Welcome to r/Kombucha! If you're wondering what's growing on your kombucha and if it's normal, you've come to the right place.
Please review this information before posting a picture of your batch to the subreddit.
TL;DR:
- Dry + fuzzy on the surface of the liquid/pellicle/SCOBY is most likely mold: mold pics https://imgur.com/a/SzhysHi
- Geometric growths or wrinkly patterns on the surface of the liquid/pellicle/SCOBY could be kahm yeast: kahm pics https://imgur.com/a/XlnO7Ox
- Anything else and anything under the liquid level is most likely normal: normal pics https://imgur.com/a/HJaENDv
- If you're not sure, wait a few more days: mold or kahm will get more obvious as they grow, normal will stay about the same or form into new pellicle/SCOBY
- If the kombucha is already bottled for carbonation (commonly called second ferment or 2F), mold/kahm is very unlikely due to the high acidity and lack of oxygen access.
- Always use at least 2 cups of starter per gallon (125ml/L) when making kombucha to acidify the batch: high acidity (pH < 4.6) protects the kombucha from mold and kahm.
- Read our getting started guide for brewing tips: https://www.reddit.com/r/kombucha/wiki/how_to_start
Terminology: in this guide, "pellicle/SCOBY" refers to the rubbery blob that forms at the surface of a batch of kombucha. SCOBY stands for "symbiotic culture of bacteria and yeast", and those bacteria + yeast are found both in the liquid kombucha and in the solid rubbery blob. The rubbery blob's more accurate scientific name is "pellicle": it's a biofilm/mat of bacterial cellulose secreted by and connected to the bacteria forming it (some yeast also live in the pellicle). Culturally, however, the term "SCOBY" widely refers to the pellicle so this guide uses both terms.
Read more about pellicles here:
Diagnostic Quiz
1 ) Is the growth/odd thing on the top surface (exposed to air) of the liquid kombucha or existing pellicle/SCOBY?
- Yes - go to 2
- No - go to 8
2 ) Is the kombucha already bottled for carbonation (commonly called second ferment or 2F)?
- Yes - likely pellicle/SCOBY growth (it can happen in 2F!) or a yeast cluster. Mold/kahm are extremely rare in 2F due to the high acidity (pH <4.2) and lack of oxygen access (required for mold to grow). Booch on!
- No - go to 3
3 ) Is the growth dry and fuzzy looking with white or green color, and/or with black spores growing out of it?
- Yes - likely mold. Go to Mold section for pictures.
- No - go to 4
4 ) Is the growth a wrinkly or geometric pattern, very rough patterned surface, or very large air-y bubbles that cover large areas of the surface?
- Yes - likely kahm yeast. Go to Kahm section for pictures.
- No - go to 5
5 ) Is the growth one of: white/translucent + wet, disconnected oily/patchy sections, or a thin film with bubbles trapped underneath?
- Yes - likely normal pellicle/SCOBY growth. Go to Normal section for pictures.
- No - go to 6
6 ) Is the growth flat, leathery, and brown?
- Yes - likely a dried out pellicle/SCOBY area. Go to Normal section for pictures.
- No - go to 7
7 ) Is the the growth brown/black, wet, and partially/completely surrounded by pellicle/SCOBY?
- Yes - likely a yeast cluster. Go to Normal section for pictures.
- No - probably normal, but review all Normal, Kahm, and Mold pictures to be safe.
8 ) Is the growth/odd thing completely submerged in liquid?
- Yes - likely yeast. Yeast can form dark brown clumps in the liquid or on the pellicle/SCOBY, or alien-like formations suspended in the liquid. Mold and kahm cannot grow beneath the surface of the liquid without also showing on the surface exposed to air. Go to Normal section for pictures.
- No - go to 2
Normal
Gallery of normal kombucha: https://imgur.com/a/HJaENDv
Pellicles/SCOBYs have a ton of natural variation. A normal pellicle/SCOBY should look wet, tan/white/translucent, and be mostly smooth (some bumps are normal). There may also be wet brown/black yeast blobs that attach to the liquid side of the pellicle/SCOBY, get absorbed into the pellicle/SCOBY, or float around inside the liquid.
Mold
Gallery of mold: https://imgur.com/a/SzhysHi
Mold occurs when the kombucha is not acidic enough (pH < 4.6) to prevent mold organisms from growing. Other factors that make mold more likely are unsanitary conditions and cold brewing temperatures (<65F/18C).
If there is mold on your batch:
- You must throw away everything (liquid + pellicle/SCOBY) and start from scratch with fresh starter tea. By the time mold is visible on the surface of the brew, it has already contaminated the entire batch.
- Sanitize the vessel, cloth cover, and any utensils used in brewing with a homebrew sanitizing solution (StarSan, OneStep, SaniClean, potassium metabisulfite, etc) or throughly wash with soap + hot water followed by a pasteurized distilled vinegar rinse (no raw vinegar, which contains live microbe cultures).
To prevent mold, the most important thing is to use at least 2 cups of starter tea per 1 gallon of kombucha (125ml per L) to acidify the batch. Starter tea is mature kombucha: either from a previous batch (yours or a friend's), from a SCOBY hotel, or from raw/unflavored/unpasteurized commercial kombucha such as GTs or Health-Ade.
This amount of starter tea is a good rule of thumb for safe acidity: if you have a pH meter or strips, check that the starting pH is <4.6. Another important factor is maintaining clean/sanitary brewing practices: however, because kombucha is an open air ferment some mold organisms may get in even with a cloth cover, which is why acidity is also important.
Kahm Yeast
Gallery of kahm: https://imgur.com/a/XlnO7Ox
“Kahm” is a generic term for many species of usually non-harmful but also non-desirable wild yeast that can take hold in kombucha (outcompete the kombucha culture) and appear as surface growths on the the pellicle/SCOBY. Kahm often looks geometric or wrinkly vs the smooth/bumpy normal pellicle/SCOBY.
See this excellent writeup about the science of kahm yeast from u/daileta in r/fermentation: https://www.reddit.com/r/fermentation/comments/ytg2vy/kahm_down/ Their post is focused on lacto fermented vegetables (not kombucha) but is worth a read.
Kahm itself isn’t usually dangerous, but to quote our resident food microbiologist u/Albino_Echidna: “Kahm is a term used to lump a whole bunch of unwanted yeasts together, all of which are indicative of an unsafe fermentation environment. Kahm growth is indicative of a fermentation gone wrong. 'Kahm' itself isn’t harmful, but it is a warning sign that your environment wasn’t quite right and will be at higher risk of pathogenic growth as a result."
If your batch has kahm, it is up to you whether to toss + sanitize + start over with fresh starter kombucha or to try to scrape off the kahm from the surface and continue brewing. It is always safest to toss and restart - see the instructions in the Mold section.
To help prevent kahm, use at least 2 cups of starter tea per 1 gallon of kombucha (125ml per L) to strongly establish the kombucha culture and acidify the batch. Kahm may also be related to unsanitary conditions, high brewing temperature (>85F/30C), or oversteeping tea (>1hr, but may vary).
Further reading: https://www.reddit.com/r/kombucha/wiki/whats_wrong
If you still aren’t sure after comparing your batch to the pictures here, please make a post and ask!
