To denature has two quite different meanings. In food and biochemistry it means to unfold a protein — heat, acid, or agitation breaks the weak bonds holding the protein’s three-dimensional shape, so it loses its biological function. Egg white setting in the pan, milk curdling with lemon, and the tenderising of meat in marinades are all denaturation at work; it is usually irreversible and it is the basis of cooking itself. In the drinks trade, to denature alcohol means to add a poisonous or foul-tasting substance — traditionally methyl alcohol — to ethyl alcohol so it becomes unfit for humans to drink. Denatured alcohol escapes beverage duty and is sold for industrial, cleaning, and laboratory use. The two senses share only the idea of altering nature; one transforms proteins, the other poisons spirits for the taxman.