(Updated September 2018 with new bacterial species!)
(December 2018 update: Got white film on the surface of kimchi? Check out this article to learn about five harmless yeast species that have been identified and analyzed for toxin formation!)
In fermentation, we talk a lot about the dreaded “kahm yeast.” This is a surface growth (a “pellicle”, for your word geekery) of unwanted organisms that can impart unpleasant aromatics and flavors to our lovingly crafted foods. But anyone who’s tried to explore the topic further has quickly learned that good definitions of kahm are lacking. It isn’t as though there’s a yeast called Saccharomyces kahm.
So what is Kahm Yeast?
From what I’ve been able to gather, “Kahm yeast” is a term that’s been used in industrial brewing and food processing for ages. It probably came into use before microscopy allowed for any sort of real identification of microorganisms. I believe the term originated in German beer making (see this 1898 etymological discussion from England). I’ve also seen it referenced in old journals of the Japanese liquor industry as well. One reference that I’ve lost track of described it as “kahm hefe,” which is pretty German. So my supposition is that “kahm” was essentially an industry term among brewers. It indicated an infection by an unwanted organism. These infections produce a surface growth and cause negative effects to the finished product, usually off odors or flavors. Kahm can also affect the texture of fermented foods. Kahm’s no fun in the home environment, but obviously terrible for commercial manufacturers.
2018 Update: Pellicle Forming Bacteria
This paper, “Pellicle formation, microbial succession and lactic acid utilisation during the aerobic deteriorating process of Sichuan pickle” by Yu Rao, et al, identifies nine bacterial species responsible for pellicle formation in their experiments with aerobic decomposition of several batches of ferments. These include: Citrobacter fruendii, Enterobacter cloacae, Klebsiella oxytoca, Enterococcus malodoratus, Bacillus subtilis, Bacillus amyloliquefaciens, Morganella morganii, Providencia rettgeri, and Staphylococcus cohnii.
Most of these organisms could only form a pellicle at a given pH, and many of these organisms could utilize (i.e., break down) lactic acid, raising the pH of the ferment as part of its deterioration process. Okay, back to the original post for more on all that….
What does Kahm do?
I ran across a very cool resource last night. I was curious about how kahm damages the fermentation process. From the link, attributed to Frazier W.C.: Food Microbilogy, McGraw-Hill Book Company, Inc., New York, 1958:
Genera of Film Yeasts. The genera Pichia, Hansenula, and Debaryo-myces (as well as Mycoderma and Candida of the false, or asporogenous, yeasts) are film yeasts which grow on the surface of acid products like sauerkraut and pickles (Figure 2-4), oxidize the organic acids, and en¬able less acid-tolerant organisms to continue the spoilage. Hansenula and Pichia tolerate high amounts of alcohol and may oxidize it in alcoholic beverages. Pichia species are encouraged to grow on Jerez and Arbois wine, to which they are supposed to impart distinctive flavors of esters. Debaryomyces is very salt-tolerant and can grow on cheese brines with as much as 24 percent salt. The film yeasts produce little or no alcohol from sugars.
Let me pause to note that I’m not a microbiologist. Wikipedia tells me that the group Hansenula has now been merged into the Pichia group. Debaryomyces appears to remain independent. I imagine an actual microbiologist working in 2016 would be able to fill in other details about this 1958 passage, and maybe one will chime in below.
But to explain what kahm yeast does, based on my limited knowledge: Kahm (which is in fact one of a variety of pellicle-forming yeasts and even some bacteria) infects a batch of, say, sauerkraut. The surface of the brine develops a pellicle, much like the one created by Acetobacter in producing vinegar. The pellicle allows the yeast organisms to fix oxygen to the acids in the brine, breaking them down and raising the pH of the batch. Aside from yeasts, bacteria like lactobacillus and pediococcus can also produce “kahm yeast” infections.
Is Kahm a problem?
Kahm is generally described as harmless by fermentation gurus, and I tend to agree. There’s one big caveat though – given enough time, a kahm infection might raise the pH out of safe zones and allow spoilage to occur. Trust your nose on this one. It won’t steer you wrong.
How do I treat or prevent a Kahm infection?
Once a ferment has a kahm infection, it’s advisable to scrape or scoop off the growth. I’ve seen advice ranging from, “sprinkle some salt into the brine after you clear the surface,” to “give it a good stir and hope for the best.” Some people blot the surface with a paper towel to remove kahm islands. And sometimes these methods work, and the kahm doesn’t grow back. But I think in most cases, once a batch is growing yeasts on the surface, it’s likely to be an ongoing issue, and it might just be time to refrigerate and start eating.
For the most part, kahm is an infection that is dependent on oxygen. If your ferments develop nuisance surface growths often, you should strongly consider investing a few dollars in airlock systems. They are by no means foolproof, but letting the fermentation develop a CO2 layer over the brine really short-circuits the mechanism for infection.
I’m going to link again to this page on Yeasts and Yeast-Like Fungi. It has great info on the difference between fermentative and oxidative yeasts, among other things.
Finally, Amanda Feifer has a great write up with pictures of kahm, how it happens, and what to do about it at her excellent Phickle blog.
Have you had kahm? How did you handle it?
(Featured image by Marsha Halstead, kahm on mulberry vinegar.)