Last night I was serving a friend dinner. Roast lamb and veggies with homemade sour cream and sauerkraut. My 3 gallon batch of fermented cabbage, burdock, apple, cranberry, seaweed and herbs was ready to harvest after bubbling for several weeks. I lifted the lid off the crock and found the surface of the brine covered with a thin white layer of yeast. This is the same yeast that I find on top of my sourdough starter, so I just wiped it away and pulled out a cup of kraut for us to eat.
My friend curiously looked into the crock and asked about the white film. I told her it was yeast and that it’s not a problem. There is lots of yeast living in the air of my house since I have been fermenting for so long. ( this picture shows little bits of the yeast after I scooped most of it off)
But this morning I woke up with some nagging doubts. Is the yeast really not a problem? It is what I remember hearing from my fermentation teacher, Sandor Katz, years ago. And I have been eating it and not getting sick. But when I am serving other people I like to be sure that I am being safe.
A little online search brought me to this fabulous post on the science behind fermentation by Nourishing Treasure. This is what she has to say about yeast and the white film on top of the kraut
A Word on Yeasts
Yeast is another major inhibitor which requires an abundance of oxygen for growth. It’s often one of the first signs that you’re allowing too much oxygen near your sauerkraut. In the presence of oxygen yeasts can be oxidized to form vinegar – not something we want in our sauerkraut. Yeasts can also cause off-flavors and discoloration, visible signs you need a better seal on your sauerkraut. Pink sauerkraut (not from red cabbage) is a sign of yeast. This could be due to too much salt, or an uneven distribution of salt, or too much oxygen exposure. If you see a creamy film on top and/or one that smells yeasty, throw it out.
There is one yeast, however, that is helpful. Saccharomyces cerevisiae, a member of the ascomycetous yeast family (as opposed to the candida family) is probiotic and can help with candida overgrowth. Interestingly, Saccharomyces cerevisiae has the ability to shift its own metabolism from fermentive to oxidative. Do you know what causes it to make the switch? Oxygen. When present, oxygen will cause Saccharomyces cerevisiae to oxidize; keep the oxygen out, and this friendly yeast can help your sauerkraut to ferment and provide you with delicious probiotics.
White Film, Slime
Although mold is harmful, white film is not. This is just your friendly probiotic yeast at work. This happens more often when cabbage isn’t properly submerged under the brine, or the container isn’t sealed well.
Slime, however isn’t to be tolerated. This is most often the cause of too little salt, or salt that wasn’t evenly mixed into the cabbage.
There is my answer. The white, non slimy, film on top of my kraut is indeed yeast which has probably started to grow more because I stirred it up last week and let some fresh oxygenated air in. I had been scooping cups of kraut out while I was waiting for the fermentation to complete. In the future I will avoid stirring it up so much, to keep the oxygen out. This batch is ready to go in jars and be eaten.
In case you are wondering what the yeast looks like before it is disturbed, one of the tell tale signs is that the film crinkles up when you push it to the side and it smells fresh. I didn’t think to take a picture of it before I scooped it off, but Herbangardener has a nice picture of the white film on top of her Beet Kvass. It is towards the end of the post under troubleshooting.