecule between the smaller glycolic acid and the larger fruit acids.

This dual nature explains everything else. The acid group triggers exfoliation; the hydroxyl group retains water. It is the only alpha-hydroxy acid that exfoliates and hydrates through two distinct mechanisms, and this is what makes it interesting for skin that cannot tolerate more aggressive acids.

It exists in two mirror forms, L and D. The L-form is produced by the human body; it is also the form used in serious formulations, as it is better recognized by the skin. A mixture of the two, known as racemic, is cheaper and found in entry-level formulations.

The name comes from its identification in the 18th century in sour milk. This has created lasting confusion: many believe that a lactic acid product contains milk, which is false in the vast majority of cases.

Two subjects share the same name

The question "what is lactic acid" covers two unrelated areas, and this is the primary source of confusion.

In exercise physiology, lactic acid refers to what muscles produce when oxygen is scarce. During intense physical exertion, muscle cells break down glucose without oxygen and produce lactate. This molecule is not a waste product: it is a fuel. The liver converts it back into glucose through the Cori cycle, and the muscle itself can reuse it as an energy source.

The idea that lactate causes muscle soreness is also false and has long been corrected: it is eliminated within one to two hours after exercise, whereas muscle soreness appears the next day. Muscle micro-tears are responsible for this.

In cosmetics, the same molecule is produced by fermentation, measured and formulated at a controlled pH to act on the stratum corneum. There is no link to muscle: neither the origin, nor the quantity, nor the effect is the same.

One thing connects them nonetheless: it is the same chemical compound, and the body knows how to metabolize it in both cases. This is one of the reasons for its good skin tolerance—the skin is not dealing with a foreign molecule.

The rest of this page deals with the cosmetic aspect.

Where it actually comes from

Industrial production relies on fermentation. Lactic acid bacteria—primarily Lactobacillus—transform a simple sugar into lactic acid in the absence of oxygen. This is the same mechanism as for yogurt, sauerkraut, or sourdough.

The starting substrate is most often plant-based: corn, beet, or cane glucose. A formula can therefore be labeled vegan while containing lactic acid, which often comes as a surprise when reading a label.

On the INCI, it appears as Lactic Acid. Its neutralized form, sodium lactate, appears as Sodium Lactate—and this is not the same thing, as the following section explains.

The chemical route, by synthesis from petroleum derivatives, still exists but is declining. It produces a racemic mixture and is primarily intended for non-cosmetic industrial uses.

Lactic fermentation in food

The same process is found in a large part of the diet, and this helps to understand what we are talking about.

Yogurt, kefir, sauerkraut, pickles in brine, sourdough, and kimchi are all fermented foods whose bacteria produce lactic acid. It is what gives the tangy taste, and it is also what preserves: by lowering the pH, it makes the environment unfavorable for undesirable microorganisms.

As a food additive, it has the code E270. Its presence on a label does not indicate a defect or a suspicious synthetic addition.

Eating fermented foods, however, has no exfoliating effect on the skin: the molecule is digested and enters general metabolism. There is no dietary route to the cosmetic action described below.

A word about lactic acidosis, which research sometimes brings up: it is a rare medical condition, linked to an accumulation of lactate in the bloodstream, completely unrelated to cosmetic use or everyday diet. It is exclusively a matter for a doctor.

Acid or lactate: the distinction that decides everything

This is the most important point on this page, and the one that labels explain the least.

In solution, lactic acid exists in two states. In its acid form, it still carries its proton, and it is this form that exfoliates. In its lactate form, it has given up its proton and no longer exfoliates at all—it becomes a humectant, which retains water.

The proportion between the two depends on the formula's pH. Its pKa is approximately 3.86: at this precise pH, half of the molecules are in acid form, the other half in lactate form.

A practical consequence follows: a percentage alone says nothing. A 10% lactic acid formulated at pH 4.5 exfoliates less than a 5% one at pH 3.5. Brands that indicate the pH of their formula give you the decisive information; those that don't leave you guessing.

