Esterase

An ester is what you get when an acid and an alcohol are joined and a molecule of water is released. It is a standard way of making a difficult ingredient behave: attach a fatty tail to vitamin C or vitamin A and the result is steadier in air, friendlier to an oil phase and usually less irritating. The bond is also the designed weak point.

Skin carries enzymes that break exactly that bond. Their activity sits mainly in the living epidermis rather than in the dead surface layer, which puts a condition on the whole strategy: the derivative has to get past the surface before there is anything there to cut it.

The honest part is how little of this a label can promise. Esterase activity differs between people, between body sites and with the state of the skin, and most of what is known about it comes from excised tissue and cell culture rather than from faces. A formula can pick a bond that is easy to cut or one that is hard to cut. It cannot supply the enzyme.

Sometimes slow is the point. An ester chosen so that it is not cut quickly is a way to reduce irritation, or to keep a material on the surface where it was wanted. Same chemistry, opposite intention — worth knowing when two products list what looks like the same vitamin at the same percentage.