The use of at least one sulphur-containing amino acid, at a total concentration of about 5 to 75 mg/ml, in particular of about 5 to about 50 mg/l, in particular of about 5 to about 30 mg/l, in particular of about 5 to about 20 mg/l, in free form, for implementing a method for preparing a fermented food product fermented by ferments containing bifidobacteria, the food product has acceptable sensory properties, contains more than about 5×107, in particular more than about 108 bifidobacteria per gram of food product fermented for a shelf lifetime of at least 30 days, in particular a shelf lifetime of at least 35 days, and containing no more than 0.5% of yeast extract or of yeast autolysate.