What is the name of the acid derived from milk used in skincare exfoliants?

The acid derived from milk and used in skincare exfoliants is called lactic acid.

Lactic acid is an alpha-hydroxy acid, or AHA, that occurs naturally in fermented milk and other biological systems. In skincare, it helps loosen the bonds between dead cells at the surface of the skin, making it a chemical exfoliant. Products containing lactic acid are commonly promoted for improving the appearance of dullness, uneven texture, and some discoloration. Because it is generally considered milder than glycolic acid at comparable use levels, it is often included in products aimed at people who want an AHA but may be sensitive to stronger exfoliation.

The connection between milk and skincare has a long history. Ancient Egyptian beauty practices are described as using sour milk, whose lactic acid content could contribute to exfoliation. Modern cosmetic formulations use purified or manufactured lactic acid, and the ingredient may be suitable for both leave-on products and professional chemical peels.

Lactic acid is not the same as salicylic acid, a beta-hydroxy acid commonly used for oilier or acne-prone skin. Glycolic acid is another AHA, while citric acid is also an AHA but is more commonly associated with citrus fruits.