Inoculation of airborne conidia of Penicillium chrysogenum on the surface of a solid medium

• Food products are often contaminated with few airborne conidia.
• A method to dilute dry conidia of Penicillium chrysogenum was described.
• Conidia were attached to glass beads transferred to successive Petri dishes.
• Experimental conditions allowing low colony counts, and decimal dilutions were provided.
• It was explained how the method can be extended to other fungi.

To reproduce a fungal contamination of food products by airborne conidia, a method to inoculate a few number (in the range 1–9) of conidia on the surface of agar media was developed. This technique would allow to determine accurately the time to detection of fungal colonies, then the mould free shelf-life of food products by using dry conidia. The method was based on dry-harvesting the conidia in the lid by gently taping the bottom of the dishes where sporulating mycelium was grown, retaining the conidia on glass beads, and, aseptically transferring the beads to successive Petri dishes to “dilute” the samples. Among the eleven factors tested by means of an experimental design, the most important factors were the incubation time of the sporulating mycelium, the resting time to dislodge as many conidia as possible from the lid, the number of beads and the number of successive dishes. The other factors were method used to produce conidia, the number of taps to remove as many conidia as possible from the sporulating culture before harvesting, the drop height of the harvest device, the number of taps to detached the harvested conidia from the lid, and the mixing times to attach conidia from the lid to the beads, and to detach conidia from the beads to the agar media. Decimal “dilutions” were achieved by transferring 10 beads to the successive dishes with a mixing time of 10 s. It was shown for Penicillium chrysogenum that an average of 3 colonies per dish were counted on the fourth of the successive dishes, for 3 days incubation time at 25 °C, 24 h resting time, and 10 beads.