A study to determine whether on-floor ambient drying systems are conducive to the formation of ochratoxin A in grain

Abstract

The problem of mould developing in grain has always been one of the risks associated with storage. However, the commercial significance of this problem has become more acute with the discovery that some species of mould may produce toxic mycotoxins during their growth. For example, ochratoxin A. is produced by a species of Penicillium mould that develops only during storage.

Surveys of UK grain have shown that ochratoxin A is found in about 10% of samples tested and sometimes at levels that exceed the likely EU tolerance level of 3-5ppb. The reason for these occurrences is not completely understood as the mould that produces the toxin will only grow at moisture contents above 16 - 17%. One possibility is that the conditions occurring during on-floor drying might be suitable for mould growth and toxin development. Therefore, the HGCA funded an investigation into mould and toxin development during on-floor drying. The work was commissioned in June 1998 and required the assessment of grain stored and dried on farm during the 1998 harvest, with the results being available before the following harvest.

24 farms spread from the South coast to North-East Lincolnshire were selected, all using on-floor drying systems. Samples of grain were collected from set points in each grain bulk immediately after harvest and then at intervals during drying and storage. The moisture content and temperature of the grain at the sample points was recorded and a number of selected samples were analysed for ochratoxin A and checked for mould flora.

240 samples were collected and out of 108 analysed, ochratoxin A was detected in only 8 on 4 of the farms, mostly at a low level. The mould associated with ochratoxin A formation was found in samples from 3 of these farms. The highest level detected was 5.9 ppb. In every case where mould increased and toxin was detected, the moisture content of the grain was above about 19% and drying was slow. These results show that on-floor drying does not carry an inherent risk of the development of ochratoxin A but when grain enters storage at a moisture content of 18% or more and drying is too slow, there is a serious risk that mould will develop and toxins may be produced.

Immediately after harvest grain was infected with a range of field fungi but these usually died out over time. In some cases, when the moisture content remained above 16 - 17%, a range of storage fungi replaced the field species. At higher moistures, these sometimes included a species of Penicillium known to produce ochratoxin A. The rate at which mould grows in stored grain is affected by temperature as well as moisture. During on-floor drying grain temperatures generally fell and this cooling is likely to have extended the drying/storage period before any toxin was detected.

The results from this work validate limited laboratory studies done to explore the conditions under which ochratoxin A can be produced in stored grain. This has allowed a risk assessment system to be offered that should provide effective advice to farmers, enabling them to minimise risks of mould or mycotoxins developing in their grain during on-floor drying and subsequent storage

This work could not have been completed without the assistance and co-operation of the farmers involved and sectors of the grain trade.