Influence of pH

The Effect of pH on the Formation of Volatiles During the Heating of a HMF/Cysteine Model System 

The salt (buffer) type and concentration may also influence reaction rate. While buffers vary in their effect on the Maillard reaction, it is generally accepted that phosphate is the best catalyst [27], The effect of phosphate on reaction rate is pH dependent with it having the greatest catalytic effect at pHs between 5-7. Potman and van Wijk [27] found the Maillard reaction rate in a phosphate buffered model system increased from 10- to 15-fold compared to a phosphate free reaction system. 

In some unpublished work, we chose to thermally process (extrude) low and normal salt (NaCl) lots of a cooked cereal base. We then analyzed the volatile profile of the two extruded products by gas chromatography. We found that the low salt formulation contained substantially less volatiles (quantitatively) than the normal salt product. It appears that the salt levels used in extruded cereal products influenced the rate of the Maillard reaction. This observation is important in efforts to manufacture thermally processed low salt foods. It appears that taking the salt out of a food may influence both aroma and taste (saltiness). 

There is very little work published on the kinetics of aroma formation via the Maillard reaction. However, what work has been done has indicated that volatile formation is very sensitive to both compositional and processing changes. One can generate kinetic data for volatile formation in a simple model system, include another precursor that should have no direct effect on the kinetics of interest, and find major changes in reaction kinetics. 

No organized experiments were performed to obtain kinetic data, i.e., studies where the researcher chose to heat a model system at several temperatures over sufQcient times to gather reliable kinetic data, until the late 80s. The available literature will be presented. 

The relevance of the pH-value was already seen in the chain reaction of ozone, especially in the initiation step. It also plays an important role in all the acid-base equilibrium by influencing the equilibrium concentrations of the dissociated/nondissociated forms. This is especially important for the scavenger reaction with inorganic carbon, which will be discussed further in Section B 4.4.4. 

For the combined oxidation processes, the effect of pH is even more complex. Experimental results have shown a steady increase in the reaction rate of micropollutants with increasing pH, as well as optima at various pH values. 

When the pH is varied, the rates of enzyme catalyzed reaction shows a maximum value at certain pH called the optimum pH as shown in Fig. 6.7. 

The influence of pH can be explained on the basis that the active center of the enzyme exists in three states of ionization as 

EH2 bears one more positive charge (or one less negative charge) than EH and EH has one more positive charge than E. For simplicity, charges have not been shown. Ka or Kb are the dissociation constants. Each of these three forms of enzyme can interact with the substrate to give the complex. The following mechanism may be proposed for reaction  

It is assumed that it is the only (EHS) form of the complex which gives rise to products. In acidic solution, the equilibrium will lie over to left, EH2S form of complex will dominate and rate of reaction will be low because EHS form is the reactive form. 

In highly basic solution the ES form will be dominating and again the rate will be low. At some intermediate pH, when EHS is dominating, the rate will be maximum. Application of steady state treatment to this scheme of mechanism leads to a very complicated rate law and it is difficult to apply it to the experimental data. 

The pH values may influence significantly on the formation of intermediate complexes during the preparation by precipitation or precipitation-deposition. For example, gold catalysts are prepared with a gold precursor (HAuCLi solution), and pH affects the formation of gold complexes, the maximum content, and the presence of chlorine. The ions [AuCU] form different complexes, as shown in Fig. 8.1. Complexes like [AuCl4 c(OH) c] x = 1-3) are adsorbed at the surface with the formation Au(OH)3 species, which are the precursors of gold nanoparticles [8,12-14]. 

In many biogeochemical reactions, appears either as a reactant or a product. Easy experimental tools are available to measure the activity of H+, thus scientists routinely use aH+ rather than molar concentration. The logarithmic expression of commonly used is 

For a standard state, H+ concentration is set as 1M, which is equal to pH = 0. However, biological reactions do not occur at pH = 0, thus for standard state reactions, scientists adopted pH of 7 (10 M). For a generalized reaction involving H+  

In biogeochemistry, pH is a master variable that can be used in reactions involving oxides, hydroxides, sulhdes, carbonates, and many enzymatically mediated biological reactions. 

