Contamination — diagnosis, recovery, prevention

Is it actually contaminated?

A surprising number of “infected” homebrew is just an ester or phenol the brewer didn’t expect. Confirm contamination by checking for:

• A pellicle — a thin, often patchy white-to-grey film on the beer surface. Not the same as krausen (the foam during active fermentation, which is yeast).
• Sourness — confirmed by tasting (when in doubt, taste sober and through a clean glass).
• Gushing on opening — a sign of continued fermentation in the bottle, often from wild yeast.
• Visible mould — fuzzy growth, usually distinct from yeast residue.

What’s actually growing

Recovery

A mildly soured beer can sometimes be saved as a kettle-sour-style or blended. But trying to “rescue” a clearly infected beer usually wastes more grain. If pellicle + sour, dump it.

Sanitise the fermenter ruthlessly before reuse:

1. Hot wash with PBW
2. Inspect for scratches — scratched plastic harbours microbes permanently and should be replaced
3. Soak for 30 minutes in fresh Star-San
4. Optional: 10% bleach soak for 1 hour for stubborn cases (rinse exhaustively after, then sanitise again with Star-San)

Prevention

See sanitation for the full procedure. The top three preventable causes we see:

1. Bottling/transfer equipment shortcuts. Bottling wand, racking cane, tubing — all need to be clean and sanitised, every time.
2. Worn plastic fermenters. Replace at the first sign of scratching, typically every 2-3 years of regular use. Glass and PET don’t have this issue.
3. Open-air transfers in dusty environments. The garage brew shed is a contamination magnet. Cover everything with foil or a sanitised lid during transfers.