Figure 3. Composite relationships between environmental stressors and microbial growth/function with optimal windows highlighted in grey.

(A) An optimal feeding frequency promotes high diversity and fermentative activity. For starters with fermentation times that are too short (i.e., feeding too frequently), the sourdough culture will exhibit suboptimal fermentation activity and slow-growing organisms (i.e., yeasts) will be inhibited. In other words, baking with a starter that has not reached its full fermentative potential may produce bread that is not well-leavened or has an inconsistent crumb texture or structure, among other undesirable sensory qualities. (B) Intermediate fermentation times strike a balance between pH and selection for acid-tolerant microorganisms. Refeeding the starter removes some organic acid from the sourdough system, temporarily relieving acid stress. With longer fermentation times, only acid-tolerant species of lactic acid bacteria (LAB) and yeast will prevail. (C) The ideal fermentation time allows both LAB and yeasts to reach optimal growth rate and cell density. The initial sourdough medium contains slightly more LAB cells compared to yeasts cells. Intermediate fermentation times maximize the growth rate and cell density of both LAB and yeasts. Beyond that window, however, organic acid accumulation inhibits yeast growth and activity.