Organic acids have been used for years to control fungal spoilage of foods. They find wide use because of solubility, taste, and low toxicity. The mode of action of organic acids is attributed to depression of intracellular pH by ionization of the undissociated acid molecule or disruption of substrate transport, by alteration of cell membrane permeability. In addition to inhibiting substrate transport, organic acids may inhibit NADH oxidation, thus eliminating supplies of reducing agents to electron transport systems (Doors 1993; Liewen and Marth 1985).
Since, the undissociated portion of the acid molecule is primarily responsible for antifungal activity, effectiveness is dependant upon the dissociation constant of the acid and pH of the food to be preserved. Because the dissociation constant of most organic acids is between pH 3 and 5, organic acids are generally most effective at low pH values. This along with solubility properties determines the foods in which organic acids may be effectively used.
A few fungal species possess mechanisms of resistance to organic acid preservatives. Saccharomyces baili is resistant to high concentrations of sorbic and benzoic acids (Warth 1977). Some molds in the genus Pénicillium can grow in the presence of high concentrations of sorbic acid and decarboxyated sorbic acid to 1,3-pentadiene, a volatile compound with an extremely strong kerosene-like odor (Liewen and Marth 1984; 1985; Tsai et al. 1988). When resistance to or metabolism of an organic acid is a problem, other preservative systems must be used.
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