glucose 2 ADP 2 Pi 2 lactate 2 ATP + +® +

Some representative homolactic LAB genera include *Lactobacillus*, *Lactococcus*, *Enterococcus*, *Streptococcus*, and *Pediococcus* species [38].

Conversely, in the heterofermentative lactate fermentation pathway, lactate is not the only end product; significant amounts of CO2 and ethanol, or acetate, are also produced. In this pathway, lactate is produced by the decarboxylation and isomerization reactions of the PPP. Glucose is oxidized to ribulose-5-phosphate that is isomerized to xylulose-5-phosphate, which in turn is cleaved to form phosphoglyceraldehyde and acetyl phosphate. The phosphoglyceraldehyde molecule is oxidized to pyruvate by reactions of glycolytic pathway, whereas the acetyl phosphate is reduced to ethanol [45, 46]. The overall reaction is as follows:

<sup>2</sup> glucose ADP Pi ethanol lactate CO ATP + +® + + +

Some representative heterolactic LAB genera include *Leuconostoc*, *Oenococcus*, and *Weissella* [38]. It is worth mentioning that heterofermentative lactate fermentation produces only one ATP molecule per glucose, while the homofermentative lactate fermentation produces two ATP molecules per glucose.

## *1.1.2.2. Bifidum pathway*

The Bifidum pathway is a particular metabolic route found in *Bifidobacterium bifidum*, which uses reactions of the PPP and homofermentative pathway, producing primarily acetate and lactate [38, 45]. In this pathway, 2.5 ATP molecules are produced per glucose. As such, ATP yields are greater than for the homofermentative or heterofermentative pathways, due to the presence of key enzymes, fructose-6-phosphate phosphoketolase and xylulose-5-phosphate phosphoketolase. These proteins catalyze two important steps: the cleavage of one molecule of fructose-6-phosphate, yielding one molecule of erythrose-4-phosphate and one of molecule acetyl phosphate, and the cleavage of two xylulose-5-phosphate into two glyceraldehyde 3 phosphate and two acetyl phosphate, respectively [45, 46]. The overall reaction is as follows:

2 glucose 5 ADP 5 Pi 3 acetate 2 lactate 5 ATP + +® + +

## *1.1.2.3. LAB—beverage industry applications*

Over the years, LAB has been explored on a large scale in several food industry seg‐ ments (processing of meats, vegetables, and beverages) occupying a central role in these niches [43, 48–50]. Withal, there are some reasons that explain their use in the food production industry. Among these are the following: the production of antimicrobial substances, which restricts the growth of harmful microorganisms, and the production of

glucose 2 ADP 2 Pi 2 lactate 2 ATP + +® +

Some representative homolactic LAB genera include *Lactobacillus*, *Lactococcus*, *Enterococcus*,

Conversely, in the heterofermentative lactate fermentation pathway, lactate is not the only end product; significant amounts of CO2 and ethanol, or acetate, are also produced. In this pathway, lactate is produced by the decarboxylation and isomerization reactions of the PPP. Glucose is oxidized to ribulose-5-phosphate that is isomerized to xylulose-5-phosphate, which in turn is cleaved to form phosphoglyceraldehyde and acetyl phosphate. The phosphoglyceraldehyde molecule is oxidized to pyruvate by reactions of glycolytic pathway, whereas the acetyl

<sup>2</sup> glucose ADP Pi ethanol lactate CO ATP + +® + + +

Some representative heterolactic LAB genera include *Leuconostoc*, *Oenococcus*, and *Weissella* [38]. It is worth mentioning that heterofermentative lactate fermentation produces only one ATP molecule per glucose, while the homofermentative lactate fermentation produces two

The Bifidum pathway is a particular metabolic route found in *Bifidobacterium bifidum*, which uses reactions of the PPP and homofermentative pathway, producing primarily acetate and lactate [38, 45]. In this pathway, 2.5 ATP molecules are produced per glucose. As such, ATP yields are greater than for the homofermentative or heterofermentative pathways, due to the presence of key enzymes, fructose-6-phosphate phosphoketolase and xylulose-5-phosphate phosphoketolase. These proteins catalyze two important steps: the cleavage of one molecule of fructose-6-phosphate, yielding one molecule of erythrose-4-phosphate and one of molecule acetyl phosphate, and the cleavage of two xylulose-5-phosphate into two glyceraldehyde 3 phosphate and two acetyl phosphate, respectively [45, 46]. The overall reaction is as follows:

2 glucose 5 ADP 5 Pi 3 acetate 2 lactate 5 ATP + +® + +

Over the years, LAB has been explored on a large scale in several food industry seg‐ ments (processing of meats, vegetables, and beverages) occupying a central role in these niches [43, 48–50]. Withal, there are some reasons that explain their use in the food production industry. Among these are the following: the production of antimicrobial substances, which restricts the growth of harmful microorganisms, and the production of

phosphate is reduced to ethanol [45, 46]. The overall reaction is as follows:

*Streptococcus*, and *Pediococcus* species [38].

114 Food Production and Industry

ATP molecules per glucose.

*1.1.2.3. LAB—beverage industry applications*

*1.1.2.2. Bifidum pathway*

**Figure 4.** Schematic representation of the metabolism of hexoses by lactic acid bacteria (adapted from [47]).

metabolites, which influences the nutritional, texture, and organoleptic qualities of the end products [36, 51]. Moreover, LAB have also been used as probiotics, which shows several potential health benefit [52]. Thus, in general, LAB enhances the shelf life and microbial safety of end products [43]. However, based on the microorganisms profile present in the raw material, their effects may be either beneficial or disadvantageous to the food process‐ ing. For instances, malolactic fermentation (MLF) is a secondary fermentation in wine normally carried out by LAB, especially by *Leuconostoc oenos,* which usually occurs at the end of alcoholic fermentation by yeast [53]. In this metabolism, L-malic acid is decarboxy‐ lated to L-lactic acid and CO2, a reaction catalyzed by the malolactic enzyme without the release of intermediates. As a consequence of this pathway, the acidity is reduced which turn it a crucial process in wine and cider production [53]. However, it is noteworthy that MLF is not only important as a deacidification process in wine, but for the aroma and microbial stability of wine [53–55]. Additionally, this fermentation prevents the malic acid utilization by other undesirable microorganisms. Another industrial application of LAB is the use of starter cultures as inoculants during the malting process, a complex biological process essential to the production of fermented beverages, in order to improve the malt quality and safety. In these conditions, LAB can improve the extraction, fermentability, and nitrogen yield of wort and the foam stability, color, and flavor of beer [50]. Moreover, another important effect of the LAB in the malting process is their ability of antimicrobial substances production (e.g., bacteriocins) that restricts the growth of harmful bacteria to malting [50, 56–58].
