Chemical strategies applied to maize and peanut storage agroecosystem in Argentina to prevent aflatoxin contamination.

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

Food Storage

Nova Science Publishers, Inc.

Lugar: New York; Año: 2011; p. 65 - 98

Studies in different components of maize agroecosystem in Argentina showed that toxigenic Aspergillus section Flavi strains are widely distributed, and have a high probability of being transferred to the storage ecosystem. We have also found a good distribution of species of the section Flavi, mainly A. flavus and A. parasiticus in soil for the cultivation of maize subjected to different cultivation practices. Studying the sensitivity of A. flavus and A. parasiticus to changes in the matric potential and osmotic potential of the substrate, was observed accumulation of compatible solutes in the fungal mycelium, when reducing these potentials. The accumulation of these reserve substances promotes the growth and survival of these fungi in environments with low matric potential. Mycological analysis of different maize genotypes showed the presence of three principal genera of filamentous fungi Fusarium (100%), Penicillium (67%) and Aspergillus (60%). In the genus Aspergillus, the species identified were A. flavus and A. parasiticus, with a percentage o infection of 78 and 21% respectively. Aflatoxin B1 contamination was detected in same maize genotypes samples. In maize storage in experimental units consisted of silos field (plastic mesh or netting), 100 kg of maize kernels were storage during six month. Aspergillus section Flavi and Penicillium were principal filamentous fungi isolated; and A. flavus was the most frequently isolated (69.6%). Although no aflatoxin B1 was detected during the storage period, A. flavus and A. parasiticus strains showed a variable ability to produce aflatoxins. A. flavus S and L strains contributed to silo community toxigenicity. In other storage system, which consisted of small silo-bags used to store 100 kg of maize kernels during four month, toxigenic Aspergillus section Flavi was detected in the range of 2,9 to 4,4 log. Twenty-five percent of maize samples were contaminated with insects (Sitophilus zeamais), however, these insects were not contaminated with Aspergillus section Flavi. Insects appeared not to be an important source of infection in this type of storage system. Nevertheless, in a study conducted in maize stored in microcosm we have shown that Sitophilus zeamais, Rhyzopertha dominica and Tribolium confusum that attack stored grains have the ability to disperse toxigenic A. flavus in those grains. In a 5-month (2005) study carried out in an experimental container filled with 200 Kg of bulk peanuts, a total of 10,997 fungal isolates were identified from the control pod and seed tissues in the six samplings done. Ninety-eight percent of all fungi isolated were Deuteromycetes and Ascomycetes. Fungal isolation was greater from pod (91.9%) than from seed tissues. The most common fungi identified included Penicillium, Aspergillus, Monascus, Eurotium and Fusarium spp. Within Aspergillus genus, the section Flavi had the greatest mean counts (3.7x103 and 2.0x102 for pod and seed tissues, respectively) during all storage period. Analyses of fungal populations from 95 peanut seed samples collected from two stockpiled warehouse that contained 26,000 ton of bulk peanut pods harvested in 2007/2008 did not demonstrate significant differences between the incidences in each sampling period. Aspergillus section Flavi were isolated during all incubation periods. Cryptolestes spp. (Coleoptera: Cucujidae) were collected in August, September and October with 18, 16 and 28% of peanut samples contaminated, respectively. Insects isolated during August showed 69% of Aspergillus section Flavi contamination. A. flavus was the most frequently isolated (79%) from peanut seeds and from insect (59%). The greater levels of AFB1 were detected in September and October with a mean of 68.9 ìg/kg and 69.1 ìg/kg respectively. The highest proportion of A. flavus toxigenic strains (87.5%) was obtained in June. A real-time PCR (RT-PCR) system directed against the aflD (nor-1) gene of the aflatoxin biosynthetic pathway as target sequence was applied to monitor and quantify Aspergillus section Flavi population in peanuts conditioned at four water activity (0.94, 0.88, 0.84 and 0.76 aW) levels and stored in big bags from July to November 2008. Sensitivity tests demonstrated that DNA amounts accounting for a single conidium of A. parasiticus RCP08300 can be detected. A correlation between cfu data obtained by RT-PCR and conventional count (CC) methods was observed (r=0.613; p<0.0001); and the former always showed values higher by 0.5–1 log units. A decrease of fungal density from 6.1 to 1.7 log units was observed throughout the storage period, regardless of the quantification methodology applied and peanut aW. Total aflatoxin levels ranging from 1.1 to 200.4 ng/g were registered in peanuts conditioned at the higher aW values (0.94–0.84 aW). At the highest water stress condition assayed (0.76 aW) and throughout the storage period there was no detection of aflatoxins. The RT-PCR assay developed appears to be a promising tool in the prediction of potential aflatoxigenic risk in stored peanuts, even in case of low-level infections, and suitable for rapid, automated and high throughput analysis. The presence of Aspergillus section Flavi and insect vectors of aflatoxigenic fungi presented a potential risk for aflatoxin production during the peanut storage period. Therefore, the interaction between biologic and abiotic factors and substrate may promote the Aspergillus contamination and the subsequent aflatoxin accumulation in stored crops.