Penicillium verrucosum

Penicillium verrucosum is the close relative of Penicillium nordicum and the second of the two Penicillium species known to produce ochratoxin A. Its name is descriptive: verrucosum is Latin for warty, after the roughened, verrucose stipes that carry its spore-bearing brushes. It was described by the Belgian mycologist Frans Dierckx in 1901, and for much of the last century its identity was tangled, confused with Penicillium viridicatum and others until careful comparison of growth, appearance and metabolites separated them; the meat specialist Penicillium nordicum was finally distinguished from it around 2001. The two remaining ochratoxin producers among the penicillia are phylogenetically close and alike to the eye, separated reliably by their metabolite profiles and by molecular methods rather than by inspection.

It is a slow-growing, cold-loving mould. On standard laboratory media it reaches only some fifteen to twenty-five millimetres across after a week at 25 C, which is 77 F, with a white mycelium and dull green to greyish-green spores, and it grows across a broad cold range, from around freezing to about 30 C, that is from roughly 32 to 86 F, and down to a water activity near 0.80. Above all it is a mould of cereals. It is endemic in the wheat, barley, rye and oats of northern Europe and Canada, the main source of ochratoxin A in grain in cool and temperate climates, and therefore the main route of human exposure to the toxin through cereal foods. It can also occur on brined olives and cheeses, and, less commonly than P. nordicum, on dry-cured meats. Alongside ochratoxin A it produces a second nephrotoxic mycotoxin, citrinin. Under low-salt conditions it tends towards citrinin, and as salt rises its output shifts towards ochratoxin A, but it is generally a more moderate and more condition-dependent ochratoxin producer than P. nordicum, and less at home in salt: its growth falls away as salt increases, roughly halving near eight per cent, where P. nordicum is instead at its most vigorous.

**How it endangers cured meat.** On cured meat it plays the same part as P. nordicum, though it is the less frequent of the two: a surface mould that can produce ochratoxin A on a ripening rind or casing, with the salted surface tending to shift its metabolism towards the toxin. Ochratoxin A is nephrotoxic and is classed by the International Agency for Research on Cancer as a possible human carcinogen, group 2B, and it is stable enough to survive later processing, so it cannot be removed once formed.

The control is the same as for the other ochratoxigenic moulds: a noble white mould, usually Penicillium nalgiovense, inoculated to hold the surface, a protective house flora including the yeast Debaryomyces hansenii, and management of water activity, temperature and humidity through ripening. Because it is chiefly a grain mould, sound dry storage of any cereal-based ingredients and of the raw materials also reduces the chance of carrying it onto the product in the first place.