What are the microbes in my cheese and what are they doing? Part 2

Last month we started talking about the microbes that are used in cheesemaking, and explored many of the bacteria used in different cheeses. Here we will talk about the other major microbe group – fungus, including moulds and yeasts.

Let’s start with the cheesemaking mould you are probably all already familiar with; the appearance and taste of blue cheeses is rooted in the use of Penicillium roqueforti, which is of course used in the making of Roquefort, as well as all other blue cheeses. P. roqueforti is typically added as a starter culture to the milk at the start of the cheesemaking process, and it is during the aging period that it starts to take effect. P. roqueforti is a mould and therefore aerobic, which means that it can only grow in the presence of oxygen; this is why part of the aging of blue cheeses involves piercing the cheese with a metal rod to introduce air holes to allow the blue mould living inside the paste of the cheese to grow, and why blue cheeses are often wrapped in tin foil – by doing so, you remove air exposure to stop the blue mould growing too much.

When you look at a section of blue cheese, you may be able to see straight lines where the piercing took place, surrounded by veins of blue mould created due to the formation of fissures during the piercing which can resemble the shape of the pieces of curd that formed together during the cheesemaking process. The distinctive flavour of blue cheese is created by the breakdown of fat and protein (lipolysis and proteolysis) caused by the enzymes released by P. roqueforti (1).

There are several different strains of P. roqueforti with different properties; some can grow faster than others, some produce a stronger blue or green colour, and some can give slightly different flavours; for example, Gorgonzola is made with a sub-species of P. roqueforti, known as P. glaucum which produces slightly milder flavours. (2) Blue cheese is thought to have originally come about from contamination by a household blue mould that thrived in cold and damp cheese caves, and over the thousands of years that we’ve made and eaten cheese we have domesticated that species of blue mould (P. roqueforti) into one that creates the appearance and flavour of the blue cheese we eat today (3).

Whist now we inoculate our milk with freeze dried cultures out of a packet and cultured in a lab, traditionally blue cheeses would have been inoculated by crumbling up dried bread (on which the mould would have been propagated) into the milk (4). Evidence shows that we’ve been making and eating blue cheese made with P. roqueforti from as early as the Bronze age; investigation into 14th Century BC paleofaeces found in a mine in Hallstatt in Austria, found P. roqueforti DNA, indicating that that particular miner had some blue cheese for lunch (5)!

There is another mould in the Penicillium family that you would probably also recognise – Penicillium candidum, also known as Penicillium camemberti, which is the white mould often used in conjunction with Geotrichum candidum, and these two together create a textured white rind on the cheese and by breakdown of fats and proteins from the rind inwards can create the flavour compounds associated with soft white cheeses such as brie, camembert, and also Bix (6)! P. candidum, is also distantly related to the mould that produces penicillin (Penicillium rubens), and whilst P. candidum does have some mild antimicrobial properties, it, as well as P. roqueforti and P. commune (more on that in a bit!), does not produce penicillin or any molecule analogous to it, therefore cheeses made with P. candidum or P. roqueforti should be perfectly safe to eat if you have a penicillin allergy* (7). G. candidum, like many of the Penicillium species, is ubiquitous in the natural environment; technically classified a yeast, it is also commonly found in the soil, in the human gut microbiome, and in raw milk (8).

On some cheeses it is actually considered a spoilage organism and can cause rind slippage if allowed to grow unchecked (9). Both P. candidum and G. candidum tend to be added to the cheese starter cultures at the start of the cheesemaking process, but as moulds they produce spores as they grow which are released into the environment and thereby become part of the microenvironment of the cheese-aging room and can then go on to grow on cheeses that were not inoculated with these cultures at the start. This is why our Highmoor and Witheridge grow a light coating of P. candidum on the surface prior to packing, despite it not being one of the starter cultures we use during the cheese-make.

Another important aspect to consider when talking about cheese mould is spoilage moulds. The blue mould you sometimes have growing on cheese that has been in the fridge too long is not the same blue mould we’ve evolved to grow in a Stilton or Roquefort; although related to P. roquefortii, this is P. commune, a household mould that is the same one you might find on old bread (10). One of the roles of P. candidum and G. candidum is the prevention of the growth of these sorts of spoilage organisms on the rind (11). Rind washing with brine, as discussed in the last blog, is another method of protecting the rind against these moulds.

