The invention concerns a ferment activator based on lactic acid bacteria, characterised in that it comprises at least: a nitrogenous substance, a buffer system capable of maintaining the activity pH of the lactic acid bacteria with which said activator is to be associated at a value ranging between 5 and 7, and free of added sugar(s) capable of being metabolised by said lactic acid bacteria. The invention also concerns a method for preparing a dairy product characterised in that it consists in using said activator.