Fermentation science
Inside the SCOBY: The Microbiology of Kombucha
What the pellicle really is, which organisms live in the liquid, and why acidity — not the disc — keeps kombucha safe.
8 min read ·
What is the SCOBY actually made of?
The rubbery disc that forms on kombucha is bacterial cellulose, secreted at the air–liquid interface by acetic acid bacteria, principally Komagataeibacter xylinus. It provides a partial oxygen barrier and a physical surface, but it is a by-product of the fermentation rather than its engine.
The real consortium is suspended in the tea: osmotolerant yeasts such as Brettanomyces bruxellensis and Zygosaccharomyces bailii hydrolyse sucrose and produce ethanol, which acetic acid bacteria then oxidise into acetic acid. That two-step relay is why kombucha smells simultaneously of cider and vinegar.
Why does kombucha need starter liquid?
Adding 10–20% mature kombucha to fresh sweet tea drops the starting pH below 4.2 immediately. That acidity is the primary safety control: it suppresses moulds and pathogens during the vulnerable first days, before the culture has produced acid of its own.
A batch started with a clean pellicle but no starter liquid is markedly more likely to grow mould. If you only have a dry or gifted pellicle, use bottled unflavoured vinegar-free kombucha or distilled white vinegar at 5% of volume to acidify the tea.
How much of the sugar in kombucha is actually consumed?
Sucrose is inverted by yeast into glucose and fructose. Yeasts preferentially consume fructose; acetic acid bacteria prefer glucose. A finished kombucha at pH 2.9 has typically consumed 30–50% of the added sugar, which is why kombucha is tart but rarely dry.
Camellia sinensis tea supplies nitrogen and polyphenols the culture requires. Herbal-only infusions lack these and cause culture decline over successive batches; blend at least 50% true tea to keep a culture healthy long term.
How does kombucha get carbonated?
Bottling with a small amount of fruit or juice in a sealed vessel restarts yeast activity, producing CO2 that dissolves under pressure. Two to four days at room temperature is usually sufficient; longer risks over-pressurised bottles.
Refrigeration does not stop fermentation, it slows it. Bottles held warm for a week can reach hazardous pressures — burp them or use pressure-rated vessels.