When egg whites are whipped they stiffen into a meringue. the change that occurs in the protein is called?

It is called Denaturation. Denaturation is a procedure in which proteins or nucleic acids lose the quaternary structure, tertiary structure and auxiliary structure which is available in their local state, by use of some outer anxiety or compound, for example, a solid corrosive or base, a concentrated inorganic salt, a natural dissolvable, radiation or warmth.