If you’ve ever spotted yellow fluff on the surface of a cheese rind, you were probably looking at the mold Chrysosporium sulfureum. Considered a typical fungal species on the rinds of many classic French cheeses including Tomme de Savoie and Saint-Nectaire, this cave-loving fungus is widespread, but enigmatic. Here’s a summary of the little that we do know about this cheese fungus.
In the lab and on the surfaces of cheeses, Chrysosporium sulfureum colonies are white at first and eventually turn yellow when the fungus decides to reproduce and make spores. It tends to be very fluffy when it grows and can look like little yellow clouds. Old growths, especially those that have been flattened during handling of the cheese, can look like lichens growing on a rock. It generally grows distinct patches on the rind surface, but I have seen a few cheeses that are almost entirely yellow from growth of this mold. I have isolated other species of this fungus from cheese rinds that are white, not yellow. In the lab, this fungus secretes a considerable amount of an unknown yellow pigment into the medium, and a similar diffusion of yellow can be seen under the surface of heavily colonized cheese rinds.
Chrysosporium sulfureum is generally only found on natural rind cheeses. The fungus grows on cheeses where other fungi have already grown, meaning that it usually pops up on older cheeses after molds such as Penicillium and Scopulariopsis. I suspect (although have no data to prove!) that it may be a mycoparasite, using living or possibly dead fungi as a food source. It may also depend on these early colonizing molds to deacidify the cheese surface because it prefers a higher pH for growth (6-8). Relative to many other cheese molds, growth is quite slow both on the cheese and in the lab. This fungus prefers the cool temperatures of caves (~14-18°C).
The frequent washing of washed rind cheeses prevents the colonization of this and many other species of mold. Bloomy rind cheeses are generally not aged long enough for this mold to establish, although I have heard of it colonizing bloomy rind cheeses aged > 30 days when in the same facility as natural rind cheeses.
It’s unclear where in the environment the mold comes from, but Chrysosporium molds are quite common in natural caves so these fungi are typically only found on cheeses aged in a natural cave environment. It is quite common on cheeses that are aged in natural caves in the Auvergne and Pyrenees regions of France, but I have also seen it growing on cheeses in the UK as well as a few cheeses made and aged in the United States.
Interestingly, this fungus is not just limited to growth on cheese. In addition to being found in natural caves around the world, it has also been reported in the nests of leaf-cutting ants and in mushroom farms where it is considered a pest.
Our understanding of what this fungus might be doing to the cheese and how it makes a living is surprisingly limited. The French generally consider this mold a sign of a good cheese aging environment, and have quaintly named the mold “fleur jaune” (yellow flower). Some cheesemakers encourage its growth for these aesthetic reasons, although it’s possible that in some regions (e.g. the United States) consumers may be frightened by the appearance of this mold on food. I have heard from several different cheese producers or distributors that this fungus can impart a very bitter, medicinal flavor to the paste of smaller format cheeses if Chrysosporium sulfureum is abundant.
If you observed anything interesting about this mold, please share your experiences in the comments section below.
This fungus was formerly known as Sporotrichum aureum. It is also sometimes misspelled as Chrysosporium sulphureum. The currently accepted name and spelling is Chrysosporium sulfureum.
While clear reports (with DNA sequencing confirming the ID) of this mold as a pathogen of humans are not known from the literature, its potential impacts on human health and production of mycotoxins have not been evaluated.
Finally, it is not a bacterium, as some sources suggest. This is a fungus. And an enigmatic one that desperately needs some more research.
For more details on Chrysosporium sulfureum, check out the following papers:
Ropars, Jeanne, et al. “A taxonomic and ecological overview of cheese fungi.”International journal of food microbiology 155 (2012): 199-210. http://www.sciencedirect.com/science/article/pii/S0168160512000852
Ratomahenina, R, et al. “Study of Chrysosporium sulfureum, the mould responsible for Fleur jaune on Saint-Nectaire cheese. Milchwissenschaft 50 (1995): 266-267.
Post written by Benjamin Wolfe.