Improper handling of grain may result in high levels of Ochratoxin A
By Susanne Elmholt, Maiken Schioldann Haase, Department of Agroecology, Danish Institute of Agricultural Sciences
Improper handling of grain may result in the formation of the mycotoxin, Ochratoxin A (OTA). In temperate conditions, OTA is produced by Penicillium verrucosum. It is the cause of porcine nephropathy and regarded as being carcinogenic. It is, however, not produced in all conditions and an increased knowledge on the ecology of P. verrucosum will improve our possibilities to prevent fungal dispersal, growth and mycotoxin production.
Surveillance reports from the Danish Veterinary and Food Administration indicate more severe problems with OTA in organic than in conventional farms, although problems seem decreasing during recent years.
At the Danish Institute of Agricultural Sciences, we have been studying the ecology of P. verrucosum. Our results indicate that the fungus is more frequently occurring in organically than in conventionally cultivated soils, but in conclusion our results do not support the hypothesis that the organic farming system as such presents a higher risk of OTA contamination. Rather, the problems seem related to improper management, either pre- and/or post-harvest. The individual organic farmer may address OTA problems by assuring that the grain is quickly and efficiently dried. It is also important that drying and storage facilities are properly cleaned in order to prevent pockets of old grain from contaminating the newly harvested grain.
Fungal dispersal via dryers
During autumn 2000, sampling was performed in rye and oats at a farm, where we had earlier found very high abundancies of P. verrucosum in dried grain. The study was part of a Master's Thesis at the University of Aarhus and associated to a DARCOF project on mycotoxin prevention in organic farming. The farm had a home-made natural air-drying system (7.5 kW). The main duct was made of chipboard, internally covered by plastic. The side-ducts were made of boards with bottoms of plywood and covered by old grain sacks made of hessian, some of which being 20 years old. The rye was placed to a height of approx. 0.7 m and oats to a height of approx. 1 m. The farmer judged that heating of the drying air was unnecessary. Using a grain sampler, we sampled in the top and bottom layer of the rye (46-82 and 13-24 cm over floor level, respectively) and in the top layer of the oats. From August to November, a statistically significant increase was found in the number of both rye and oats kernels that were contaminated with P. verrucosum (Figure 1). The bottom layer of the rye contained more kernels with P. verrucosum than the top layer. It is most likely that contamination has taken place via the dryer as both the ducts and the hessian were found to contain large amounts of P. verrucosum conidia. These will inevitable be dispersed into the grain.
Large amounts of OTA in some samples
When the fungal analyses were completed, the remaining grain was placed at 2˚C. The relative humidity in the room was high, and because the packaging was not completely air tight, the moisture content of the grain increased. After 18 months it was approx. 23 % in all samples of rye. At this time The Veterinary and Food Administration analysed the OTA content in some of the samples. Both top- and bottom samples of rye from the first four samplings were below the accepted limit of grain for human consumption (5µg OTA/kg). So were all bottom samples of rye from the fifth sampling while all top samples from the fifth sampling exceeded this limit by 10-70 times! We do not know for sure whether OTA was produced during drying or during the following cool storage - or perhaps both. After a further six months' storage in these conditions, however, some of the top samples from the fifth sampling were analysed again and the same level of OTA was found this time, indicating that no OTA had been produced during these six months. It is known, however, that P. verrucosum is able to grow at 2˚C, and German studies have shown that the fungus can actually produce OTA at 4˚C in grain with water contents above 20 %.
Preventing OTA problems
Improper handling of grain may lead to OTA levels far beyond the Danish limit of 5 µg/kg grain. We are, however, still lacking knowledge on, why P. verrucosum - under apparently similar conditions - produces OTA in some cases and not in others. This emphasizes the need to prevent grain from being contaminated with P. verrucosum if at all possible and based on so-far obtained knowledge we can give the following pieces of advice:
- P. verrucosum is present in some soils, apparently more frequently in organically than conventionally cultivated soils. Therefore, seek to avoid soil contamination as much as possible during harvest
- Field trials do not indicate that P. verrucosum contamination of seed constitute a risk of grain contamination in the field. Yet we often find P. verrucosum on newly harvested grain, and especially damaged kernels will be vulnerable to infection. Assure that harvest equipment is properly cleaned and adjust the combine harvester so kernels are minimally damaged
- Rapid drying is very important. Earlier studies of P. verrucosum at three organic farms with on-farm drying showed major differences. At two farms with natural air-drying and in-bin drying, respectively, we found high contaminations by P. verrucosum after drying, while the third farm with re-circulation drying had no contamination. This pattern was reproduced during three consecutive years. Problems were obviously not related to farming system but rather to improper drying practice
- Winnowing the grain is often necessary - especially in organic farming - in order to reduce the risk of establishing humid "pockets" of weed seeds, unripe grain, soil etc. during drying and storage. If such pockets contain P. verrucosum this may lead to OTA-formation perhaps even at very low temperatures. The latter awaits further studies
- Results from the Master's Thesis show that dryers that are difficult to clean, can contaminate the grain with conidia of P. verrucosum - especially of course if the drying process is slow. Large amounts of conidia will not always result in high levels of OTA but they do constitute a risk if the fungus obtains suitable outer conditions for toxin formation. This was shown in our experiments - no matter whether OTA was formed during on-farm drying or during the following laboratory storage. Therefore: It is important that the drying facilities are properly cleaned, so that conidia are removed and not just stirred up. Using an efficient vacuum cleaner can be recommended, preferably with a HEPA filter or similar to assure that conidia are actually trapped in the vacuum and not blown through the exhaust
Elmholt, S. 2003: Ecology of the ochratoxin A producing Penicillium verrucosum: Occurrence in field soil and grain with special attention to farming system and on-farm drying practices. Biological Agriculture and Horticulture 20, 311-337.
Elmholt, S. & Hestbjerg, H. 2000: Field ecology of the ochratoxin A-producing Penicillium verrucosum: Survival and resource colonisation in soil. Mycopathologia 147, 67-81.
Elmholt, S., R. Labouriau, H. Hestbjerg & J.M. Nielsen (1999) Detection and estimation of conidial abundance of Penicillium verrucosum in soil by dilution plating on a selective and diagnostic agar medium (DYSG). Mycological Research 103, 887-895.
Haase, M.S. (2003). Forebyggelse af mykotoksindannelse i økologisk brødkorn -med fokus på Penicillium verrucosum og dannelse af ochratoksin A (Preventing Mycotoxin Problems in Organic Farming - with Focus on Penicillium verrucosum and Ochratoxin A). Master's Thesis (in Danish), University of Aarhus, Aarhus. The thesis will be available from May 26th .