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The charge on a droplet surface produces a repulsive barrier to coalescence into the London-van der Waals primary attractive minimum (see Section VI-4). If the droplet size is appropriate, a secondary minimum exists outside the repulsive barrier as illustrated by DLVO calculations shown in Fig. XIV-6 (see also Refs. 36-38). Here the influence of pH on the repulsive barrier between n-hexadecane drops is shown in Fig. XIV-6a, while the secondary minimum is enlarged in Fig. XIV-6b [39]. The inset to the figures contains t,. the coalescence time. Emulsion particles may flocculate into the secondary minimum without further coalescence. 

Fig. 1. Influence of pH on A, the addition reaction of urea and formaldehyde (1 1) and B, the condensation of methylolurea with the amino hydrogen of a...

Consistent with this, even KI3 is rapidly decolorized in alkaline solution. The example is a salu-tory reminder of the influence of pH, solubility, and complex formation on the standard reduction potentials of many elements. 

The influence of pH on the residual metal concentration [M] was studied at a constant polymer concentration of 10 mg/1 and copper sulphate concentration of 10 gm/1, results are shown in Table 4. It is clear that [M] decreases with increasing pH value. Results are given in Table 4, which shows that the concentration of Cu decreases with an increase in the pH value for each polymer used. This is attributed to the effect of the pH value on the active groups, which are distributed along the polymer chains. At a low pH value the amide groups... 

Figure 4.34 illustrates, by means of potential/anodic current density curves, the influence of pH and Cl ions on the pitting of nickel The tendency to pit is associated with the potential at which a sudden increase in anodic current density is observed within the normally passive range ( b on Curve 1 in Fig. 4.34). It can be seen that in neutral 0-05 M Na2S04 containing 0-02m Cl" (Curve 1) has a value of approximately 0-4 V h- When pitting develops, the solution in the pits becomes acidic owing to hydrolysis of the corrosion product (see Section 1.6) and when this occurs the anodic current density increases by at least two orders of magnitude and tends to follow the curve obtained in 0 05 m H2SO4-t-0-02 m NaCl (Curve 2). Comparison of Curves 2 and 3 illustrates the influence of Cl" ions on the pitting process.

Figure 4.36 shows the influence of pH on the breakdown potential of nickel in alkaline solutions containing Cl ions, and it is apparent that the breakdown potential becomes more positive as the pH increases, i.e. breakdown is unlikely unless the solution has a very high redox potential. 

Fig. 4.36 Influence of pH and Cl ion on (he breakdown potential of commercial nickel in alkaline solutions (0-001-5 m NaOH) de-aerated with N2 (after Postlethwaite )...

The choice of a satisfactory chelating agent for a particular separation should, of course, take all the above factors into account. The critical influence of pH on the solvent extraction of metal chelates is discussed in the following section. 

Fig. 3.2.2 Influence of pH on the initial light intensity of euphausiid luminescence when the fluorescent compound F and protein P are mixed in 25 mM sodium phosphate buffers of various pH values, each containing 1M NaCl, at near 0°C. Both F and P were obtained from Meganyctiphanes norvegica. From Shimomura and Johnson, 1967, with permission from the American Chemical Society.

Fig. 3.3.2 Influence of pH on the activity of luciferase ( ) and the quantum yield of coelenterazine (o) in the bioluminescence of Oplophorus. The measurements were made with coelenterazine (4.5 pg) and luciferase (0.02 pg) for the former, and coelenterazine (0.1 pg) and luciferase (100 pg) for the latter, in 5 ml of 10 mM buffer solutions at 24° C. The buffer solutions used sodium acetate (pH 5.0), sodium phosphate (pH 6.0-7.5), Tris-HCl (pH 7.5-9.1), and sodium carbonate (pH 9.5-10.5), all containing 50 mM NaCl. Replotted from Shimomura et al., 1978, with permission from the American Chemical Society.

Fig. 4.1.4 Influence of pH on the total light emission and initial light intensity of aequorin. Buffer solutions containing 0.1 mM calcium acetate, 0.1 M NaCl, and 10 mM sodium acetate (for pH < 7) or 10 mM Tris-HCl (for pH > 7) were adjusted to various pH with acetic acid or NaOH, and then 2 ml of the solution was added to 3 pi of aequorin solution containing 1 mM EDTA to elicit luminescence, at 22°C. The data shown are a revision of Fig. 9 in Shimomura et al., 1962. The half-total time is the time required to emit 50% of total light.

