In 1925 his interests shifted to the nitrogen-fixing bacteria in the root nodules of leguminous plants. The improved methods of butter preservation, by adding disodium phosphate to prevent acidic hydrolysis. This method was in use for several decades in Finland. His research from 1925 till 1932 included the invention of a fodder preservation method (AIV Fodder). The method, patented in 1932, was basically a kind of silage that improved the storage of green fodder, which is important during long winters. The process includes adding dilute hydrochloric or sulfuric acid to newly stored grain. Increased acidity stops harmful fermentation and has no adverse effect on the nutritive value of the fodder or the animals it is fed to. In 1945, Virtanen received the Nobel Prize in chemistry "for his research and inventions in agricultural and nutrition chemistry". His later years studies included the development of partially synthetic cattle feeds. The nitrogen for the synthesis of amino acids normally comes from proteins in the fodder. A special bacterial environment in the rumen of cattle allows them to use urea and ammonium salts as source for the nitrogen instead of plant proteins like soybean or meat and bone meal. He also headed the Valio Laboratory from 1921–1969. The prestige conferred by the Nobel Prize brought Virtanen invitations, honorary doctorates and membership in foreign academies of science. He was a member of the Finnish, Norwegian, Swedish, Flemish, Bavarian, and Pontifical Academies of Science, and of the Swedish and Danish Academies of Engineering Sciences. He was an honorary member of learned societies in Finland, Sweden, Austria, Edinburgh, and the US, and received honorary degrees of the Universities of Lund, Paris, Giessen, and Helsinki, the Royal Technical College at Stockholm, and the Finland Institute of Technology. The asteroid 1449 Virtanen, discovered by the renowned Finnish astronomer and physicist Yrjö Väisälä, was named after him. The lunar crater Virtanen is also named after him.