Abstract: A method, termed “bionative” method, for preparing a leaven, in which: a raw milk is selected from the group formed from raw milks of which the pH is capable of reaching, during a step of stabilising fermentation by acidifying lactic fermentation of the raw milk, at least one selection temperature, a value of 5.5 in more than 15 h, and a minimum value of 5.0 in less than 48 h, and then a raw milk-stabilising treatment is carried out. The treatment includes at least one step, termed stabilising fermentation step, of acidifying lactic fermentation of a fermentation medium, including a first stabilising fermentation step carried out using a fermentation medium including the raw milk, in which at least one stabilising fermentation step is interrupted when the pH of the fermentation medium reaches a value of between 4.9 and 6.0.