Undissociated organic acids

Undissociated organic acids entering the bacterial cell. A schematic representation of this action is illustrated in Figure 5.1. 

Antimicrobial action of organic acids is dependent on various factors. The following are discussed (Chaveerach et al., 2002) number of undissociated organic acids that enter the bacterial cell, acidity constant (piCa value), water activity (aw), temperature, and production of H202. 

Number of undissociated organic acids that enter the bacterial cell... 

Organic acids can enter the microbial cell only in their undissociated forms, which diffuse across the microbial cell membrane. This entrance of the acid molecule then lowers the intracellular pH (pH,) of the cell (Carrasco et al., 2006). The concentration of the undissociated form of an organic acid and the pH of the environment are interdependent variables, linked by the Henderson-Hasselbach equation (Breidt, Jr., Hayes, and McFeeters, 2004). As the extracellular pH decreases, the number of undissociated organic acids increases, and so do their activities toward the microbial cells (Kwon and Ricke, 1998). This undissociated state of the acid molecule is primarily responsible for any antimicrobial activity and effectiveness is dependent on the dissociation constants (pKa) of the acid (Barbosa-Canovas et al., 2003). This undissociated state of the organic acid is extremely important in the capacity to inhibit a microbial cell (Gauthier,... 

In 1983, Eklund developed a mathematical model for the antimicrobial activity of organic acids, which described the antimicrobial action of the dissociated as well as the undissociated organic acid. In contrast to a model assuming the activity of the acid form only, this model provided a good description of the actions of a variety of organic acids (Eklund, 1985). The model was suggested to have practical value, because the determination of MICs of a specific substance at only two different pH levels could be used to predict the MIC of that same substance at other pH levels. However, the model failed to consider the antimicrobial effect of the low pH on its own (Lambert and Bidlas, 2007). 

The pH chauges iu wetlaud soils siguificautly iuflueuce hydroxide, carbonate, sulfide, and silicate equilibria. These equilibria regulate the precipitation and dissolution of solids, the sorption and desorption of ions, and the concentrations of nutritionally significant ions or substances such as AT+, Fe, H2S, H2CO3, and undissociated organic acids. 

Distribution of the undissociated organic acids between the phases. 

Simple diffusion probably accounts for the uptake of undissociated organic acids by the yeast cell. Facilitated diffusion may be involved in the transport of sugars [17], although the fact that some are taken up against a concentration gradient could indicate that active transport occurs [18]. Active transport processes are used to transport amino acids and the ions of potassium, magnesium, phosphorus and sulphate. 

Clostridia grow best at pH 7.0-7.4. They cannot tolerate acid conditions, and a pH of 4.2 is usually considered to be low enough to inhibit their growth. The nature of the acids responsible for lowering the pH is important, with undissociated organic acids being the most potent. Clostridia are particularly sensitive to water availability and require very wet conditions for active growth. Mth very wet crops (i.e. those... 

Organic acids may inhibit growth when present in the undissociated form because of their abiHty to change the pH inside the ceU. The most efficient are benzoic acid and sorbic acid, but formic, acetic, and propionic acid also have this effect. The parabens, ie, -hydroxy benzoic acid esters, are also used because of their antimicrobial effect over a broad pH range. 

Organic acids convert the blue mesomerically stabilized phenolate anion to the red undissociated acid. Reductones (e.g. ascorbic acid) reduce the reagent to a colorless salt. 

Superior antimicrobial activity in alkaline pH (seawater is always above pH 8), in the presence of nitrogenous organic matter, and due to lower volatility has been documented for bromine antimicrobials3 4. The pKa acid dissociation constants for HOC1 and HOBr are 7.4 and 8.7, respectively the dissociated acids are less effective antimicrobials4,5. Undissociated hypohalous acids are more effective because they are far better halogenating agents compared to the dissociated anion (hypohalite). Table 1 shows the effect of acid dissociation on antimicrobial performance in well-controlled laboratory experiments. 

The pH of the finished product may have a strong influence on the type of preservative used. A good example of this can be seen with the use of organic acids which may exist in a predominantly dissociated or an undissociated form as a consequence of the product pH. The undissociated form is considered to confer the antimicrobial activity and the effect of pH on benzoic, sorbic and dehydroacetic acid is described in the graph below. It can be seen that, at the normal pH of most personal care products ie. 5.5 to 7.0, there is little activity remaining. Hence organic acids would be suitable preservatives for predominantly acidic products, such as astringent washes made with lemons. 

Various approaches to the analysis of dissolved silicon have been tried. Most of them are based on the formation of /J-molybdosilic acid [ 199-203 ]. Dissolved silicon exists in seawater almost entirely as undissociated orthosilicic acid. This form and its dimer, termed reactive silicate , combine with molybdosilicic acid to form a- and /I-molybdosilicic acid [180]. The molybdosilicic acid can be reduced to molybdenum blue, which is determined photometrically [206]. The photometric determination of silicate as molybdenum blue is sufficiently sensitive for most seawater samples. It is amenable to automated analysis by segmented continuous flow analysers [206-208]. Most recent analyses of silicate in seawater have, therefore, used this chemistry. Furthermore, reactive silicate is probably the only silicon species in seawater that can be used by siliceous organisms [204]. 

Thus, on measuring a partition coefficient of HA, it is imperative to know which species is being measured, i.e., neutral (undissociated, HA) or charged species (A ). Mathematical procedures can be used to take into account the complicating equilibria, and partition coefficients can be calculated for both the nonionized and ionized species of organic acids. The difference in partition coefficient between the two species is approximately... 

In extraction of weak organic acids (abbreviated HA) from acidic aqueous solutions, the concentration of undissociated acid, HA, exceeds the concentration of its dissociated anions, A, as long as pK > pH. The acid may then act as adductforming nonelectrolyte see section 4.5 and Example 1. 

Sodium nitrite has multiple bacteriostatic effects, and is inhibitory to both aerobic and anaerobic organisms. Nitrite is most inhibitory to both aerobic (Tarr, 1941a,b Castellani and Niven, 1955) and anaerobic (Shank et al., 1962) microbes over the pH range of 5.0-5.5, consistent with the hypothesis that undissociated nitrous acid (pK, = 3.4) is the active compound. Shank et al. (1962) concluded that rapid nitrite loss due to conversion of nitrous acid to gaseous NO... 

Offline precolumn derivatization is the most common alternative in this respect it involves separating the esters obtained from the organic acids by reversed-phase chromatography, which amply surpasses solvophobic chromatography (i.e., the use of undissociated acids as such) and allows gradient elution techniques to be applied, thanks to the wider lipophilicity range covered by the derivatized compounds. 

A. The use of indicators and indicator test papers An indicator is a substance which varies in colour according to the hydrogen-ion concentration. It is generally a weak organic acid or weak base employed in a very dilute solution. The undissociated indicator acid or base has a different colour to the dissociated product. In the case of an indicator acid, Hind, dissociation takes place according to the equilibrium... 

Perel man (1968) points out that in neutral waters true dissolved molecules of undissociated Fe(OH)3 can also occur in predominant amount (compared to Fe " ion). Probably the molecules of Fe(OH)3 are stabilized by organic acids. It is important that transport in the form of true ionic solutions is possible only for divalent iron, while trivalent iron migrates mainly in the form of colloids and organic compounds. However, Castano and Garrels (1950) believe that aerated waters provide conditions adverse to the preservation of organic matter itself. The available data suggest that organic matter decomposes at the same rate at which it arises. 

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