Safety aspects in soybean food and feed chains: Fungal and mycotoxins contamination

BARROS G.G.; OVIEDO M.S; RAMIREZ, M.L; CHULZE S.N.

Soybean - Biochemistry, Chemistry and Physiology

Intech Open Access Publisher

Año: 2011; p. 7 - 20

Soybean (Glycine max L.) is a main source of protein and is used worldwide both as food and feed. Soybean production reached 47.5 million tons during the 2006 to 2007 harvest season, ranking Argentina third in the world among soybean producers. Around 70% of the soybean harvested is processed, providing 81% of the world´s exported soybean oil and 36% of soybean meal. Most of the soybean production is exported to the European Union as oil, seeds, and flour. Soybean is often attacked by fungal infections in the field, or post-harvest (in transit or in storage), significantly affecting its productivity. Hygienic safety of soybean and by-products depends on fungal contamination, among other microorganisms. According to the European Union legislation is needed in the short time for mycotoxins in soybean and by products. Numerous efforts have been done in our country in relation to soybean and by products quality and safety. In a first approach, the examination of fungal and mycotoxin contamination on soybean used in the soy meal production allow us to identify critical control points (CCPs) in the process. Among the trichothecenes, deoxynivalenol (DON) was detected in different points of the process at ppm levels. However, Fusarium graminearum, the main responsible for cereal contamination with DON in Argentina, was not recovered. Based on these results, the dynamic of potentially toxigenic Fusarium species at different soybean growth stages and natural occurrence of several trichothecenes (DON, nivalenol, T-2 and HT-2 toxins) was analysed. High isolation frequency of Fusarium species was found in pods and seeds at stage R6 (full seed), being the aw of immature seeds 0.992.  At stage R8 (full maturity), the water content of the seeds dropped dramatically to 0.700 and the percentage of Fusarium species also diminished compared to stage R6. Among the Fusarium species identified, F. equiseti was the most frequently recovered from flowers, pods and seeds followed by F. semitectum and F. graminearum. DON was detected at ppm levels in a few samples obtained from R6 and R8 stages of soybean development. This is the first report that demonstrates DON contamination on soybean during grain ripening. Based on the DON contamination on soybean seeds at stages R6 and R8, we considered interesting to investigate the toxigenic ability of strains belonging to F. graminearum species complex isolated from soybean agroecosystem. Strains of F. graminearum usually express one of three sets of trichothecene metabolites either: nivalenol and acetylated derivatives (NIV chemotype), (ii) deoxynivalenol and 3-acetyldeoxynivalenol (3-ADON chemotype), or deoxynivalenol and 15- acetyldeoxynivalenol (15-ADON chemotype). Surprisingly, Fusarium isolates that produce both DON and NIV (NIV/DON chemotype) have been reported and described as “unknown” chemotypes. Examination of trichothecene chemotype among the strains isolated in soybean agroecosystem revealed that 15-ADON was the dominant chemotype identified by chemical and PCR analysis. However, a 10% of the isolates showed an unusual pattern of trichothecenes production (DON and NIV producers). Similar to Fusarium species, Alternaria spp. showed high mean isolation frequency on flowers, pods and seeds, at similar levels across all stages evaluated. Although this organism is considered to be nonpathogenic on soybean plant, Alternaria species are well known for the production of toxic secondary metabolites. Based on these findings the ecophysiology of A. alternata on soybean was evaluated. During this study we compared the impact of water activity (aW) and temperature on growth and alternariol (AOH), alternariol monomethil ether (AME) and tenuazonic acid (TA) production on soybean-based media by strains of A. alternata isolated from soybeans in Argentina. Both variables affected growth, and the pattern obtained was independent of the strains evaluated. Maximum growth rates were obtained at the highest aw (0.995) and 25ºC, with growth decreasing as the water availability of the medium was reduced. Maximum amount of AOH was produced at 0.98 aW and 25ºC for both strains. Maximum AME production was obtained for both strains at 30ºC but different aW values, 0.92 and 0.94, for the strains RC 21 and RC 39, respectively. Maximum TA production was obtained for both strains at 0.98 aW, but at 30 and 25ºC for the strains for RC 21and RC 39, respectively. The concentrations of three toxins varied considerably depending on the aW and temperature interactions assayed. The three metabolites were produced from 5 to 30 ºC and at aW values of 0.92 to 0.995. Although at 5 and 18 ºC little mycotoxin was produced at aW lower than 0.94. Two-dimensional profiles of aW by temperature interactions were developed from these data to identify areas where conditions indicate a significant risk from AOH, AME and TA accumulation on soybeans. All the conditions of aW and temperature that resulted in maximum production of both toxins are those found during soybean development in the field. Thus, field conditions are likely to be conducive to optimum A. alternata growth and toxin production. Natural occurrence of Alternaria toxins (AOH and AME) in soybean seeds harvested in Argentina was evaluated by using HPLC analysis coupled with a solid phase extraction column clean-up. From a survey of 50 soybean seed samples evaluated for AOH and AME contamination, it was found that 44% of them were contaminated with AME. AME was found in levels ranging from 62 to 1153 ng/g. Although a limited number of samples were evaluated, this is the first report on the natural occurrence of Alternaria toxins in soybean seeds and is relevant from the point of view of animal public health. Further studies on molecular characterization of the potential toxigenic fungal species isolated from soybean and by products are in progress. Also chemical detection of natural mycotoxin contamination is needed. These data will improve the knowledge on safety through the soybean food and feed chains.