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 ·
The pellicle is a by-product, not the culture
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 starter liquid matters more than the disc
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.
Sugar, tea and what actually gets 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.
Secondary fermentation and carbonation
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.