Organic acids Antimicrobial effects

With its wealth of information and rare focus solely on the subject, this book provides practical tools to help readers better understand these and other problems and then develop optimal solutions and strategies. It discusses mechanisms of organic acid antimicrobial action and facilitates troubleshooting techniques regarding problems with preservatives and microorganisms. It also shows how to predict the effectiveness of a preservation method or application. 

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. 

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. 

Different organic acids, primarily lactic acid, have been successfully used for decontamination of whole livestock carcasses, and the application of different organic acids used for decontamination has also been tested in the fruit and vegetable industry. Organic acids other than lactic acid that are known to have bactericidal effects are acetic, benzoic, citric, malic, propanoic, sorbic, succinic and tartaric acids (Betts and Everis 2005). The antimicrobial action is due to a reduction in the pH in the bacterial environment, disruption of membrane transport, anion accumulation or a reduction in the internal pH in the cell (Busta et al., 2001). Many fruits contain naturally occurring organic acids. Nevertheless, some strains, for example E. coli 0157, are adapted to an acidic environment. Its survival, in combination with its low infective dose, makes it a health hazard for humans. 

In Table 3.1 some factors are stipulated that should be considered in the decontamination of meat (European Union, 1996). Spraying with 1.5-2.5% organic acids such as acetic or lactic acid is effectively applied on red meat carcasses (Canadian Food Inspection Agency, 2004). Lactic and acetic acid solutions are commonly used by the red meat slaughtering industry as an antimicrobial spray wash on freshly slaughtered beef carcasses. These spray washes are used in the early steps of beef carcass processing, usually applied to carcasses after hide removal, before and after evisceration, but before chilling (Berry and Cutter, 2000). 

It is not realistically possible to make general statements regarding the antimicrobial activity of organic acid salts and spice combinations. Such combinations should each be evaluated separately to determine the appropriate level of organic acid salt to effectively control the growth of C. per-fringens (Sabah, Juneja, and Fung, 2004). 

The antimicrobial effect and toxicity of organic acids have been attributed to (1) a combination of the hydrogen ion concentration (lowering the pH), (2) consequent decreases in proton motive force, and (3) the action of undissociated molecules that may freely permeate the cell membrane... 

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,... 

Comparison of different inhibitory effects of an antimicrobial agent on microbial activity necessitates the determination of parameters that could express the influence of a toxic compound (Banerjee and Sarkar, 2004). Two such parameters have been identified kir which is the exponential inhibition constant, and is inversely correlated with tolerance to antimicrobials, and C50%, which refers to the toxic concentration that would inhibit 50% of any pysiological mechanism (Liewen and Marth, 1985 Banerjee and Sarkar, 2004). To form a good picture of the action of organic acids k, should be taken into account together with xmin values, in order to evaluate the inhibition of fermentation. However, the value of the reciprocal of k, plus the value of xmin could be adapted if needed, for example, for a more practical industrial approach. There is a fair correlation between k, and C50o/o. It is, however, easier to determine the latter parameter, and it... 

In a study done on the antimicrobial effects of ozone alone or combined with organic acids in the control of E. coli 0157 H7 and L. monocytogenes inoculated on mushrooms, it was found that ozone treatment alone had minimal influence on the numbers of E. coli 0157 H7 and L. monocytogenes, whereas a combined treatment of 3 ppm ozone with 1% citric acid significantly reduced the population numbers (Yuk et al., 2007). 

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). 

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