Ceviche is cooked without heat, because acid unfolds proteins just like fire
Boil an egg and the clear, runny white turns firm and opaque. Pour raw egg white into acetone and much the same thing happens, no stove needed. Latin American cooks exploit the same chemistry in ceviche, soaking raw fish in citrus juice until it firms up. In each case, proteins are being unravelled, a process chemists call denaturation.
A protein starts life as a chain of amino acids strung together by the cell's ribosomes, following instructions copied from a gene. The chain then folds into a precise three-dimensional shape, tucking its water-shy parts inside and leaving water-loving parts on the surface. That shape is what lets a protein work. An enzyme, for instance, depends on a pocket of exactly the right form to grip the molecule it acts on.
The trouble is that the forces holding the fold in place, hydrogen bonds and other weak attractions, are easily disturbed. Heat, strong acids or alkalis, concentrated salts, solvents such as alcohol, radiation and even vigorous shaking can all knock a protein out of shape. That fragility is one reason living things work so hard to keep their internal conditions steady.
Denaturation happens at several levels. Neatly coiled helices and pleated sheets collapse into random tangles; the folded bundle loses its side-chain bonds, including the sulfur bridges between cysteines; and proteins built from several subunits fall apart. What survives is the basic string itself, since the covalent peptide bonds linking the amino acids stay intact. Even so, the protein can no longer do its job, and denatured proteins often clump together or stop dissolving.
Everyday life is full of examples. Cooking firms meat for the same reason it sets eggs, and the skin that forms on curdled milk is a mat of denatured protein. Inside a living cell, widespread unfolding disrupts normal activity and can kill the cell. Some unusual proteins, by contrast, work while floppy and fold only once they latch onto their target.
Source: Denaturation (biochemistry)
