Fungal population succession in stored peanut seeds

PASSONE, M.A.; RUFFINO, M.; NESCI, A.; ETCHEVERRY, M.

Cairns Convention Centre, Queensland, Australia,

Congreso; 8th International Mycological Congress; 2006

Organisin Commitee of the 8th Mycological Congress

Carcinogenic aflatoxin produced by A. flavus and A. parasiticus are common contaminants of peanuts. Soil serves as a reservoir for primary inoculum of A. flavus and A. parasiticus (Horn and Dorner, 1998) and peanut pods are in direct contact with soil populations of aflatoxigenic fungi. Postharvest contamination may also occur when stored products are not maintained at a safe moisture and temperature level. Other fungi, dominated by the genera Penicillium and Fusarium colonized stored peanut seeds primarily at water activities (aW) and temperatures suboptimal for section Flavi species and A. niger. Thus the object of the present study was to compare the native fungal population succession in silo control and silo treated with A. flavus / A. parasiticus mixture during five months of storage in peanut seeds. The environmental factors including aW, temperature and pH were monitored. A. flavus / A. parasiticus mixture (104 spores ml-1) was applied on 200 kg of stored peanut. A control peanut without inoculate was made. Representative samples (10 samples per month) were taken with a device once a month during five months. Temperatures changes were recorded once a week. Water activities (aW) and pH values of the grains were determined. Quantitative enumeration of fungal propagules was done on the solid media (dichloran-Rose Bengal-chloramphenicol and dichloran-18% glycerol) using the surface spread method by blending 50 g portion of each sample with 450 ml of peptone water solution. The results were expressed as CFU per gram of peanut. Taxonomic identification of the different genera was made according to Pitt and Hocking (1997) and Samson et al. (2002). When compared total fungal count between nature and inoculated seeds (Table 1), significantly difference was not observed during all storage period. However, fungal count increased 1 log unit from 1th to 6th sampling in both peanut containers. An increase of temperature from 7 to 20°C was registered along the experience, while aW and pH of the grains were relatively constant about of 0.73 and 6.7, respectively. Predominant genera in nature peanut seeds at 1th sampling were Penicillium spp. (81.7%) and Aspergillus section Flavi (18.3%) among other genus isolated in minor percentage. At the end of the storage period a high prevalence of Penicillium spp. (70.8%) following by Aspergillus section Flavi (35%), Aspergillus section Nigri (20.5%) and Fusarium spp. (4.2%) was observed. Mycological analyses in inoculated seeds showed similar results during the six sampling, with 47.6% of Penicillium spp., 38% of Aspergillus section Flavi and 4.8% of Aspergillus section Nigri. Aspergillus section Flavi population was isolated during all storage period in both natural and inoculated peanut seeds in the order 1.5x103 CFU g-1, showing a high adaptability to environmental conditions. The inoculation not showed impact o mycological population succession. The results showed that peanut pods protect the entry of additional inoculum to the seeds and environmental conditions allowed to maintain the stability of inoculum level in the seeds.       Table 1. Comparison of means temporal changes in total fungal count (log CFU) between nature peanut grains and inoculated peanut grains over a 5-months storage period. Treatments Log CFU / g   1th sampling 2th sampling 3th sampling 4th sampling 5th sampling 6th sampling Nature peanut 3.6x103 a 6.6x103 a 2.7x103 ab 1.9x104 a 9.4x103 ab 1.7x104 a Inoculated peanut 1.9x103 ab 8.0x103 ab 2.7x103 b 1.1x103 b 2.1x103 ab 2.1x104 a     Data with the same letter for each count are not significantly different according to Tukey test (P<0.05).