Susceptibility of Aspergillus spp. to acetic and sorbic acids based on pH and effect of sub-inhibitory doses of sorbic acid on ochratoxin A production

Abstract

The use of organic acids as preservatives in foods is an alternative for restricting the development of moulds in food and subsequently reducing the possibility of mycotoxin contamination. The objective of this study was to investigate the influence of acetic and sorbic acids on the growth of species of Aspergillus in culture media at different pH values, besides determining the influence of sub-inhibitory concentrations of sorbic acid on the production of ochratoxin A. To assess the sensitivity of fungal isolates to acids, factorial arrangement was used with nine isolates of Aspergillus, four pH levels (4.5, 5.0, 5.5, and 6.0) and two types of acids in the concentrations: acetic acid (0; 12.5; 25; 50; 100; 200; 400 and 800mM) and sorbic acid (0; 0.5; 1; 2; 4; 8; 16; 32mM). The experiments were conducted in triplicate, with incubation for six days at 25°C. A 2×2 factorial arrangement with two ochratoxigenic isolates and two pH values (4.5 and 5.0) was used to determine the inhibitory concentration (IC) and the influence of sorbic acid on the production of ochratoxin A. The sorbic acid concentrations used for ochratoxin A production studies were the same used for testing susceptibility. This experiment was conducted in duplicate, with incubation for seven days and determination of the IC for each isolate, where the controls and two concentrations below IC (25% and 50% of the IC) were selected for the analysis of ochratoxin A. Ochratoxin A was quantified by high performance liquid chromatography with fluorescence detection. There was variability in the sensitivity of the species tested for the acids assessed. Aspergillus carbonarius, Aspergillus parasiticus and Aspergillus flavus were more sensitive to the tested acids while Aspergillus niger and Aspergillus tubingensis were the most resistant when compared with the other isolates. A direct relationship between the pH values and the inhibitory concentration was observed and, in general, each increase of 0.5 in the pH value doubled the acid concentration necessary to inhibit the growth of a same isolate. It was also observed that sorbic acid under-dosing, in addition to not preventing fungal growth, can stimulate the production of ochratoxin A in isolates of A. carbonarius and A. niger.