The curious case of the purple cheese rind

When you buy an aged cheese at the store, you expect it to have a specific appearance. Camembert should be white and fuzzy. Limburger should be orange and sticky. Cheese makers work hard to manage aesthetics, but sometimes unplanned colors or textures cause cheese defects. In the past few years, cheese rinds around the world have started turning an unusual purple color. In this Science Digested, I’ll explain how my lab discovered the microbial cause of this purple rind defect.

In 2015, we received a call from a cheese maker in the United States. They had been making a washed-rind cheese using raw cow milk. Washed-rind cheeses normally have an orange appearance due to the growth of specific ripening microbes that produce yellow, orange, and red pigments. But all wheels of cheese across multiple batches were turning an intense purple color (see photo below). We named this phenomenon purple rind defect.


The left photo shows multiple wheels of cheese with purple rind defect. The orange wheel in the front is most similar in color to a typical washed-rind cheese. The photo on the right shows one section of a wheel heavily impacted by the defect. Photos by Benjamin Wolfe.

This cheese defect was a surprise to both the cheese maker and to me. I called many cheese makers and microbiologists, and none of them had observed purple rind defect before. Purple is not a very common pigment in the microbial world. There are some specific groups of bacteria, including the purple sulfur and purple non-sulfur bacteria that can appear purple. The bacterium Chromobacterium violaceum is also purple. But neither of these groups of bacteria grow on cheese.

When we plated out the cheese in the lab, we noticed that purple sections of the cheese had a high abundance of the bacterium Proteus (see images of Petri dishes below). After exploring previously published research, we quickly realized that Proteus may be the culprit. Proteus species are sometimes associated with a striking medical phenomenon known as purple urine bag syndrome. When hospitalized patients with urine bags get a urinary tract infection with Proteus or another species of Gammaproteobacteria, the bacteria can convert the amino acid tryptophan into the pigments indigo and indirubin. Indigo is the blue dye used to add color to blue jeans and other fabrics. Indirubin is a pigment with a similar chemical structure, but is more red in color. The mix of blue indigo and red indirubin leads to purple pigmentation.

Petri dishes showing colonies from normal orange rinds (left) and rinds with purple rind defect (right)

Petri dishes showing colonies from normal orange rinds (left) and rinds with purple rind defect (right). B = Brevibacterium sp.. Ps = Psychrobacter sp. S = Staphylococcus equorum. M = Marinilactibacillus. Pr = Proteus sp. Photos by Benjamin Wolfe.

To confirm that Proteus causes purple rind defect, we performed two different experiments. Using chemical analyses, we isolated the pigments and determined the purple pigment produced by Proteus was indeed composed of indigo and indirubin. In the process, we also discovered that another bacterium from this cheese rind, a Psychrobacter species, can also make indigo and indirubin. We also used experimental cheese communities to try to recreate purple rind defect. Control cheese communities with standard cheese rind bacteria turned orange brown as expected (see photo below). Cheese communities with Proteus added turned purple. Cheese communities with only Psychrobacter added turned a reddish-brown color. All of these results confirmed that Proteus can produce indigo and indirubin and is likely the main cause of purple rind defect.

Experimental cheese communities demonstrated that Proteus causes purple rind defect.

Experimental cheese rind communities demonstrated that Proteus can cause purple rind defect. On the left shows the color of the control cheese community with a typical bacterial composition. On the right shows purple pigment production when Proteus was added to experiment cheese communities. Photos by Benjamin Wolfe.

Purple rind defect is not isolated to this one incident in the United States. On this website, we have received a report of purple rind defect caused by Proteus in Turkey (see comments section of this page). Rinds of a washed-rind cheese in the United Kingdom have also been reported to turn purple (see photo below). It is unclear whether purple rind defect is a new problem that cheese makers will continue to face or if it simply had not been formally described before.

IMG_1188 (1)

A washed-rind cheese in the UK with signs of purple rind defect. Photo by Bronwen Percival.

What can cheese makers do to manage purple rind defect? Our research did not specifically address this, but we do make several suggestions for potential management strategies. We found high amounts of Proteus in raw cow milk used in the production of this cheese suggesting that herd hygiene and milk quality management can help control purple rind defect. The addition of starter cultures that might outcompete Proteus may also provide a reduction in purple pigment production. Proteus, Psychrobacter, and other Gammaproteobacteria that live on cheese rinds like high moisture conditions. Working to dry out the rind surface may also help reduce purple rind defect.

