Last updated:
Identifying a beer infection by appearance and smell is a learnable skill that saves batches, knowing what you’re looking at tells you whether to dump immediately, whether the beer is salvageable for a different purpose, or whether what you see is actually harmless. I’ve encountered most common homebrew infections over the years, and the single most important diagnostic principle is this: not everything unusual-looking in a fermenter is dangerous, and not everything dangerous looks alarming at first. A systematic visual and olfactory check gives you the information you need.
Normal fermentation appearances (not infection)
Before diagnosing infection, know what healthy fermentation looks like. Krausen (the thick white/tan foam on top of active fermentation) is normal, it can look unusual and even discolored slightly from hop material. Yeast rafts floating on the surface during secondary are clumps of flocculated yeast and are harmless. CO2 bubbles and slight cloudiness are expected. White sediment at the bottom of the fermenter is dead and settling yeast. None of these indicate infection.
Infection identification guide
Kahm yeast (white flat film on surface)
Appearance: Flat, matte white or off-white film on the surface of the beer. Not raised, no fuzzy texture. May have a slightly wrinkled or powdery surface. Distinct from krausen (which is foamy) and from pellicle (which is thicker and more structured).
Smell: May be odorless or have a slightly yeasty, mildly off aroma. Rarely smells strongly unpleasant.
Risk: Low, kahm yeast is a wild Saccharomyces or Pichia species. It produces minor off-flavors if left unchecked but rarely ruins a beer. Skim it off and taste the beer.
Pellicle (Brettanomyces or Pediococcus)
Appearance: Structured, often thick, sometimes ropy or web-like film on the surface. Can be white, cream, tan, or slightly brown. Often has a raised, three-dimensional structure unlike the flat kahm film. May have “islands” or a continuous film. Develops over weeks rather than days.
Smell: Distinctly funky, barnyard, leather, sour, fruity, or combination. Can be pleasant (for intentional sour/wild fermentation) or harsh and unpleasant.
Risk: Depends on intent. In a beer meant to be a clean lager or pale ale, a pellicle means significant contamination with wild organisms. The beer will become sour and funky. It may or may not be drinkable depending on the organism and your taste. In an intentional wild or sour fermentation, a pellicle is expected and desired.
Pink or red discoloration
Appearance: Pink, red, or magenta patches or tint in the beer or on the krausen surface.
Risk: High, pink or red growth is a strong indicator of Serratia marcescens or other bacterial contamination not normal in brewing. Discard. This is not a typical brewing organism and indicates a sanitation failure.
Fuzzy mold growth (green, black, or blue-green)
Appearance: Visibly fuzzy, raised, distinctly colored (green, black, blue-green, or orange). Usually appears on surfaces above the liquid line, on the inside of carboy necks, under lids, or around airlocks.
Risk: High, true mold growth above the beer line is a contamination event. Discard the batch. While the liquid below may technically be unaffected, mold on any surface in contact with the beer headspace can produce mycotoxins. The risk is low but not worth it for a 5-gallon homebrew batch.
Ropy or viscous beer
Appearance: Beer that pours like thin syrup, has visible “strings” or viscous threads, or has a slimy texture. The liquid pulls apart in threads.
Cause: Pediococcus damnosus, a lactic acid bacteria that produces exopolysaccharides creating the “sick” or “ropy” character. Common in traditional Belgian lambic production (where it’s a normal phase) and in unintended contamination. The condition usually resolves itself over months as Brettanomyces metabolizes the exopolysaccharides, in intentional sour fermentation, wait it out. In a clean beer batch, it indicates contamination and the beer should be discarded.
Common Questions
My beer smells like vinegar, is it ruined?
Vinegar aroma (acetic acid) comes from Acetobacter bacteria, which require oxygen to produce acetic acid from ethanol. A strongly vinegary beer indicates significant oxygen exposure with Acetobacter present, typically from a poorly sealed fermenter, repeated opening, or a contaminated airlock. If the vinegar character is mild and the beer was meant to be a sour style, it may be usable as a blending component or vinegar in cooking. If vinegar character is strong and the beer was meant to be clean, it’s not recoverable for drinking. Future prevention: ensure airtight seals, don’t repeatedly open fermenters, and sanitize airlock liquid regularly.
