Aspergillus westerdijkiae

Aspergillus westerdijkiae belongs to Aspergillus section Circumdati, the yellow-spored aspergilli long grouped as the Aspergillus ochraceus group. The genus name comes from the Latin aspergillum, the brush used to sprinkle holy water, which the spore-bearing head resembles. The species name honours Johanna Westerdijk, the Dutch mycologist and long-serving director of the CBS fungal collection in Utrecht, the reference collection now named the Westerdijk Institute.

The species has a short but significant history. Ochratoxin A was first isolated in 1965 from a culture then identified as Aspergillus ochraceus, and for decades A. ochraceus was named as the principal ochratoxin-producing Aspergillus. In 2004 Frisvad and Samson used DNA to separate two species that had been grouped under the ochraceus name, A. westerdijkiae and A. steynii, and showed that most of the strains producing large amounts of the toxin were these. They also reassigned the original 1965 isolate to A. westerdijkiae. Much of the older literature that attributes ochratoxin on cured meat to A. ochraceus is now thought to describe this species.

Its importance to the curesmith is the toxin. Ochratoxin A is nephrotoxic, damaging chiefly to the kidney, and the International Agency for Research on Cancer classes it as a possible human carcinogen, group 2B. A. westerdijkiae produces it under the conditions of a ripening room. Toxin production is greatest at a water activity of about 0.94 to 0.97 and a temperature of about 20 to 25 degrees C, and none is produced at a water activity of 0.85. Warmth favours it, so it is most of concern in warmer climates and in the earlier, moister phase of ripening; the cold-tolerant Penicillium nordicum is the counterpart that continues in a cool cellar. It has been found on dry-cured hams and on the casings of dry fermented sausages, with the toxin recovered from the rind, so it is a current rather than a theoretical risk.

**How it endangers cured meat.** It grows on the surface, on the rind of dry-cured ham and on sausage casings during ripening, and produces ochratoxin A there. Growth is not a reliable warning, because the mould can produce toxin while forming little visible colony, so the control is preventive rather than by inspection.

The defence is competition on the surface rather than anything applied to the meat. A known noble white mould, usually Penicillium nalgiovense, is inoculated so that it establishes first and occupies the rind, and a protective house flora, including Debaryomyces hansenii and Staphylococcus xylosus, suppresses both this species and Penicillium nordicum. The physical controls are the ones the Safety Triangle already names: bring the water activity down, hold the microclimate steady, and remove any wild growth that appears.