Effect of relative humidity on infection of almond kernels by Aspergillus flavus and A. parasiticus and levels of aflatoxin contamination

Aspergillus flavus and A. parasiticus can contaminate almond nuts with aflatoxins. Humid conditions during the drying process of the nuts on the orchard floor or storing in stockpiles may affect aflatoxin levels in almonds. Almond kernels were inoculated with the toxigenic isolates of A. flavus A224 and A. parasiticus A408, wound-inoculated, incubated at 30C under relative humidity (RH) of 70, 75, 80, 85, 90, 95, and 100%, respectively, and sampled 1 week later. A real-time PCR assay was used to quantify the number of spores per kernel. The aflatoxin level for each sample was quantified by high performance liquid chromatography (HPLC). A positive linear relationship was observed between the number of spores per kernel and RH for each species. Toxigenicity of A408 was about 10-times higher than that of A224. Aflatoxin levels were lower under 70, 75 and 80% RH with A224, but were higher under 85, 90, 95, and 100% RH. The A408 produced aflatoxins under all RH treatments. For this isolate, no quantitative relationship between RH and aflatoxin levels was observed, but aflatoxins were significantly higher under 100% RH than under the other RHs. No correlation was observed between the detected levels of spores and aflatoxins for either species, but in a few cases, significantly higher numbers of spores per kernel were associated with high aflatoxin levels. Drying the nuts quickly on the orchard floor should reduce aflatoxin contamination in almonds.