Proteins were recognized as a distinct class of biological molecules in the eighteenth century by Antoine Fourcroy and others.  Members of this class (called the "albuminoids", Eiweisskörper, or matières albuminoides) were recognized by their ability to coagulate or flocculate under various treatments such as heat or acid; well-known examples at the start of the nineteenth century included albumen from egg whites, blood serum albumin, fibrin, and wheat gluten. The similarity between the cooking of egg whites and the curdling of milk was recognized even in ancient times; for example, the name albumen for the egg-white protein was coined by Pliny the Elder from the Latin albus ovi (egg white).
With the advice of Jöns Jakob Berzelius, the Dutch chemist Gerhardus Johannes Mulder carried out elemental analyses of common animal and plant proteins.  To everyones surprise, all proteins had nearly the same empirical formula, roughly CHNO with individual sulfur and phosphorus atoms.  Mulder published his findings in two papers (1837,1838) and hypothesized that there was one basic substance (Grundstoff') of proteins, and that it was synthesized by plants and absorbed from them by animals in digestion.  Berzelius was an early proponent of this theory and proposed the name "protein" for this substance in a letter dated 10 July 1838
Mulder went on to identify the products of protein degradation such as the  amino acid, leucine, for which he found a (nearly correct) molecular weight of 131 Da.