Influence of storage conditions of apples on growth and patulin production by Penicillium expansum

Penicillium expansum causes blue mould rot, a serious post-harvest disease of apples, and is the main producer of the mycotoxin patulin. The present study aimed to determine the influence of storage conditions (i.e. temperature and O2 level) on growth and patulin production by different P. expansum strains on a simulation medium and on apples. Growth was strongly influenced by the temperature, while the used atmosphere (20, 3, and 1% O2; < 1% CO2) had no effect. Optimal growth was observed at 25 °C for every strain tested. Patulin production was stimulated when the temperature decreased (from 20 to 10 or 4 °C), while a further decrease of the temperature to 1 °C caused a reduction in patulin production. The temperature at which the stimulation was changed into suppression was strain dependent. Similar results were observed for the O2 level. A reduction of the O2 level from 20 to 3% O2 could stimulate or suppress patulin production depending on the strain, while a clear decrease of the patulin production was observed when the O2 level was reduced from 3 to 1%. These results show that the induction of limited stress to the fungus, such as lowering the temperature or lowering the O2 levels stimulates patulin production. However, the combination of different stress conditions (e.g. low temperature and low O2) will result in a reduced formation of the toxin. The combination of stress conditions, at which the transition from stimulation to suppression is observed, is strain dependent. Moreover, patulin production is characterized by a high natural variability. The presented results show that the temperature and O2 level has to be as low as possible during the storage of apples in order to suppress patulin production and to guarantee food safety.