Kahm is a type of yeast which is often characterised by cyclic patterns in the pellicle. Often the patterns are wavey, liney or circular. It can sometimes be tricky to distinguish between Kahm and Mould as well (or you could have both!).
The reason why it grows is usually:
• SCOBY not covered at all or with the right type of cloth
• SCOBY is left sitting next to something ‘yeasty’ such as beer.
• SCOBY is contaminated during preparation.
Kahm is an unfriendly yeast to consume, therefore even though some may say to scrape the pellicle off and let a new one form. It is hard to guarantee that the Kahm yeast did not contaminate the liquid as well. To be safe, its always best to dispose and clean everything with vinegar. Then start a new batch with a new SCOBY (see our Golden Recipe).
Happy brewing
Chris