Fusarium spp. and storage fungi in suboptimally stored wheat: Mycotoxins and influence on wheat gluten proteins.

When wheat is stored under suboptimal conditions, a further mycotoxin increase of deoxynivalenol (DON), but especially of mycotoxins produced by storage fungi, e.g. ochratoxin A, is possible, lowering wheat quality and food safety. Different storage trials were conducted under suboptimal storage conditions.Fusarium survival during suboptimal storage was monitored by cultural technique and multiplex-PCR and set into relation to DON contents. Furthermore, XANES spectroscopy was applied on a selected storage trial in order to characterize sulfur speciation in low molecular weight (LMW) subunits of glutenin isolated from suboptimally stored wheat samples highly infected withFusarium and from wheat infected withAspergillus andPenicillium. Distinct changes in sulfur speciation were observed in grains infected with storage fungi, especially a significant increase of higher oxidation states (sulfoxide state, sulfonate state).

Significance of different inoculum sources for the Fusarium infection of wheat ears.

From 1997 to 1999 the occurrence ofFusarium spp. on wheat grain and its contamination with the mycotoxins deoxynivalenol and invalenol were investigated under organic farming conditions in the Rhineland, Germany. For comparison, some trials were also run under integrated farming conditions. The importance of the seed contamination withFusarium spp. as well as the impact of farming system, previous crop and soil preparation on the inoculum sources ofFusarium spp. in the soil were investigated. The data on the inoculum sources was compared to the Fusarium infection of grains and their content of DON and NIV. The crop residues in the soil were the most important inoculum source for the Fusarium species infecting wheat ears and grains. The amount of potential inoculum in the soil largely depended on the previous crop and the system of soil preparation.

Rheological and breadmaking properties of wheat samples infected withFusarium spp.

Rheological and breadmaking properties of untreated and suboptimally stored wheat samples (grain moisture: 20%, temperature: 20°C) and also of wheat which was inoculated withFusarium spp. were investigated. The deoxynivalenol (DON) content of the stored and inoculated wheat samples ranged between 820-12,000 µg/kg. Gluten proteins were isolated with different extraction solutions and the fractions obtained were analysed by means of RP-HPLC. Microextension tests and micro-baking tests were used for the determination of dough properties (maximum resistance (MR) and extensibility (EX)) and bread volume, respectively. In spite of the extremely high DON concentrations of some wheat samples contaminated withFusarium spp. they showed only a slight decrease of the amount of gluten proteins. Extension tests of dough led to a slight decrease of MR, bread volumes stayed almost the same compared with the non-contaminated grain. The contamination of wheat withAspergillus andPenicillium led to a high decrease of gluten proteins, which resulted in an extremely decreased MR of the dough and a very low bread volume.

Deoxynivalenol and ochratoxin A in German wheat and changes of level in relation to storage parameters.

The occurrence of the mycotoxins deoxynivalenol (DON) and ochratoxin A (OTA) in the winter wheat of 1997 and 1998 grown under organic farming conditions was investigated using ELISAs (R-Biopharm) for quantification. The influence of delayed drying of the grain after harvest on the development of DON and OTA was determined in storage trials (moisture: 17% and 20%; temperature: 20 degrees C; duration: four and six weeks). The Tox5 PCR assay was used both to detect Fusarium species with the potential to produce trichothecenes and as a measure of their relative DNA content during the storage trials. The intensity of the PCR signals was correlated with the DON concentration. Fusarium species were identified microscopically by standard methods. All the freshly harvested grain samples were contaminated with DON and showed further increases in the DON concentration during storage. OTA contamination was found in 14.3% of the 1997 samples and in 24.1% of the 1998 samples. OTA increased during storage trials of the 1997 samples but not in the 1998 samples.

Impact of growth conditions on the occurrence ofFusarium spp. and the mycotoxin content of wheat.

In 1997-99 the occurrence ofFusarium spp. on winter wheat and the contamination with mycotoxins was investigated at three locations in the Rhineland, Germany. All cultivation methods investigated had an effect on the level ofFusarium infection, however, rainfall during flowering was the most important factor. The choice of cultivar and soil cultivation proved to be the most promising tools to reduce head scab severity and mycotoxin contamination.

Development of deoxynivalenol contents in relation to the PCR detection of potentially trichothecene producingFusarium spp. during storage of wheat.

Development of deoxynivalenol (DON) in wheat with a low contamination withFusarium spp. was investigated under suboptimal storage conditions (17% and 20% grain moisture, 20°C). The influence of storage on the relative DNA content of potential DON producers was also determined. The DON contents were quantified using an ELISA. The Tox5 PCR was used for the detection of potential trichothecene producers and for the estimation of their relative DNA content. ThegaoA gene was subsequently amplified by PCR to detect specificallyFusarium graminearum. The concentration ofF. graminearum DNA was semiquantitatively determined using a Light Cycler™. The DON concentrations increased during storage trials but the intensity of PCR signals decreased.