Suppressed Production of Aflatoxin B1 by Aspergillus flavus and Aspergillus parasiticus on California In-hull Almonds and Hull Fragments

Almond hulls are economically important by-products marketed as dairy cattle feed in California. This study investigated the ability of Aspergillus flavus and Aspergillus parasiticus to grow and produce aflatoxins on almond hulls. In-hull almond kernels and hull fragments were incubated at 0.92-0.98 a_w and 20-35 °C for 20 days. There was no growth and aflatoxin production at 0.92 a_w by either fungus. Both A. flavus and A. parasiticus were able to grow at 0.95-0.98 a_w and 20-35 °C on almond hulls. While Aspergillus flavus grew well on the in-hull almonds at 0.95-0.98 a_w and 27-35 °C, it did not produce any aflatoxins under any of the study conditions. In contrast, A. parasiticus produced aflatoxins B₁, G₁, B₂_, and G₂. On the in-hull kernels, A. parasiticus synthesized the highest levels of total aflatoxins (>100 µg/kg) at 27 °C and 0.95-0.98 a_w. On the hull fragments, 20 °C and 0.95 a_w were the ideal conditions for aflatoxin production, where the average total aflatoxin production was 35 µg/kg. The predominant aflatoxin synthesized by A. parasiticus was AFG₁, accounting for up to 100% of the total aflatoxin produced on the hulls, which highlights the need to measure and report total aflatoxins (rather than AFB₁ only) for almond hulls. Further studies are needed to determine the factors that limit AFB₁ synthesis by A. flavus and A. parasiticus on almond hulls. Also, the types of phenolic acids present in California almond hulls need to be identified.