European regulations also govern this use: beyond certain concentrations and below certain pH levels, a cosmetic product falls outside the scope of daily care.

How it exfoliates

The cells of the stratum corneum, corneocytes, are held together by protein structures called corneodesmosomes. Their natural degradation is what allows for invisible daily desquamation.

Lactic acid accelerates this degradation. It does not dissolve the skin or "burn" it: it breaks down an assembly that would break down anyway, just faster. The result is a thinner and more regular stratum corneum, which refines skin texture and improves how light reflects—hence the immediate brightening effect.

This acceleration has a trade-off: the stratum corneum contributes to the skin barrier. Exfoliating too often weakens it, leading to tightness, redness, and increased sensitivity to other products. This is the most common mistake with acids.

How it hydrates

This is the aspect that other alpha-hydroxy acids do not possess to the same degree.

Lactate is part of the natural moisturizing factor, a blend of small molecules that the skin produces itself to retain water in the stratum corneum. Amino acids, urea, PCA, and lactate make up the essential part of it.

Supplying lactate therefore amounts to supplementing a component that the skin already recognizes. At high pH and low concentration, a lactic acid formula primarily acts in this way: it hydrates, softens, and exfoliates only marginally.

Studies have also shown that it stimulates ceramide production in the epidermis, which strengthens the barrier instead of weakening it. This is what distinguishes a well-conducted exfoliation from harsh stripping.

Compared to other acids

Glycolic acid, being smaller, penetrates faster and deeper: it acts more, it irritates more. Salicylic acid, being oil-soluble, goes where the other two don't—inside the pore. See our page on salicylic acid.

Lactic acid occupies the intermediate position, and precisely for this reason, it is the first acid recommended for skin that has never used one before.

How to introduce it

Gradual introduction is more important than the chosen concentration.

• Weeks 1 and 2: once a week in the evening, at 5%, on clean and perfectly dry skin.
• Weeks 3 and 4: twice a week in the evening if no reaction has appeared.
• Afterward: a maximum of three evenings per week, or a higher concentration, never both at the same time.

Three rules govern its use. Always in the evening, because an exfoliating acid increases sensitivity to UV rays for several days. Always followed by a moisturizing product, which closes what exfoliation has opened. And always accompanied by sun protection the next morning, without exception. See our exfoliation module.

What should never be combined on the same evening: another acid, a retinoid, or mechanical exfoliation. Chemical exfoliation replaces mechanical exfoliation; it does not add to it.

A simple signal serves as a safeguard: if the skin feels tight, red, or tingles for more than an hour after application, the frequency or concentration is too high. Successful exfoliation is not felt the next day.

Reading a label

Four elements are enough to evaluate a lactic acid formula.

Its position in the INCI list. Ingredients are listed in descending order up to one percent. Lactic Acid mentioned after preservatives is present in trace amounts.

The stated pH. This is the most useful and rarest information. Between 3.5 and 4, the formula exfoliates without excess. Above 5, it mainly hydrates.

The presence of Sodium Lactate. Its presence indicates partial neutralization, thus a deliberately gentler formula.

What accompanies it. An acid combined with ceramides, glycerin, or panthenol is better tolerated than an acid alone in an aqueous base.

What the definition doesn't say

Knowing what lactic acid is does not allow one to predict whether a product will be gentle. Two formulas with the same percentage can behave very differently depending on their pH, their base, and the associated active ingredients.

It does not erase an established wrinkle. It refines skin texture and improves radiance, which diminishes the appearance of fine lines—a real effect, distinct from action on the dermis.

It is not suitable for everyone. On very reactive or already compromised skin, it is better to restore the barrier before introducing any acid. Our pages on lactic acid and skin pH detail these two points.

Acid or lactate: what the label should say

A product that lists sodium lactate does not exfoliate—the neutralized form hydrates, and that's all. To obtain the two functions described above, free acid at low pH is required, which is what an AHA peeling with lactic acid and hyaluronic acid claims to do, and which lactate alone will never achieve.