Validation of a model to avoid conditions favouring Ochratoxin A production during ambient-air drying

Abstract

Ochratoxin A (OTA) is a fungal mycotoxin that may form when grain moisture content exceeds 18%. EU food limits are set at 5ppb. In a previous HGCA project the risk of OTA formation during drying was assessed using the simulation tool 'Storedry', substituting the old biodeterioration criteria of visible mould or germination loss with a new model for avoiding OTA formation.

This model was based on the time taken before fungal growth enters the rapid growth phase. Shorter safe storage times were predicted using the new model. The present study aimed to compare these predictions with real, full scale drying processes.

Four bins each of 30 tons of wheat were dried using ambient air in two seasons. Physical parameters (e.g. moisture and temperature in the wheat and biological changes including mycotoxin production) were measured to provide data to validate the model. Valuable data on changes in populations of fungi were also collected.

The simulation, 'Storedry', gave good predictions of the general drying behaviour and final moisture content. However, it underestimated drying time by about 20%, mainly because the exhaust air in the simulation was less saturated than in the experiment and so the prediction of exhaust humidity by Storedry needs to be improved.

The prediction of biodeterioration using the new OTA model was found to be too rapid. Experimental data indicated a safe storage time at least two times that predicted by the model even when the wheat had been inoculated with Penicillium verrucosum, the main cause of OTA in UK cereals.

Based on this, simulations were run with safe storage times increased by a factor of two. From these simulations it was concluded that, for drying with continuous ventilation, the new OTA model was no more demanding than the well established model based on visible mould and significant loss of viability. Therefore, when drying by continuous ventilation, the risk of OTA appears no greater than the risk of visible mould or significant loss of viability.

For drying by continuous ventilation, the time for the drying front to pass through the bed does not need to be reduced. Hence, recommendations for bed depth and airflow rate in The Grain Storage Guide (HGCA, 2003) remains appropriate for continuous ventilation.

Drying rate does not need to depend on the amount of inoculum present.