Impacts of gaseous ozone (O3) on germination, mycelial growth, and aflatoxin B1 production in vitro and in situ contamination of stored pistachio nuts

Pistachio nuts can become colonized by mycotoxigenic fungi, especially Aspergillus flavus, resulting in contamination with aflatoxins (AFs). We examined the effect of gaseous O3 (50–200 ppm; 30 min; 6 L/min) on (a) in vitro germination, (b) mycelial growth, and (c) aflatoxin B1 (AFB1) production on a milled pistachio nut-based medium at different water activity (aw) levels and at 30 °C. This was complimented with in situ studies exposing raw pistachio nuts to 50–200 ppm of O3. Exposure of conidia to gaseous O3 initially resulted in lower germination percentages at different aw levels. However, 12 h after treatment, conidial viability recovered with 100% germination after 24–48 h. Growth rates of mycelial colonies were slightly decreased with the increase of the O3 dose, with significant inhibition only at 0.98 aw. The production of AFB1 after O3 treatment and storage for 10 days was stimulated in A. flavus colonies at 0.98 aw. Raw pistachio nuts inoculated with A. flavus conidia prior to O3 exposure showed a significant decrease in population after 20 days of storage. However, AFB1 contamination was stimulated in most O3 treatments. The relationship between exposure concentration, time and prevailing aw levels on toxin control needs to be better understood for these nuts.