Penicillium nordicum

Penicillium nordicum is the toxigenic mould most associated with dry-cured meat. It is a Penicillium, one of the brush moulds, but on a salted rind it forms a pale surface growth that closely resembles the noble white mould Penicillium nalgiovense used to season sausages and hams. The two are difficult to tell apart by eye, and reliable separation needs the laboratory. That resemblance is the centre of the problem: a mould that looks like the desirable one can produce ochratoxin A.

It is adapted to salt and to protein-rich foods, and it is found chiefly on dry-cured meats and salted cheese, occasionally on salt itself. It has been recovered from the air of meat-processing rooms and, in several cases, from the seasoning salt used on the product, so both the environment and the salt are possible sources. It was distinguished from its close relative Penicillium verrucosum around 2001. The two are phylogenetically close and look alike, but differ in their ecology and their metabolites. P. nordicum is the more consistent and higher ochratoxin producer of the two, and the main ochratoxin-producing species on cured meat in temperate regions.

Its most important feature for the curesmith is unexpected. Salt, which is added to control bacteria and to lower the water activity, increases ochratoxin A production in this species. It produces the toxin across a wide range of salt concentrations, and adding salt within that range raises the amount, because salt induces the genes for ochratoxin synthesis. It is also cold-tolerant, producing toxin during long, cool ripening below 15 degrees C, so a cold cellar does not prevent it, unlike the warm-favouring Aspergillus species. Ochratoxin A is nephrotoxic, damaging to the kidney, and the International Agency for Research on Cancer classes it as a possible human carcinogen, group 2B. The toxin is stable and is not removed by later processing.

**How it endangers cured meat.** It colonises the rind of dry-cured ham and the casing of dry fermented sausage during ripening and produces ochratoxin A there. Because it resembles the noble mould it can establish unnoticed, and because it tolerates salt and cool temperatures the ordinary conditions of a curing cellar do not hold it back; salt, if anything, encourages the toxin. Visible growth is not a reliable guide, since toxin can be present without heavy colonisation.

The control is to occupy the surface deliberately rather than leave it to chance. A known noble white mould, usually Penicillium nalgiovense, is inoculated so that it establishes first, and a protective house flora, in particular the yeast Debaryomyces hansenii, suppresses P. nordicum and its toxin. Managing water activity, temperature, humidity and air movement during ripening reduces production, and replacing part of the sodium chloride with potassium chloride, or adding sugar, has been shown to lower it.

A word on strains, to match the noble moulds it imitates. Toxin production in these Penicillia is a strain-level trait, not simply a species one: ochratoxin output varies between Penicillium nordicum isolates, and genuinely atoxigenic strains exist, some studied as biocontrol agents that crowd out their toxigenic relatives on ham. That variation is no comfort at the bench, though, because the strains cannot be told apart on the rind, only by molecular typing or metabolite analysis in a laboratory, and because P. nordicum also resembles the noble Penicillium nalgiovense, an unidentified pale mould on a product must be treated as a toxin risk whatever its strain. The lesson is the same one the noble species teach, read the other way round: since toxigenicity travels with the strain and not the look, safety comes from establishing a known, screened culture on the surface, never from trusting the wild mould that happens to settle.