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Aspergillus on tree nuts: incidence and associations
Paul Bayman1, James L Baker, Noreen E Mahoney
1USDA ARS Western Regional Research Center, 800 Buchanan St., Albany, CA 94710, U.S.A. pbayman@upracd.upr.clu.edu
Mycopathologia
|March 6, 2003
Summary
Fungi commonly colonize tree nuts, with Aspergillus species often found together. Postharvest handling significantly impacts the fungal communities on almonds, pistachios, and walnuts.
Area of Science:
- Agricultural Science
- Food Microbiology
- Mycology
Background:
- California tree nuts (almonds, pistachios, walnuts) face strict aflatoxin regulations in export markets.
- Concerns over aflatoxin contamination drive research into eliminating fungi like Aspergillus flavus.
Purpose of the Study:
- To investigate the incidence and associations of fungal species on various tree nuts.
- To differentiate between superficial and internal fungal colonization in nuts.
Main Methods:
- 1100 tree nuts (pistachios, almonds, walnuts, brazil nuts) without visible damage were analyzed.
- Nuts were plated on salt agar, with half the samples surface-sterilized to distinguish colonization types.
- Fungal growth was observed and identified.
Main Results:
- Aspergillus, Rhizopus, and Penicillium were the most common fungal genera identified.
- Each nut type exhibited a distinct mycoflora.
- Surface sterilization reduced fungal populations on most nuts, indicating superficial contamination, except in brazil nuts.
Conclusions:
- Strong positive associations were found among Aspergillus species, suggesting co-colonization.
- Penicillium showed negative associations with Aspergillus niger and Rhizopus in some instances.
- Harvest and postharvest handling significantly influence nut mycoflora, with multiple fungi often colonizing single nuts.