Most of these spoilage moulds are perfectly safe to eat in small quantities (if there’s one little spot on the rind of your white cheese – some cheesemakers like to refer to these as “beauty spots”!) but best avoided in larger amounts as they can produce some ‘off’ flavours; it used to be thought that some of these spoilage moulds, including P. commune, have the potential to produce mycotoxins (a type of toxin produced by fungi) that could make someone ill, however studies have shown that P. commune does not produce “significant or persistent quantities of mycotoxins” and so it will not cause harm (12).

I hope you enjoyed this foray into all things cheese mould, be sure to check in in the New Year to learn about how probiotics can improve your health!

* Penicillium nalgiovense does produce penicillin however (albeit in very very small amounts), and while these aren’t typically used in cheesemaking they can be found on cured meats, so if you have a penicillin allergy it would be best to avoid any cured meats with a white dusting on the surface!

References

1. Caron T, Piver M Le, Péron AC, Lieben P, Lavigne R, Brunel S, et al. Strong effect of Penicillium roqueforti populations on volatile and metabolic compounds responsible for aromas, flavor and texture in blue cheeses. Int J Food Microbiol [Internet]. 2021 Sep 16 [cited 2022 Oct 9];354. Available from: https://pubmed.ncbi.nlm.nih.gov/34103155/

2. Gillot G, Jany JL, Coton M, Le Floch G, Debaets S, Ropars J, et al. Insights into Penicillium roqueforti morphological and genetic diversity. PLoS One. 2015 Jun 19;10(6).

3. Ropars J, Caron T, Lo YC, Bennetot B, Giraud T. [The domestication of Penicillium cheese fungi]. C R Biol [Internet]. 2020 [cited 2022 Nov 23];343(2):155–76. Available from: https://pubmed.ncbi.nlm.nih.gov/33108120/

4. Blue cheese – what is the blue mould, and how does it come about? [Internet]. [cited 2022 Nov 23]. Available from: https://www.thecourtyarddairy.co.uk/blog/cheese-musings-and-tips/blue-cheese-what-is-the-blue-mould-and-how-does-it-come-about/

5. Maixner F, Sarhan MS, Huang KD, Tett A, Schoenafinger A, Zingale S, et al. Hallstatt miners consumed blue cheese and beer during the Iron Age and retained a non-Westernized gut microbiome until the Baroque period. Curr Biol [Internet]. 2021 Dec 12 [cited 2022 Nov 2];31(23):5149. Available from: /pmc/articles/PMC8660109/

6. Molimard P, Lesschaeve I, Issanchou S, Brousse M, Spinnler HE. Effect of the association of surface flora on the sensory properties of mould-ripened cheese. Lait [Internet]. 1997 [cited 2022 Oct 9];77(1):181–7. Available from: http://dx.doi.org/10.1051/lait:1997112

7. Laich F, Fierro F, Martín JF. Production of Penicillin by Fungi Growing on Food Products: Identification of a Complete Penicillin Gene Cluster in Penicillium griseofulvum and a Truncated Cluster in Penicillium verrucosum. Appl Environ Microbiol [Internet]. 2002 [cited 2022 Nov 23];68(3):1211. Available from: /pmc/articles/PMC123731/

8. Tinsley CR, Jacques N, Lucas M, Grondin C, Legras JL, Casaregola S. Molecular Genetic Analysis with Microsatellite‐like Loci Reveals Specific Dairy‐Associated and Environmental Populations of the Yeast Geotrichum candidum. Microorganisms [Internet]. 2022 Jan 1 [cited 2022 Oct 9];10(1):103. Available from: https://www.mdpi.com/2076-2607/10/1/103/htm

9. Eliskases-Lechner F, Guéguen M, Panoff JM. Yeasts and Molds | Geotrichum candidum. Encycl Dairy Sci Second Ed. 2011 Jan 1;765–71.

10. Penicillium commune | NBN Atlas [Internet]. [cited 2022 Nov 23]. Available from: https://species.nbnatlas.org/species/BMSSYS0000012954

11. Decker M, Nielsen PV. The inhibitory effect of Penicillium camemberti and Geotruchum candidum on the associated funga of white mould cheese. Int J Food Microbiol [Internet]. 2005 Sep 25 [cited 2022 Oct 9];104(1):51–60. Available from: https://pubmed.ncbi.nlm.nih.gov/16083983/

12. Neaves P, Williams AP, Law BA, Tamime AY. Microbiological Surveillance and Control in Cheese Manufacture. Technol Cheesemaking Second Ed [Internet]. 2010 May 11 [cited 2022 Dec 2];384–412. Available from: https://onlinelibrary.wiley.com/doi/full/10.1002/9781444323740.ch11