Synergistic Antimicrobial Effect of Ethanol, Sodium Chloride, Acetic Acid and Essential Oil Components

Abstract

Ethanol, NaCl, acetic acid and essential oil components were examined for their synergistic antimicrobial effects, using air-borne microorganisms and pure cultures of fungi. Antimicrobial assays were carried out at 27°C, using 2% glucose Sabouraud agar. In order to completely suppress the growth of all the contaminating air microorganisms for over 20 days, more than 8% (by volume) ethanol, 25% (wt/vol) NaCl, or 0.2% (by volume) acetic acid was required in the medium. All essential oil components examined, at a concentration of 1 or 2mM, were only moderate or very weak in antimicrobial effect. However, when these substances were applied in pairs, in threes and in fours, they exhibited a potent antimicrobial effect at relatively or considerably lower concentrations.
The combination of 3% ethanol, 0.05% acetic acid and 1mM perillaldehyde, and the
combination of as low as 0.5% ethanol, 2% NaCl, 0.05% acetic acid and 0.5mM perillaldehyde were sufficient to suppress completely the growth of all contaminating air microorganisms for over 20 days. The combination of 3% ethanol, 3% NaCl and 0.5mM perillaldehyde or 0.05% acetic acid was also fairly potent in this respect. Other essential oil components, such as citral, citronellol, geraniol, cuminaldehyde and eugenol, were as effective as perillaldehyde ; L-menthol, linalool and D-carvone were somewhat less effective ; and hydrocarbons (D-limonene, α-pinene, β-pinene, camphene, β-myrcene and p-cymene), citronellal, 1, 8-cineole, anethole and vanillin were very weak or almost completely ineffective in this respect. Similar synergistic antimicrobial effects of these substances were also observed when using pure cultures of fungi as inocula.
Our results strongly suggest that combinations of ethanol, NaCl, acetic acid and certain essential oil components, each at a considerably lower concentration, are applicable for effective preservation of foods without applying synthetic preservatives or other antimicrobial substances uncommon in usual foods.