**1. Introduction**

72 Lactic Acid Bacteria – R & D for Food, Health and Livestock Purposes

*Streptococcus thermophilus*. J. Bacteriol. 2008;190(4) 1401–1412.

[156] Horvath P, Romero DA, Coûté-Monvoisin AC, Richards M, Deveau H, Moineau S, Boyaval P, Fremaux C, Barrangou R. Diversity, activity, and evolution of CRISPR loci in

> Lactic acid bacteria can be found in a diversity of ecosystems, which is consistent with their ability to adapt to highly variable environments. Among the various parameters that characterize these environments (temperature, pH, water activity), redox is relatively recent. It has however already been addressed indirectly in studies relating to the impact of oxidative stress on lactic acid bacteria. Indeed, the concept of oxidation has often been associated with the presence of oxygen; however, oxidoreductive effects on microorganisms must not be limited to oxygen.

> A broader vision could be proposed concerning the adaptation of lactic acid bacteria to extracellular redox. The metabolism of lactic acid bacteria, chemosynthetic organisms, involves a series of dehydrogenation (oxidation) and hydrogenation (reduction) reactions. This metabolism follows the principle of conservation of energy and matter, and therefore requires the availability of a terminal electron acceptor. In lactic acid bacteria, a carbon metabolic intermediate is reduced (mainly pyruvate). In homofermentative lactic acid bacteria, redox coenzymes (NAD+/NADH) enable coupling between oxidation and reduction reactions. During anaerobic glycolysis, glucose is oxidized to 2 moles of pyruvate with the formation of 2 moles of NADH, which then further reduces pyruvate to form lactic acid. Consequently, the typical equation of homolactic fermentation is: 1 glucose 2 lactate.

> Such a perfect matching, theoretically consistent, must be qualified according to the environmental conditions, including the redox state of the extracellular medium. The

© 2013 Cachon et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

adaptation of lactic acid bacteria to extracellular redox depends on their ability to positively or negatively interfere with oxidants (electron acceptors) or reducing molecules (electron donors). Carbon and electron flow management by the cell will thus be highly dependent on the ability of the microorganisms to interact with the redox environment.

Redox Potential: Monitoring and Role in Development of Aroma Compounds, Rheological Properties and Survival of Oxygen Sensitive Strains During the Manufacture of Fermented Dairy Products 75

Ox ne Red (1)

(3)

<sup>h</sup> E defined as the standard redox potential at pH 7, which

(2)

(4)

**Reduction** is a reaction in which a molecule, atom or ion, gains electrons.

substance that removes electrons from another reactant in a redox reaction.

0 h h

Eh = redox potential (mV) (in relation to a normal hydrogen electrode).

donates an electron to another species in a redox reaction.

and oxidizing characteristics.

the oxidised and reduced species:

F = Faraday constant (96500 C.mol-1) n = number of electrons exchanged R = gas constant (8.31 J.mol-1.K-1)

<sup>F</sup> 59 mV (at 25 °C)

From Equation (2) it can be written:

Equation (4) is used to determine 0'

0 h h

is closer to biochemical and biological processes (Figure 1).

m = number of protons involved in the reaction

T = temperature in K

reduced species (Red):

where:

0

RT 2.3

reaction:

An **oxidant** (also known as an oxidizing agent, oxidizer or oxidiser) can be defined as a

A **reductant** (also known as a reducing agent or reducer) can be defined as a substance that

In the same way pH defines acid-base characteristics of a solution, Eh defines the reducing

Presented below is the reduction half-reaction of an oxidant (Ox) to its corresponding

The Nernst equation gives the relationship between the redox potential and the activities of

E E 2.3 log nF Red 

*<sup>h</sup> E* = standard redox potential (mV) (in relation to a normal hydrogen electrode) at pH 0

However, chemical reactions in aqueous media involve protons, and the following half-

Ox mH ne Red H O2

E E 2.3 pH 2.3 log nF nF Red 

mRT RT Ox

RT Ox

Potentially, all biochemical reactions in the cell, and therefore the enzymatic activity, may be influenced by the redox state of the environment. Dissolved oxygen is an oxidizer and can reach concentrations of 8 mg.L-1 of medium (equilibrium with air). Despite the strict anaerobic metabolism of some lactic acid bacteria, the majority are aerotolerant and can react with dissolved oxygen at varying levels. Lactic acid bacteria provided with NADH oxidase can reduce oxygen to water (reduction reaction coupled with the re-oxidation of NADH). This process influences both the intracellular and extracellular redox environment, and will result in a change in the metabolism, cellular physiology and physico-chemical environment surrounding the microorganism.

Changes in the extracellular environment can be monitored by measuring the redox potential (Eh). This parameter plays a key role in the quality of fermented dairy products, but is still rarely taken into consideration or is completely ignored during the manufacturing process. The reasons for this lack of interest can be attributed to difficulties associated with its measurement and control. Over the past ten years, several studies advocate the monitoring and control of Eh in fermented products using lactic acid bacteria selected for their reducing ability, redox molecules, or heat treatment. In terms of food applications, the variation in Eh must involve compounds that do not alter the product characteristics. So, modifying the Eh using gas, which enables the product characteristics to be maintained, may be advantageously exploited in industry.

The aim of this chapter is to present the latest knowledge concerning the adaptation of lactic acid bacteria to their redox environment, and the interest of modifying Eh using gas for lactic acid bacteria applications in the food industry.
