[1] Following a suggestion by Jöns Jacob Berzelius, Mulder used the term protein in his 1838 paper, "On the composition of some animal substances" (originally in French but translated in 1839 to German).

[3][4][5] Mulder "was the first to propose a theory concerning the causes of the differences between albumin, casein, and fibrin, and other substances more or less similar to them in physical properties and in their chemical behavior when exposed to reagents.

Analyses of these substances showed that their percentage contribution with respect to carbon, hydrogen, nitrogen and oxygen were so similar as to suggest that they contain one common radical.

The variations in albuminous substances were attributed to peripheral bonds of protein to sulfur and/or phosphorus.

Justus Liebig and his students sought to determine the structure of proteins, but until the methods of Emil Fischer and Franz Hofmeister became available, the amino acid decompositions were unknown.