Finnish biochemist whose investigations directed toward improving the production and storage of protein-rich green fodder, vitally important to regions characterized by long, severe winters, brought him the Nobel Prize for Chemistry in 1945.

As a chemistry instructor at the University of Helsinki (1924–39), where he became professor of biochemistry (1939–48), Virtanen studied the fermentation processes that spoil stores of silage. Knowing that the fermentation product, lactic acid, increases the acidity of the silage to a point at which destructive fermentation ceases, he developed a procedure (known by his initials, AIV) for adding dilute hydrochloric or sulfuric acid to newly stored silage, thereby increasing the acidity of the fodder beyond that point. In a series of experiments (1928–29), he showed that acid treatment has no adverse effect on the nutritive value and edibility of the fodder and of products derived from animals fed the fodder.

Virtanen was also a professor of biochemistry at the Helsinki University of Technology (1931–39) and director of Finland's Biochemical Research Institute, Helsinki, from 1931. He did valuable research on the nitrogen-fixing bacteria in the root nodules of leguminous plants, on improved methods of butter preservation, and on economical, partially synthetic cattle feeds. His AIV System as the Basis of Cattle Feeding appeared in 1943.