Further Observations on the Relation of pH Value to Toxicity of Preservatives to Microörganisms

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Abstract
The approximate concentrations of the weak salicylic, sulphurous and acetic acids required to prevent growth at various pH values, of Mucor spp., Penicillium glaucum, Saccharomyces ellipsoideus and a mixed culture of acetic and lactic bacteria were determined. Above pH 4.5 the concentrations required rapidly increased, and in some cases at pH 7 more than 150 times as much preservative was required as at pH 2.5. The preservative actions of NaCl and formaldehyde were not markedly affected by pH, contrary to earlier findings on Na ben-zoate. The salicylate, sulphite and acetate resemble the benzoate in this respect. Evidently the undissociated acid rather than its negative ion is responsible for inhibiting growth. Similar results were obtained in exps. on the effect of pH in the retarding action of these preservatives on rate of fermentation by S. ellipsoideus, using small initial inocula.

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