**4.1 fermentation of glucose**

Fermentation of glucose with LAB occurs in two pathways which include either the pentose phosphate pathway or the Embden–Meyerhof–Parnas (glycolysis) pathway. These two major pathways are used to phosphorylate and metabolize glucose sugar [24]. Except for *Leuconostocs*, the obligately heterofermentative Lactobacilli, *Gonococci,* and *Weissellas,* the Embden–Meyerhof–Parnas pathway is found in all LAB. In this process, 1 mole of glucose produces 2 moles of lactic acid and results in a net gain of two ATPs [23]. According to de Oliveira [25], glycolysis can result in heterotactic fermentation under certain conditions, and some homofermentative LAB utilizes the pentose phosphate pathway to metabolize substrates. The phosphoketolase split of xylulose-5-phosphate to glycerol aldehyde-3-phosphate (GAP) and acetyl-phosphate is regarded as an essential and important step in the pentose phosphate pathway. After that, GAP is converted to lactate, and acetyl-phosphate is also metabolized to acetate and ethanol [24]. In sourdough fermentations, Lactobacilli are largely heterofermentative and use the pentose phosphate pathway to break down glucose. Oxygen and fructose may both be used as electron acceptors in microaerophilic circumstances, resulting in the creation of other metabolites such as acetate and mannitol [23].

### **4.2 Fermentation of fructose**

Fructose, a hexose sugar abundant in many plants, is one of the most important monosaccharides for bacterial growth in most plant-associated habitats, and it is usually fermented by the two Lactobacilli metabolic pathways [26]. *Lactobacillus sanfranciscensis* and *Lactobacillus pontis* can thus use fructose as a carbon source; nonetheless, when maltose is present, they utilize it most as an electron acceptor to create mannitol [27], especially when oxygen is limited [27]. The acetate kinase process generates additional ATP during the conversion of fructose to mannitol, resulting in a shorter lag phase with a greater growth rate and biomass production. The predominant product, acetic acid, has a molar ratio of 4:1 (fructose: maltose) [27]. Fructose is converted to mannitol by *L. sanfranciscensis,* but a little amount of lactic acid and ethanol are produced by *L. pontis* [28].

#### **4.3 Fermentation of lactose**

Lactose is mostly regarded as the major carbohydrate in dairy products, and it is the only carbohydrate that is usually used by Lactobacilli for the rapid growth, development, and production of acid [7]. The utilization of lactose by Lactobacillus is usually determined by the system of transport and enzymes involved in the hydrolyzation of the lactose sugar. The Lactobacillus species transports the lactose sugar into the cell through the permease and after the transportation sugar is then hydrolyzed (broken down) into glucose and galactose by an enzyme known as β-galactosidase. As the result of lactose fermentation, the simple glucose and galactose are next metabolized using the Embden–Meyerhof pathway to release lactic acid. This lactose fermentation process can also use a pathway known as 6-phosphogluconate, and in this process lactic acid, CO2, and ethanol are produced as the end product [29].
