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Beyond B. linens: How cheese rinds get their color

When it comes to giving credit for those pungent orange rinds, most people cite the bacterium B. linens (formally Brevibacterium linens). But recent research suggests that B. linens may be getting way too much credit for cheese rind color. Numerous other bacteria and fungi can create distinct pigments on the rinds of cheeses.

My own experience isolating microbes from cheese rinds and other published research suggests that some highly pigmented rinds have very little B. linens present. For example, in a washed rind cheese from California with a typical orange colored surface, we didn’t detect Brevibacterium. Most of the bacteria we found weren’t even added to the cheese as a starter culture, but probably came from the sea salt used in the brines for the cheese.

The surface of a washed rind cheese (left) from California has a typical orange color. But orange colonies of Brevibacterium linens (commonly called “B. linens”) are nowhere to be found when we look at the microbes that live in the rind (right). These are Proteobacteria, which are light brown, clear or tan in color. This observation and other recent research suggest there are many more microbes out there that give washed rinds their characteristic pigments. Left photo by Adam Detour, right photo by Benjamin Wolfe.

 

So if Brevibacterium isn’t always responsible for washed rind color, what microbes are? Here’s an overview of some of the lesser known pigment producers:

Fusarium domesticum, known in the starter culture world as Mycodore and previously known as Trichothecium domesticum, is a mold commonly added to washed rind cheeses. When grow in the lab in a cheese-like medium, it produces orange pigments on the cheese surface. Eventually the mold surface is covered in short white fuzz. The specific compound responsible for orange coloration of this mold has not yet been identified, but Fusarium species can produce carotenoid compounds similar to those produced by Brevibacterium. Many washed rind cheeses have this mold growing on their surface and likely get some of their color from Fusarium.

The lab cheese on the left is not colonized by microbes. The cheese on the right with the pigmentation is colonized by Fusarium domesticum. Photo by Benjamin Wolfe.

 

Rhodosporidium, known in the starter culture world as R2R, is a red yeast that is also commonly added to washed rind cheeses. This yeast is incredibly slow-growing and is sometimes hard to get to establish on rinds. When it does grow it makes beautifully pink washed rinds. The yeast is pink in color because it produces a type of carotenoid.

Colonies of the red yeast Rhodosporidium growing in the lab

Colonies of the red yeast Rhodosporidium growing in the lab. This pink yeast contributes reddish-pink pigments to the surfaces of washed rind cheeses. Photo by Benjamin Wolfe.

 

Some Staphylococcus species, including Staphylococcus xylosus, are orangish-yellow color and can cause orange pigmentation on the surfaces of cheeses. These benign (non-pathogenic) Staphylococcus species tend to thrive on the surfaces of drier natural rind cheeses, but Staphylococcus xylosus can thrive on wet washed rinds where it rapidly colonizes the surface early in rind development. The compound that gives these species their color is staphyloxanthin, which is also a type of carotenoid.

A colony of Brevibacterium linens growing next to Staphylococcus xylosus. Some Staphylococcus species can produce orange pigments that give washed rinds their characteristic color. Photo by Benjamin Wolfe.

 

Bacterial-fungal interactions, and not the microbes themselves, can result in pigmentation on cheese rinds. One example of microbial interactions producing pigments comes from when the bacterium Arthrobacter interacts with a mold, such as Fusarium, Penicillium, or Scopulariopsis. In our large survey of rind microbiology, we found that when grown alone, Penicillium and Arthobacter don’t produce pigments. But when we grow the two microbes together, they form a pink pigment that is secreted into the cheese curd.

When the bacterium Arthrobacter (top) or the fungus Penicillium (bottom) are grown alone, no pigments are produced. When they are mixed together, Arthrobacter secretes a pink compound.

We still don’t know what these pigments are or why the microbes produce them, but it’s likely that some of the pink colorations that form on ripening cheese rinds result from similar bacterial-fungal interactions.

 

We can’t completely ignore Brevibacterium when talking about rind pigments. It is definitely a widespread bacterium that can be found on the rinds of many cheeses. Cheesemakers can purchase Brevibacterium as a culture to be added to their cheese, but it’s also commonly found in raw milk and can be part of raw milk cheese rinds.

One overlooked aspect of Brevibacterium: there’s more biodiversity in Brevibacterium than just B. linens (“B.” being the abbreviation for the genus name Brevibacterium and “linens” being the abbreviation for the species name… see this page for a refresher on scientific naming of organisms). A quick check in GenBank (a large database that includes DNA sequences of microbial species) shows hits to numerous Brevibacterium species. Oh, and that “B. linens” that gets all the credit… some scientists have proposed we change the name to B. auranticum.

So the next time someone tells you it’s all about the B. linens, tell them there’s more to that orange cheese rind than they may think.

 

For more details on pigment-producing cheese rind micobes, check out the following resources:
Bachmann, H. P., et al. “Occurrence and significance of Fusarium domesticum alias Anticollanti on smear-ripened cheeses.” LWT-Food Science and Technology 38 (2005): 399-407. http://www.sciencedirect.com/science/article/pii/S002364380400163X

Buzzini, Pietro, et al. “Carotenoid profiles of yeasts belonging to the genera Rhodotorula, Rhodosporidium, Sporobolomyces, and Sporidiobolus.” Canadian Journal of Microbiology 53 (2007): 1024-1031. http://www.nrcresearchpress.com/doi/abs/10.1139/W07-068

Wolfe, Benjamin E., et al. “Cheese Rind Communities Provide Tractable Systems for In Situ and In Vitro Studies of Microbial Diversity.” Cell 158 (2014): 422-433. http://www.sciencedirect.com/science/article/pii/S0092867414007454

 

Post written by Benjamin Wolfe. Header photo by Adam Detour, with styling by Catrine Kelty.

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