White frosting on firm rye sourdough

 Following a discussion on TFL, some reported a white frosting forming on their rye sourdough cultures. Some referred to this as “kahm” yeast. I joined the discussion as I have the same issue. I had previously adopted Jeffrey Hamelman’s feeding regimen from his book: 10g culture, 16g water and 20g rye flour. The ratios vary slightly seasonally. I feed the rye sourdough every morning and it lives on our kitchen countertop. The TFL link to the discussion is Methods and Rationale for Sourdough Starter Maintenance and Elaboration | The Fresh Loaf.

 “kahm” frosting - 24 hours after feeding

Inside view of Culture - Cut section

I contacted Debra Wink regarding the TFL discussion. She was confident that what some people called ‘Kahm” was not an invading species, but the yeast in a pseudo-mold form looking for more nutrient. It’s not an invading or unwanted microbe in sourdough, but a sign the yeast is suffering from a nutrient deficiency. My observation was that the white frosting did not appear until around 15 hours and was more prevalent in higher temperatures.

Under DW’s oversight, I added some yeast nutrient at the recommended concentration. I gradually increased the concentration for two weeks (24-hour feeds) without any effect on the “kahm”. I took a photograph each morning before feeding. 

DW summarized what Jeffrey Hamelman's maintenance was in May of 2024. He no longer uses 20 g of whole rye but has cut it down to 10 g flour to 8 or 8.3 g water (the lesser in summer to slow things down). Culture amounts also vary depending upon the season. He still prefers once a day feeding but adjusts the amount of culture with seasonal temperature swings which can vary a lot throughout the year. So, while he might use 2 g culture in winter, that could go down to .5 g in summer with the goal of attaining ripeness every 24 hours.

My current ambient house temperature is in the mid 20’s C (around 77F). I chose 1g rye culture, 8g water and 10g rye flour. The next morning there was no sign of the white frosting or acetone smell, and the culture was ripe. 

Rye culture now after 24 hours.

DW’s conclusion:

Going forward, I think it's safe to assume that if you see this stuff forming, it's telling you that you need to adjust by feeding it more (by using less ripe starter), slowing it down (by reducing hydration or temperature), or some combination of these things. You can also feed it twice in a day when needed to get things back on track if it matures too quickly. I tell students that sourdough is an interactive medium. It responds to what you do, or don't do, and it's up to you to decipher and respond to the signs it's giving you in return. It's what Jeffrey would do :)

A reasonable conclusion here is that, in your starter at least, the white film and acetone smell are signs of underfeeding. Perhaps because of the season you're in and temperatures being higher, and because it seems like there is something in shorter supply in rye than in wheat. I still don't know what that is, but the experiment (thank you for doing it) gave us valuable information about some things that it probably isn't: it doesn't appear to be from a lack of the type of nitrogen sources or anything provided by this yeast nutrient. 

Fortunately, we don't always need to know how and why something works in order to use it. The answer may reveal itself in time. That is often the way of things :)

Baking Results

I baked these Vermont Sourdough loaves using the revised rye culture as the “seed” for the levain.

Baked loaves 27th Feb 2025.