Fig. 5.8 Influence of pH, temperature, NaCl concentration, and the concentration of coelenterazine on the light intensity of luminescence reaction catalyzed by the luciferases of Heterocarpus sibogae, Heterocarpus ensifer, Oplophorus gracilirostris, and Ptilosarcus gruneyi. Buffer solutions used 20 mM MOPS, pH 7.0, for Ptilosarcus luciferase and 20 mM Tris-HCl, pH 8.5, for all other luciferases, all with 0.2 M NaCl, 0.05% BSA, and 0.3 p,M coelenterazine, at 23°C, with appropriate modifications in each panel. Various pH values are set by acetate, MES, HEPES, TAPS, CHES, and CAPS buffers.

Fig. 7.1.6 Influence of pH and temperature on the luminescence of Cbaetopterus photoprotein elicited by old dioxane and Fe2+ in 20 mM phosphate buffer. Left panel the effect of pH in phosphate buffer solutions of various pH values, at 22°C. Right panel the effect of temperature at pH 7.2. Luminescence was initiated by the injection of Fe2+. The time lag of the light emission after the Fe2+ injection was also shown in the right panel. From Shimomura and Johnson, 1966.

Fig. 7.2.8 Influence of pH on the luminescence intensity of the Odontosyllis luciferin-luciferase reaction at room temperature. From Shimomura et ai, 1963d, with permission from John Wiley Sc Sons Ltd.

Fig. 10.2.3 Influence of pH on the peak luminescence intensity of Luminodesmus photoprotein, when 0.5 ml of a solution of the photoprotein was added to 2 ml of 10 mM sodium acetate buffers (a), 10 mM sodium phosphate buffers (o) or 10 mM Tris-HCl buffers ( ), each containing 1 mM MgCl2 and 0.05 mM ATP. From Shimomura, 1981, with permission from the Federation of the European Biochemical Societies.

Fig. 10.3.1 Bioluminescence spectrum of the centipede Orphaneus brevilabiatus (left panel), and the influence of pH on the luminescence of the exudate of the same centipede in 0.1 M potassium citrate/phosphate buffers (right panel). From Anderson, 1980, with permission from the American Society for Photobiology.

The influence of pH on the affinity of Hb for oxygen known as the Bohr-effect indicates that protons retain the allosteric regulation of oxygen transport. It is also an indirect confirmation of the ability of Hb and Im Hb for transporting carbon dioxide. The values of the Bohr-effect d log P50/d pH for Hb and Im Hb are close to each other in the pH range 7.1-7.4. It is possible that the effect of the micro-environment of carboxylic CP on immobilized Hb and its polyfunctional interaction represents the interaction between Hb and the structural elements inside the red cell [99]. 

FIG. 1 Influence of pH on the surface tension of some alkyl ether carboxylic acids. (Concentration 1 g/1, Ring method.) (From Ref. 57.)... 

The influence of pH, ionic strength, and protein concentration on the extraction of a-lactalbumin and 3-lactoglobulin from an aqueous solution with water/AOT/isooctane microemulsions and their separation has been reported [168],... 

Savolainen, K. and Kuusi, T., The stability properties of golden beet and red beet pigments influence of pH, temperature, and some stabilizers, Ztschr. Lebensm. (Inters. Forsch., 166, 19, 1978. 

Beltran E, H Eenet, JE Cooper, CM Coste (2000) Kinetics of abiotic hydrolysis of isoxaflutole influence of pH and temperature in aqueous mineral buffered medium. J Agric Eood Chem 48 4399-4403. 

Other diagrams could be devised indicating the influence of pH, electrolyte concentration, etc., on the appearance of one or two phases. Three-dimensional diagrams might also be of great aid. 

Table 4. Influence of pH on particle dimension and sol stability using PVA as a protecting agent.

Skibo, E. B. Formation and fate of benzimidazole-based quinone methides. Influence of pH on quinone methide fate. J. Org. Chem. 1992, 57, 5874-5878. 

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