# Probiotics in the Sourdough Bread Fermentation: Current Status
## 3 Sourdough Fermentation: Major Pathways  
The sourdough fermentation process generates mainly acids, alcohols, aldehydes, esters, and ketones; it is the primary route of volatile organic compounds (VOCs) [36]. Sourdough bread has a complex profile, strongly influenced by the compounds generated during fermentation by the diverse array of microorganisms present, mainly yeasts and LAB. the oxidation of the coenzymes NADH + and H+ by the lactate dehydrogenase, and CO\({}_{2}\) from a glucose molecule until ribulose 5 phosphate (heterolactic fermentation). This fermentation is used by genera such as _Leuconstoc_, _Bifidobacterium_, _Weissella_, and some _Lactobacillus_. Some glycolytic enzymes are missing in these bacteria, and they use the pentose-P pathway to degrade the glucose (Figure 2A2). Classification according to the type of lactic fermentation defines three major _Lactobacillus_ groups: group I: obligate homofermature lactobacillus. Hexoses are exclusively fermented to lactic acid via the Embden-Meyerhof-Parnas pathway. The bacteria have the enzyme fructose-1,6-bisphosphate aldolase, but phosphoketolase is absent. Due to this, pentoses and gluconate are not fermented. This group includes the species _L. acidophilus_, _L. delbrueckii_, and _L. salivarius_; group II: facultative heterofermature lactobacillus. Hexoses are fermented to lactic acid almost exclusively via the Embden-Meyerhof-Parnas pathway. The bacteria  
Figure 2: Major sourdough fermentation pathways. (**A**) Lactic fermentation. (**B**) Alcoholic fermentation. (**A1**), homolactic fermentation; (**A2**), heterolactic fermentation. (**B3**), alcoholic fermentation from yeast. The formation of ethanol and CO\({}_{2}\) from the reduction of pyruvate characterizes alcoholic fermentation. possess both aldolase and phosphoketolase enzymes, and ferment not only hexoses but also pentoses. In the presence of glucose, the enzymes of the phosphogluconate pathway are inhibited. This group includes _L. casei_, _L. paracasei_, _L. plantarum_, and _L. curatatus_. Organic acids, CO\({}_{2}\), alcohol, and H\({}_{2}\)O\({}_{2}\) could be produced; group III: composed by the obligate heterofermetative Lactobacillus. They have the enzyme phosphoketolase but not aldolase and ferment sugars in a heterofermetative mode. Hexoses are fermented via the phosphogluconate pathway, producing lactate, ethanol (acetic acid), and CO\({}_{2}\) in equimolar amounts. Pentoses enter this pathway and can also be fermented [39; 40; 41; 42]. After the enzymatic hydrolysis of pentosans and other complex carbohydrates, pentoses such as d-xylose, ribose, and l-arabinose are liberated in rye and wheat flour. Heterofermetative strains can ferment pentoses through part of the 6-phosphogluconate pathway [43]. Pentoses and hexoses are simultaneously rather than successively fermented by sourdough Lactobacillus. Lactobacillus are important during the fermentation of sourdoughs, and they can ferment pentose carbohydrates without producing CO\({}_{2}\) because they have a constitutive phosphoketolase. Facilitatively heterofermetative lactic acid bacteria produce the phosphoketolase enzyme in response to the presence of pentoses [42; 44]. The contribution of LAB to the flavor of sourdough bread is associated with the production of lactic acid (fresh acidity) and acetic acid (sharp acidity). The conversion of amino acids such as phenylalanine (sweet), isoleucine (acidic), glycine, serine, and alanine (vinegar/sour) to aldehydes and ketones can form additional flavor compounds [36]. Alcoholic fermentation (AF) is characterized by the formation of ethanol and CO\({}_{2}\) from pyruvate reduction (Figure 2B). AF is a metabolic pathway of yeasts. In this fermentation, yeast produces gas that promotes dough conditioning and increased volume and imparts desirable aromas and flavors in baked goods. A synergy occurs between the dough and the yeast, and in this context, the rate of CO\({}_{2}\) production is determined by the activities of the glycolytic yeast enzymes.