Although citric acid is not used directly as an antimicrobial, it has activity against some molds and bacteria. Citric acid retards growth and toxin production by Aspergillus parasiticus and A. versicolor, but not Penicillium expan-sum (7). It is inhibitory to Salmonella in media and on poultry carcasses, to growth and toxin production by C. botulinum in shrimp and tomato products, to S. aureus in microbiological medium and to flat sour bacteria isolated from tomato juice (7). The mechanism of citrate inhibition has been theorized to be related to its ability to chelate metal ions (27). However, Buchanan and Golden (28) found that, while undissociated citric acid is inhibitory against Listeria monocytogenes, the dissociated molecule protects the microorganism. They theorized that this protection is due to chelation by the anion.
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