Have you observed purple rind defect before? Have you developed successful management approaches for this defect? Tell us about your experiences in the comments section below.

Post written by Benjamin Wolfe, Assistant Professor of Microbiology at Tufts University.

For more details on the study, please see Kamelamela, Noelani, et al. “Indigo- and indirubin-producing strains of Proteus and Psychrobacter are associated with purple rind defect in a surface-ripened cheese.” Food Microbiology 76:543-552.

There are 13 comments on this article

  • jessica says:

    ? very interesting. I have been working as a cheese monger for over 10 years and only recently have I encountered this issues. However my experience has been cheese that once cut open the interior or
    Paste turned deep purple . We faced the cheese and just underneath the exposed paste it would be normal color but several
    Hours later the newly faced paste turned purple again. This has happened maybe two or three times in the past two years and they were always soft ripened cheeses ( a beer washed rind and a bloomy rind… both from California). Would love to know more as you discover more information.

    Reply to this comment
  • Aliza Jacobsohn Levy says:

    Can you, please, detail the Proteus spp and Psychrobacter spp bacteria that you isolated from the cheese and caused the purple phenomena?

    Reply to this comment
  • Sara Baron says:

    I found this article searching for purple mold on food (I know now it is not mold). I had some fresh mozzarella that turned a light purple after I left it in a Tupperware for too long. It remained wet. I’m also in California. I’ve also had some of the fallen trees behind my house get a lovely purple spalting. Related maybe? Thanks for the article!

    Reply to this comment
  • Patty says:

    I bought some cheese curd yesterday that has turned purple today. Thanks for the info- we will not eat it.

    Reply to this comment
  • John Paul says:

    But can I eat it? I noticed a purple tint under the rind of a soft cow’s milk cheese from the Azores I purchased on vacation, unfortunately upon return from vacation I was back out on the road for another week or so due to a death in the family. I returned to find some slight purple under the rind.

    I am loathe to toss one of the best cheeses I’ve ever tasted.

    Reply to this comment
  • Angela Jackson says:

    I bought 3 containers as I eat it to loose weight…long story but but I eat out of container w purple …then 2nd had a quarter size ink spot…I ready thought it was just ink…tight me…just are out of other side before I got alarmed .. I have had urinary tract infection since consuming it…I kept it to take back to store but did not and this a m I looked and that ink is growing…what should I do…

    Reply to this comment
  • Shelby says:

    So, is this a new issue with cow’s milk products in general? Our household goes through quite a bit of sour cream. It is not uncommon for some of it to go moldy when an empty container gets left on the counter or forgotten in the back of the fridge. Up until about a month ago it was always the color you would expect typical food mold to be; that greenish-grey-blue tint. However, a month ago we started to notice that any mold we found on it was a very bright shade of purple. Also, it seems to show up faster than normal mold. It used to be, we use up a container, leave it on the counter and wash it the next day. Now, the very next day it’s already starting to turn purple. Does it reproduce faster? I had been unable to find anything at all about it until this article that shows cheese with mold the exact same color. If this is a fairly new issue, what does it mean? What does it mean for the dairy produced with it?

    Reply to this comment
  • Serena Laing says:

    I am so glad to see that it looks like you finally got to the bottom of the purple cheese mystery. Wish I could have been in the lab to help… hope all is well!

    Reply to this comment
  • Sunil Meka says:

    Was the cheese edible and if yes, How drastic a change was noticed in the taste and texture of the cheese? Thanks in advance.

    Reply to this comment
  • ann huang says:

    I have some purple stuff grow on my tofu, not sure if they are eatable or not. I can send a photo. I still have the tofu in the fridge.

    Reply to this comment
  • Rachel Stevens says:


    Can you please give me an idea of whether it’s safe to eat cheeses that have been affected by these indigo producing bacteria? I’m making a blue cheese and I’ve noticed a purple layer underneath the rind and I’m worried about the safety of having these bacteria in the cheese. I can’t seem to find any info online!


    Reply to this comment
    • Hi Rachel –
      There are many different types of microbes that can cause pigments in cheese. If you are concerned about the safety of your cheese, you should get it tested by a professional food safety lab.
      Ben Wolfe

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