Many organic acids are used as food additives, but not all have antimicrobial activity. The most effective antimicrobials are acetic, lactic, propionic, sorbic, and benzoic acids. The activity of organic acids is related to pH and the undissociated form of the acid is primarily responsible for antimicrobial activity (7). The use of organic acids is generally limited to foods with a pH less than 5.5, since most organic acids have piiTa's of pH 3.0 to 5.0 (7). Another factor affecting potential activity is polarity. This relates both to the ionization of the molecule and contribution of any alkyl side groups or hydrophobic parent molecules. Antimicrobials must be lipophilic to attach and pass through the cell membrane but also be soluble in the aqueous phase (8).
The mechanism of action of organic acids and their esters has some common elements. As already stated, in the undissociated form, organic acids can penetrate the cell membrane lipid bilayer more easily. Once inside the cell, the acid dissociates because the cell interior has a higher pH than the exterior (9). Bacteria maintain internal pH near neutrality to prevent conformational changes to the cell structural proteins, enzymes, nucleic acids, and phospholipids. Protons generated from intracellular dissociation of the organic acid acidify the cytoplasm and must be extruded to the exterior. This requires energy in the form of ATP, which will eventually deplete cellular energy.
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