As I always do, I posted the link on Facebook and then sat back a little surprised as the Facebook comment thread lengthened.  Penicillium glaucum is best known as the Gorgonzola mould but also, according to Ivan Larcher, very suitable for use in goats cheese and naturally occurring on rinds of Loire cheeses like Valancay.  Was this the same as was sourced from Coquardand used at Sleight Farm?  No, that’s Penicillium album, but it looks similar.  Is Penicillium glaucum also in Stilton (it’s sometimes referred to in textbooks as Roqueforti (Glaucum)).  Are Penicillium album and glaucum the same?

This was above my knowledge level, but when something’s a bit out of my league, I like to catch up and understand.  Luckily for me, Paul Thomas of ThimbleCheesemakers and a biochemist to boot, is very generous with his explanations and very good at explaining things so that the idiot (that would be me) can follow.

I will basically copy and paste what he emailed me because he says it better than I can.

 ‘As a quick background to microbiological classification, historically scientists would have peered down the microscope to attempt to assemble a vast number of species into some kind of order.

This creates some problems.  Some microbes have stages in their life cycle during which they may behave in different ways (which may lead to the identification of two species that are actually one).  There may be an element of variation within a species (that may or may not deserve subsequent separation into two species).  And, given the vast diversity of species it is possible to incorrectly identify a species of assign it to the wrong group.  If incorrectly assigned would it then perhaps lead to incorrect assumptions about the characteristics specific to the group and increase the likelihood of further errors in classification of other species?

The introduction of molecular biology techniques such as DNA sequencing makes it easier to classify species now according to genomic similarity.

So, Penicillium is a Genus (Kingdom, Phylum, Class, Order, Family, Genus, Species) and roqueforti, camemberti, album or glaucum are all species names.

P. glaucum is more commonly called P. expansum outside of cheese circles and is sometimes used to describe the light green mould found in Gorgonzola. I believe that David Jowett* was referencing Walker-Tisdale & Woodnut when he said that stilton had been described as being veined with “P. roqueforti (P.glaucum)”.  I’ve never seen it available as a culture – or at least not described as such.  I suppose it is possible that some of the milder P. roqueforti cultures may turn out to be glaucum – or it may turn out that what is often attributed to P. glaucum in Gorgonzola production is actually a low-pigment, low-methyl ketone (the group of compounds which cause the distinctive ‘blue cheese’ taste) form of P. roqueforti.  The cultures almost certainly predate any interest in sequencing them.

P. expansum/ glaucum is associated principally with decay in apples. Citrus Green Mould (P. digitatum) is very closely related to P.expansum.

P. album is now called Gliocladium album (it has been reclassified into another genus). This is the one we would commonly associate with the greyish appearance of the rind of a Loire goats cheese. Coquard sell a product described as P.album that produces a rind with an appearance compatible with that of a Loire goats cheese but, of course, it may be possible that natural mould growth in either the surface ripened goats cheese or a farmhouse gorgonzola may actually consist of several Penicillium (and Gliocladium) species.

While use of pasteurisation dramatically reduces the bacterial diversity of the finished cheese, it has less of an impact upon moulds.  Moulds present in very low numbers in the raw milk and are simply a representation of the moulds present environmentally and therefore continuously inoculated into the milk/curd/cheese. With Lyburn’s Stoney Cross, I saw considerable rind diversity (including some Sporendonema) despite the pasteurisation of the milk.

On a related subject, as I know you are keen to express the natural microflora of the milk, it may prove to be impossible to influence one Penicillium over another as the species present similar requirements with regard to temperature, pH and moisture.  Darker, more methyl ketone-producing strains seem to tend to dominate in natural blueing – either because they are more dominant and outcompete the milder strains or because they are simply more noticeable.  Termignon is a case worth studying in this instance and, I imagine, similar to the pre-industrial two-curd Gorgonzola.’

Back to my take on the situation.

At the end of the day, to my mind, there are a couple of key areas that confuse the cheesey mouldy world that Paul addressed.  Re-classification and new technologies that post date the accumulated knowledge of even pretty technical cheesemakers confuse us with new names that may or may not correlate with what has been written in the past and our sources for information may or may not be up to date with the latest nomenclature.  However also there is the environmental factor.  What we believe to be in the cheese be it Gorgonzola, Persille de Beaujolais, Fourme d’Ambert may be a glorious mixture of what you put into the milk and what the environment favours.  What you buy from Coquard in terms of a rind culture to allow it to resemble a Loire cheese, may actually not be present when your cheese is finally mature as your own environment’s influences take effect.

Back to that eternal balancing act that the cheesemaker somehow has to manage.  Cheese is an expression of your milk, your environment and your skills.  And thank God for that!