Kjeldahl, Johann Gustav Christoffer (1849–1900), Danish chemist. Kjeldahl was the director of research at the chemical laboratory of the Carlsberg brewery. He introduced his method for the estimation of nitrogen in organic substances in 1883. In 1888 he constructed the Kjeldahl flask as an aid for simplifying the Kjeldahl method. The method is very important for agriculture, medicine, and drug manufacturing, and it is still used essentially as Kjeldahl developed it.