Inhibition of growth and aflatoxin production of Aspergillus parasiticus by citrus oils.

Aspergillus parasiticus was inoculated into grapefruit juice and a glucose-yeast extract medium; both contained 500-7000 ppm of citrus oils that were incorporated into the media by sonication. Orange and lemon oil were more inhibitory to mold growth and aflatoxin production than was dlimonene, the main constituent of the two peel oils. After 7 days at 28 degrees C, 2000 ppm of lemon and 3000 ppm of orange oil in grapefruit juice afforded maximum suppression of mold growth and toxin formation. When the glucose-yeast extract medium was used, 3000 ppm of either oil were needed to achieve the same result. After 4 days at 28 degrees C, orange oil at 3500 ppm in either medium markedly inhibited mold growth (as evidenced by dry weight of mold mycelium) and aflatoxin production (only 14 and 1% of the amount normally produced in the juice and artificial medium, respectively). Higher concentrations of orange oil further reduced mold growth and aflatoxin production and also delayed the onset of sporulation, if it occurred. Although aflatoxin was detected in all samples, only 0.2 to 0.5% of the amount found in controls (without the citrus oil) was present when the medium contained 7000 ppm orange oil. The mold consistently grew, albeit very poorly, on the glass at the liquid-atmosphere interface even when the substrate contained a large amount of citrus oil.