Abstract: The extent of fermentation of a food product by a microorganism can be controlled by using a selected quantity of a nutrient for which the microorganism is auxotrophic. For example, an improved sourdough bread product was produced using a bread dough made with a starter that included a Lactobacillus fermentum strain that is auxotrophic for methionine and a quantity of whey, a source of methionine, that was selected to stop fermentation when the bread dough reached a desired acidity level.