Which of the following describes an irreversible change to proteins?

Coagulation refers to the process where proteins change their structure and form a solid or semi-solid mass. This transformation is often triggered by heat, acid, or certain enzymes, leading to an irreversible change in the protein's structure. Once proteins coagulate, they cannot return to their original state, which is a defining characteristic of this process.

In cooking, common examples include the coagulation of egg whites when heated or the thickening of milk into curds during cheese making. This change is vital in many culinary applications as it contributes to the texture and stability of various dishes.

The other processes mentioned do not result in irreversible changes to proteins. Emulsification involves mixing two immiscible liquids, blending incorporates ingredients without altering their fundamental structure, and fermenting typically involves biochemical changes mediated by microorganisms that might not lead to a permanent alteration in protein structure.