Potential Control of Mycotoxigenic Fungi and Ochratoxin A in Stored Coffee Using Gaseous Ozone Treatment

Abstract

The objective of this study was to examine the effect of treatment of Arabica green coffee beans with gaseous ozone (O₃) for the control of ochratoxigenic fungi and ochratoxin A (OTA) contamination by Aspergillus westerdijkiae, A. ochraceus, and A. carbonarius during storage. Studies included (i) relative control of the populations of each of these three species when inoculated on irradiated green coffee beans of different initial water availabilities using 400 and 600 ppm gaseous O₃ treatment for 60 min at a flow rate of 6 L^-1 and on OTA contamination after 12 days storage at 30 °C and (ii) effect of 600 ppm O₃ treatment on natural populations of green stored coffee beans at 0.75, 0.90, and 0.95 water activity (a_w) or with additional inoculum of a mixture of these three ochratoxigenic fungi after treatment and storage for 12 days at 30 °C on fungal populations and OTA contamination. Exposure to 400 and 600 ppm O₃ of coffee beans inoculated with the toxigenic species showed that there was less effect on fungal populations at the lowered a_w (0.75). However, toxigenic fungal populations significantly increased 48 h after exposure and when stored at 0.90 and 0.95 a_w for 12 days. All three species produced high amounts of OTA in both O₃ treatments of the wetter coffee beans at 0.90 and 0.95 a_w. Gaseous O₃ (600 ppm) treatment of naturally contaminated green coffee beans had little effect on fungal populations after treatment, regardless of the initial a_w level. However, after storage, there was some reduction (26%) observed in coffee at 0.95 a_w. In addition, no fungal populations or OTA contamination occurred in the 0.75 and 0.90 a_w treatments after exposure to 600 ppm gaseous O₃ and storage for 12 days. It appears that under wetter conditions (≥0.90-95 a_w) it is unlikely that fungal populations and OTA contamination of stored coffee beans, even with such high O₃ concentrations would be controlled. The results are discussed in the context of potential application of O₃ as an intervention system for stored coffee post-fermentation and during medium term storage and transport.

Keywords: Aspergillus section Circumdati; coffee beans; control; intervention; ochratoxin A; ozone treatment; section Nigri; storage.

Figure 1

Effect of 0, 400, and 600 ppm O₃ exposure (60 min; 6 L min⁻¹) on the log₁₀ + 1 populations of (a) A. westerdijkiae, (b) A. ochraceus, and (c) A. carbonarius pre-inoculated on irradiated coffee beans adjusted to 0.75, 0.90, and 0.95 a_w, compared to the control (untreated, 0 ppm); (i) 48 h after exposure and (ii) after 12 days storage at 30 °C. Vertical bars indicate the standard error of the means.

Figure 2

Combined effect of O₃ dose and a_w (0.75, 0.90, 0.95 a_w) on ochratoxin A production in coffee beans inoculated with (a) A. westerdijkiae, (b) A. ochraceus, and (c) A. carbonarius and treated with O₃ (400, 600 ppm; 6 L min⁻¹, 60 min) and stored at 30 °C for 12 days. Vertical bars indicate the standard error of the means. Key: <LOQ. Less than Limit of quantification.

Figure 3

Effect of air or 600 ppm O₃ exposure for (600 ppm; 6 L min⁻¹, 60 min) on the populations (log₁₀ + 1 CFUs g⁻¹ dry weight) isolated from (a) naturally contaminated coffee beans and (b) naturally contaminated + inoculum of condia of A. westerdijkiae, A. ochraceus, and A. carbonarius in coffee beans adjusted to 0.75, 0.90, and 0.95 a_w, compared to the controls (0 ppm O₃). (i) after exposure and (ii) 12 days storage at 30 °C. Vertical bars indicate the standard error of the means.

Figure 4

Combined effect of 600 ppm O₃ × a_w (0.98 and 0.94) on ochratoxin A production in (a) naturally contaminated coffee beans and (b) coffee beans inoculated with conidia of 3 ochratoxigenic species when stored for 30 °C for 12 days. Vertical bars indicate the standard error of the means. Key: <LOQ, less than Limit of Quantification.