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Lactic acid bacteria (LAB), particularly heterofermentative species, play a crucial role in sourdough fermentation. These bacteria contribute to acidification, producing lactic acid, CO2, acetic acid, and ethanol through heterolactic fermentation of glucose. This process lowers the dough pH to below 4.5. The ratio of lactic acid to acetic acid, known as the fermentation quotient (FQ), is a critical factor influencing sourdough bread flavor. An FQ between 2.0 and 2.7 is optimal, and this range depends on the dominant LAB species (homo or hetero-fermentative). The FQ ultimately impacts the sensory characteristics of sourdough breads.

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Lactic acid bacteria (LAB) are the predominant microorganisms in sourdough fermentation
with a cell concentration of 108–109 CFU/g and are responsible for sourdough acidification
[49]. LAB species contribute to the dough acidification process; however, heterofermentative
species also contribute partially to the leavening process and are of greater importance in
sourdough than homofermentative LAB species (commonly used in other fermented food
products). The metabolism of heterofermentative LAB produces lactic acid, CO2, acetic acid,
and ethanol by heterolactic fermentation using glucose as a carbon source. These
metabolites decrease dough pH at values lower than 4.5 . The relationship between the lactic
acid and acetic acid produced during sourdough fermentation is an important parameter
known as fermentation quotient (FQ). It indicates the molar ratio of lactic acid/acetic acid
and is calculated as FQ = (g of lactic acid in 100 g of dough/molecular weight of lactic acid):(g
of acetic acid in 100 g of dough/molecular weight of acetic acid). The FQ should be between
2.0 and 2.7 to achieve a good bread flavor. This parameter is related to the type of LAB
(homo and hetero-fermentative) that dominates in the fermentation, and its value influences
the sensory properties of sourdough breads .