Redox potential growth media

Some variables such as temperature, pH, nutrient medium, and redox potential are favorable to certain organisms while discouraging the growth of others. The major characteristics of microbial processes that contrast with those of ordinary chemical processing include the following [1] ... 

Moreover, almost in all the early steps, the redox potential of the clusters, which decreases with the nuclearity, is quite negative. Therefore the growth process undergoes another competition with a spontaneous corrosion by the solvent and the radiolytic protons, corrosion which may even prevent the formation of clusters, as mostly in the case of nonnoble metals. Monomeric atoms and oligomers of these elements are so fragile to reverse oxidation by the medium that H2 is evolved and the zerovalent metal is not formed [11]. For that reason, it is preferable in these systems to scavenge the protons by adding a base to the solution and to favor the coalescence by a reduction faster than the oxidation [53]. 

Stability means that clusters do not undergo coalescence nor corrosion by the medium, at least in the absence of oxygen. The quite negative value of ii°(MVM ) and the dependence of the cluster redox potential on the nuclearity have crucial consequences in the formation of early nuclei, their possible corrosion or their growth. As an example, the faster the coalescence, the lower is the probability of corrosion of the small clusters by the medium. The property of stability offers the means to apply to these clusters a larger amount of suitable characterization techniques than to transient oligomers. 

The key strategy when a new medium is developed is to reproduce the metabolic conditions of the cells when in vivo. Therefore, when adding media components it is important to maintain the pH, redox potential, and osmolality constant, as well as attempting to avoid generation of metabolic compounds that are toxic or growth inhibitory, such as lactate and/or ammonium. Apart from purely nutritional factors, it is important to develop a medium that is stable under normal storage conditions or in contact with light. 

Fig. 2a-c. Growth of Bacillus stearothermophilus PV72 in continuous culture on a synthetic medium containing glucose (8 gH) as the sole carbon and energy source. The dissolved oxygen concentration was controlled at 50 % and the dilution rate was 0.3 h-1. As derived from the measured process variables, variant formation started at about 15-16 h after inoculation. Shown are the measures for a Respiration activity b External and internal reduction state (redox potential and culture fluorescence) c Cell density (Reprinted from J. Biotechnol. 54, K.C. Schuster et al., p. 19,1997, with permission from Elsevier Science)... 

The stoichiometry for cell growth is very complex and varies with micra-organism/nutrient system and environmental conditions such as pH, temperature, and redox potential. This complexity is especially true when more than one nutrient contributes to cell growth, as is usually the case. We shall focus our discussion on a simplified version for cell growth, one that is limited by only one nutrient in the medium. In general, we have... 

Growth can be followed indirectly by monitoring the increase in concentration of the metal solubilized from the mineral, for example, iron from pyrite and copper from chalcopyrite. The formation of Fe(III) in cultures growing on Fe(II) sulfate can be measured by colorimetric and UV methods, and also from the Fe(III)/Fe(II) redox potential, when the concentration of Fe(III) can be determined from the Nemst equation. Assessment of growth can in theory also be made from the change in pH of the medium, but this method is subject to many errors. 

Low redox potential Nutrient depletion Crowding pH reduction of surrounding medium (pH 3.5-4.5) Growth decreases carbohydrate food content Competitive growth inhibits spoilage or pathogenic organisms... 

Sulfur compounds have long been considered as contributors to Cheddar cheese flavor. Some strains of Lc. lactis ssp. cremoris, but not Lc. lactis ssp. lactis, can absorb glutathione (y-Glu-Cys-Gly GSH) from the growth medium (Fernandes and Steele, 1993). Release of GSH into the cheese on cell lysis may affect the redox potential (Eh) of cheese, and hence the concentration of thiol compounds. Cheesemaking studies using starter strains that accumulate glutathione or those that do not are warranted. 

The redox potential of the medium may change in the relatively anaerobic growth conditions of many cultures. This can cause irreversible bleaching of some indicators. 

Figure 2.15. Schematic representation of the concept of interactions between physical transport phenomena (oxygen transfer rate, OTR heat transfer rate, H TR power consumption, P/V circulation time, and distribution, CTD residence time distribution, RTD) and biokinetics (specific rates of growth fi of consumption of substrates q, resp. consumption of oxygen, of production of heat,, of CO2 qc, of product qp and of all internal compartments (jinJ. The physical properties of the medium (temperature T, pH value, redox potential rH, density p, gas hold-up Eq, mean energy dissipation e, shear rate 7, resp. specific viscosity q — as a quantitative...

Practically all these processes occur concomitantly so that the redox values are always average values of all the ongoing processes. Another complication arises due to growth in biofilms (see Sec. 4.3.3). The redox potential in the bulk medium can be completely different from the one measured in colonies and/or under colonies close to the material s surface. This means that redox potential measurements do not indicate which processes are occurring. Inside colonies, anaerobic zones (oxygen-free) with low redox potentials often exist, where processes such as sulfate-reduction occur, despite the fact that the bulk milieu is sufficiently aerated (and possibly causes deterioration). 

Von Wolzogen Kuhr and van der Vlugt (1934) suggested that the above set of reactions is caused by SRBs. This electrochemical generalization has been accepted and is stfll prevalent During corrosion, the redox potential of the bacterial growth medium is —52 mv (Hadley 1948). After inoculation of a corrosion testing cell with SRBs, the electrochemical potential decreases from the initial value of —470 mv to approximately —538 mv O Fig. 12.6). 

When ionic liquid systems are intended to be applied for electrodepwsition their behaviour has to be assessed as comp>ared with the case of aqueous electrolytes. The main factors which affect the overall electrochemical process include viscosity, conductivity, the potential window, the ionic medium chemistry as well as the structure of the electrical double layer and redox potentials. All these prarameters will influence the diffusion rate of metallic ions at the electrode surface as well as the thermodynamics and kinetics of the reduction process. Consequently, the nudeation/growth mechanisms and the deposit morphology will be affected, too. More detailed discussions on this topic may be formd in ( Abbott et al., 2004 Abbott et al., 2004 Abbott McKenzie, 2006 Abbott et al., 2007 Endres et al., 2008 and included references). 

An interesting idea has been to prepare the photosensitive electrode on site having the liquid play the dual role of a medium for anodic film growth on a metal electrode and a potential-determining redox electrolyte in the electrochemical solar cell. Such integration of the preparation process with PEC realization was demonstrated initially by Miller and Heller [86], who showed that photosensitive sulfide layers could be grown on bismuth and cadmium electrodes in solutions of sodium polysulfide and then used in situ as photoanodes driving the... 

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