He became a professor of chemistry at Rotterdam and later at Utrecht. While at the Utrecht University, Mulder described the chemical composition of protein. He claimed that albuminous substances are made up of a common radical, protein, and that protein had the same empirical formula except for some variation in amounts of sulfur and phosphorus, long before the polymer nature of proteins was recognized after work by Staudinger and Carrothers.
He was the first to use this name, protein, coined by Jöns Jacob Berzelius in a publication, his 1838 paper 'On the composition of some animal substances'. In the same publication he also proposed that animals draw most of their protein from plants.
On the composition of some animal substances Translation of parts of Mulder's article from: Mikulás Teich, A Documentary History of Biochemistry, 1770-1940 (Rutherford, NJ: Fairleigh Dickinson University Press, 1992) "Ueber die Zusammensetzung einiger thierischen Substanzen". Journal für Praktische Chemie (in German) 16: 129–152. 1839. doi:10.1002/prac.18390160137.