Is My Beer Infected? How to Tell
Infections are rarer than nervous new brewers think. Normal fermentation produces foam (krausen) and floating “yeast rafts” that look alarming but are harmless. A real infection shows up as a pellicle — a waxy, wrinkled, sometimes bubbly film — usually paired with a sour or funky smell. The golden rule: if you have to ask, it’s almost certainly fine. Never dump a batch unless you see mold or are positive it’s infected.
Sanitation anxiety is a rite of passage. You drop an unsanitized spoon in the wort, or spot something floating on top a week later, and immediately assume the batch is ruined. Here’s the reassuring truth veteran brewers repeat constantly: it is genuinely hard to infect beer, and infections almost never happen to the paranoid beginners who worry about them most.
What’s completely normal
- Krausen — the thick, foamy, tan head that forms at the peak of fermentation. It can climb the walls of the fermenter and look chaotic. Totally healthy.
- Yeast rafts — opaque blobs, from sesame-seed to silver-dollar sized, with ragged fluffy edges. They’re lighter than the wort, rarely cover the whole surface, and usually vanish once krausen develops. A normal byproduct of fermentation, not a cause for alarm.
- A ring or brown gunk on the fermenter walls and murky trub on the bottom — all normal.
What an actual infection looks like
A true infection forms a pellicle: a film across the surface created by wild yeast or bacteria in the presence of oxygen. Tell-tale signs:
- Covers the entire surface (rafts don’t).
- Waxy, powdery, slimy, or wrinkled — sometimes with bubbles or ridges. Never fuzzy or hairy (that’s mold, a different problem).
- A distinctly funky, sour, or off smell — not the clean bready/hoppy aroma of fermenting beer.
The two-minute test
Open the fermenter and smell it. If it smells like beer, hops, or bread, you’re fine — that’s your first piece of evidence. If you’re still unsure, draw a small sample and taste it. An infected beer announces itself immediately: sharply sour, medicinal, or vinegary. If it tastes like young, flat beer, it’s just young, flat beer.
Should you dump it?
Almost never. The community consensus is blunt: never dump a batch unless it has visible fuzzy mold or you are positive it’s infected — and if you’re not sure, it isn’t. Even a mild infection often ferments out into perfectly drinkable (sometimes interesting) beer. When in doubt, let it ride, bottle it, and taste the finished product before deciding.
Frequently asked questions
There is white film on top of my beer. Is it infected?
Not necessarily. Thin patches or blobs with ragged edges are usually harmless yeast rafts. A uniform, wrinkled or waxy film covering the whole surface, paired with a sour or funky smell, is more likely a pellicle (infection). Smell it — if it smells like beer, you are almost certainly fine.
I dropped something unsanitized in my wort. Did I ruin it?
Almost certainly not. A brief contact with an unsanitized object rarely causes a noticeable infection, especially once healthy yeast has taken over. Let it ferment and taste the result before worrying.
What is the difference between mold and an infection?
Mold is fuzzy or hairy and grows in spots, often green, black, or white and cottony. A pellicle from a bacterial or wild-yeast infection is flat, waxy, or wrinkled and never fuzzy. Fuzzy mold is the one clear reason to dump a batch.