Suscetibilidade de Aspergillus sp. aos ácidos orgânicos conforme pH e a influência destes sobre a produção de ocratoxina A

Abstract

Fungal growth on food and subsequent contamination with mycotoxins is a public health problem. Ochratoxin A (OTA) is a important mycotoxin, with nephrotoxic effects and carcinogenic properties. Decreasing the fungal contamination and consequently preventing toxin development is the most efficient action to reduce human exposure through the diet. The use of organic acids as preservatives in foods is an excellent alternative. The aim of this study was to investigate the influence of acetic, citric and sorbic acid on the growth of Aspergillus species at different acidic pH values, and the influence of these organic acids on OTA production. To evaluate the susceptibility of the isolates in the analyzed acids a factorial arrangement was used with nine fungi, four pH levels (4.5, 5.0, 5.5, and 6.0) and three types of acids in different concentrations: acetic acid (0, 12.5, 25, 50, 100, 200, 400 and 800mm), citric acid (0, 50, 100, 200, 400 and 800mm) and sorbic acid (0, 0.5, 1, 2, 4, 8, 16 and 32mm). The experiments were incubated for six days and conducted in triplicate. The inhibitory concentrations (IC) were the lowest acid concentration tested where the fungi was not able to grow. To determine the influence of these acids on mycotoxin production, a factorial arrangement was used with two fungi (A. carbonarius and A. niger) and two pH (4.5 and 5.0). The acid concentrations were the same as those used in the susceptibility experiments. The experiment was carried out in duplicate by incubation for seven days at 25°C for the determination of IC for each isolate. Controls and two concentrations below the IC (0, 25 and 50% IC) were selected for the analysis of OTA. OTA quantification was performed by high performance liquid chromatography (HPLC) with fluorescence detection. Differences were found on the sensitivity of the species tested between the analyzed acids. A. flavus, A. parasiticus and A. carbonarius were more sensitive to acetic and sorbic acids whereas A. niger and A. tubingensis were the most resistant. A direct correlation between pH and the inhibitory concentration was observed. Generally, for every increase in pH by 0.5 the level of acetic acid and sorbic acid required to inhibit the growth of the same isolate doubled. In the other side, citric acid did not show an important antifungal power. It was also observed that sub-lethal dosages of sorbic acid did not prevent fungal growth, and in addition can stimulate the production of ochratoxin A on A. carbonarius and A. niger isolates. The result shows the importance of considering both the pH and the predominant fungal species in the product, when calculating the acid dosages for prevention of fungal food spoilage.