Activity of organic acids

Acidification of internal components of cell membranes by the undissociated acid molecule. 

Bacterial membrane disrupting (leakage, transport mechanisms). Therefore, loss of active transport of nutrients through the membrane. 

Disruption of acid transport by alteration of cell membrane permeability. 

Chelation as permeabilizing agent of outer membrane and zinc binding. 

Act as inhibitors of other stress responses, for example, heat-shock response (Piper et al., 2001). 

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Tamblyn, K.C. and Conner, D.E. 1997. Bactericidal activity of organic acids in combination with transdermal compounds against Salmonella typhimurium attached to broiler skin. Food Microbiology 14 477-484. 

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). 

In Figure 5.1 the transport and proposed antimicrobial activity of organic acids as antimicrobial agents is schematically illustrated. 

Organic acids are most active at an environmental pH equal to or lower than their pKa value (ionization constant) (Brul et al., 2002). Bacteriostatic effects of acids vary, and the type of acid, as well as the pH, are of utmost importance in controlling pathogens in foods. Antibacterial activity of organic acids is related to the reduction in pH, as well as its ability to dissociate, which is determined by the pKa value and pH of the surrounding area, and as a result activity increases with a decrease in pH (Jung and Beuchat, 2000). 

Little information exists on the activity of organic acids against viruses. However, enteric viruses have been shown not to possess similar susceptibility to weak organic acids compared to bacteria and consequently survive well in organic acid preserved foodstuffs such as lactate-fermented products (Adams and Nicolaides, 1997). 

Skrivanova, E. and Marounek, M. 2007. Influence of pH on antimicrobial activity of organic acids against rabbit enteropathogenic strain of Escherichia coli. Folia Microbiol (Praha) 52 70-72. 

Matsuda, T., Yano, T., Maruyama, A., and Kumagai, H. 1994. Antimicrobial activities of organic acids determined by minimum inhibitory concentrations at different pH ranged from 4.0 to 7.0. Japan Society for Food Science and Technology 41 687-701. 

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). 

Another important parameter to consider regarding the eiScacy of acids is the constant dissociation of the acid (pK), which is the pH value where eoncentrations of dissoeiated and undissociated species are equal. For example, formie acid (pK = 3.75) will be 50% dissociated and 50% undissociated at a pH value of 3.75. Thus, anti-bacterial activities of organic acids are influenced by the pH of the intestinal tract. The higher the pH value, the more they will tend to dissociate. 

Varga Z, Kiss G, Hansson HC (2007) Modelling the cloud condensation nucleus activity of organic acids on the basis of surface tension and osmolality measurements. Atmos Chem Phys 7 4601-4611... 

Appendix Calculation of Activities of Organic Acids from Reported Concentrations... 

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