In 1905, a 27-year-old Bulgarian student discovered the specific bacterium that turns milk into yogurt.

Stamen Grigorov conducted his groundbreaking research at the University of Geneva under the mentorship of Professor Léon Massol. In 1905, he successfully isolated the rod-shaped lactic acid bacterium responsible for milk fermentation. Grigorov's findings were published in the prestigious scientific journal 'Revue Médicale de la Suisse Romande' later that same year.His discovery caught the attention of Nobel Prize-winning Russian scientist Ilya Metchnikoff, who was working at the Pasteur Institute in Paris. Metchnikoff hypothesized that the longevity of Bulgarian peasants was directly linked to their high consumption of yogurt containing this specific bacterium. He believed that the bacteria prevented the putrefaction of the large intestine, a theory he detailed in his 1907 book 'The Prolongation of Life.'Today, the bacterium is officially classified as Lactobacillus delbrueckii subsp. bulgaricus. It works in a symbiotic relationship with Streptococcus thermophilus to convert lactose into lactic acid, which thickens the milk and gives yogurt its tart flavor. Grigorov's legacy is honored at the Museum of Yogurt in his home village of Studen Izvor, Bulgaria, which remains a site of historical scientific importance.