Die Wirkung von Sorbins�ure auf Saccharomyces cerevisiae: I. Beeinflussung des Wachstums sowie der aeroben und anaeroben Glucoseverwertung

4: Final Report on the Safety Assessment of Sorbic Acid and Potassium Sorbate

Sorbic acid is a straight-chain monocarboxylic acid used in cosmetic formulations as a preservative at concentrations up to 1.0%.

Sorbic acid and potassium sorbate were practically nontoxic to rats and mice in acute oral toxicity studies. In subchronic studies no significant adverse effects were observed in rats, mice, or dogs when 10% sorbic acid was included in the diet.

Sorbic acid and potassium sorbate at concentrations up to 10% were practically nonirritating to the rabbit eye. Both ingredients at concentrations up to 10% were at most only slightly irritating.

Sorbic acid and potassium sorbate have been tested for mutagenic effects using the Ames test, genetic recombination tests, reversion assays, rec assays, tests for chromosomal aberrations, sister chromatid exchanges, and gene mutations. Results have been both positive and negative.

Potassium sorbate at 0.1% in the diet or 0.3% in drinking water of rats for up to 100 weeks produced no neoplasms. In other chronic studies, no carcinogenic effect was demonstrated by sorbic acid in rats or mice fed diets containing up to 10% sorbic acid.

No teratogenic effects have been observed in pregnant mice and rats administered potassium sorbate.

In three repeat insult patch tests, sorbic acid had overall sensitization rates of 0, 0.33, and 0.8%. All of the subjects sensitized were inducted with 20% sorbic acid and challenged with 5% sorbic acid. Formulations containing up to 0.5% sorbic acid and or potassium sorbate were not significant primary or cumulative irritants and not sensitizers at this test concentration. A formulation containing 0.01% sorbic acid was not a photosensitizer.

On the basis of the available data, it is concluded that sorbic acid and potassium sorbate are safe as cosmetic ingredients in the present practices of use and concentration.

Weak-acid preservatives: pH and proton movements in the yeast Saccharomyces cerevisiae

Weak-acid preservatives commonly used to prevent fungal spoilage of low pH foods include sorbic and acetic acids. The "classical weak-acid theory" proposes that weak acids inhibit spoilage organisms by diffusion of undissociated acids through the membrane, dissociation within the cell to protons and anions, and consequent acidification of the cytoplasm. Results from 25 strains of Saccharomyces cerevisiae confirmed inhibition by acetic acid at a molar concentration 42 times higher than sorbic acid, in contradiction of the weak-acid theory where all acids of equal pK(a) should inhibit at equimolar concentrations. Flow cytometry showed that the intracellular pH fell to pH 4.7 at the growth-inhibitory concentration of acetic acid, whereas at the inhibitory concentration of sorbic acid, the pH only fell to pH 6.3. The plasma membrane H⁺-ATPase proton pump (Pma1p) was strongly inhibited by sorbic acid at the growth-inhibitory concentration, but was stimulated by acetic acid. The H⁺-ATPase was also inhibited by lower sorbic acid concentrations, but later showed recovery and elevated activity if the sorbic acid was removed. Levels of PMA1 transcripts increased briefly following sorbic acid addition, but soon returned to normal levels. It was concluded that acetic acid inhibition of S. cerevisiae was due to intracellular acidification, in accord with the "classical weak-acid theory". Sorbic acid, however, appeared to be a membrane-active antimicrobial compound, with the plasma membrane H⁺-ATPase proton pump being a primary target of inhibition. Understanding the mechanism of action of sorbic acid will hopefully lead to improved methods of food preservation.

Inhibition of spoilage mould conidia by acetic acid and sorbic acid involves different modes of action, requiring modification of the classical weak-acid theory

Fungal spoilage of many foods is prevented by weak-acid preservatives such as sorbic acid or acetic acid. We show that sorbic and acetic acids do not both inhibit cells by lowering of internal pH alone and that the "classical weak-acid theory" must be revised. The "classical weak-acid theory" suggests that all lipophilic acids with identical pK(a) values are equally effective as preservatives, causing inhibition by diffusion of molecular acids into the cell, dissociation, and subsequent acidification of the cytoplasm. Using a number of spoilage fungi from different genera, we have shown that sorbic acid was far more toxic than acetic acid, and no correlation existed between resistance to acetic acid and resistance to sorbic acid. The molar ratio of minimum inhibitory concentrations (MICs) (acetic: sorbic) was 58 for Paecilomyces variotii and 14 for Aspergillus phoenicis. Using flow cytometry on germinating conidia of Aspergillusniger, acetic acid at pH 4.0 caused an immediate decline in the mean cytoplasmic pH (pH(i)) falling from neutrality to approximately pH 4.7 at the MIC (80 mM). Sorbic acid also caused a rapid but far smaller drop in pH(i), at the MIC (4.5 mM); the pH remained above pH 6.3. Over 0-5 mM, a number of other weak acids caused a similar fall in cytoplasmic pH. It was concluded that while acetic acid inhibition of A. niger conidia was due to cytoplasmic acidification, inhibition by sorbic acid was not. A possible membrane-mediated mode of action of sorbic acid is discussed.