Infected beer typically shows visible changes on its surface, in its clarity, or in its carbonation. The most common sign is a film or unusual growth floating on top of the liquid, but infections can also make beer look ropey, hazy, or overly fizzy. Whether you’re a homebrewer checking on a fermentation or wondering about a store-bought bottle, here’s how to spot the signs.
The Pellicle: The Most Common Visual Sign
The hallmark of a beer infection is a pellicle, a layer that forms on the surface where the liquid meets the air. Pellicles are produced by wild yeast and bacteria as a protective barrier, and they come in a surprising range of appearances. Some look like an uneven, bubbly white film. Others appear ropey or weblike, almost resembling a spider web stretched across the surface. You might also see what looks like small, dry white patches floating on top, or a slimy, glossy sheen.
Pellicles can be thin and barely noticeable or thick and dramatic. A thin, smooth white layer is common with certain wild yeast strains, while bacteria like Lactobacillus or Pediococcus can produce thicker, lumpier formations. The key feature is that a pellicle covers a broad area of the surface rather than sitting in one spot. If you see something spreading across the top of your beer, that’s a strong indicator of microbial activity beyond your brewing yeast.
Ropiness and Slime
One of the more unsettling visual signs is ropiness. Beer contaminated with Pediococcus bacteria can develop a thick, viscous texture that looks stringy or slimy when poured. Instead of flowing normally, the beer pulls in long strands, almost like egg whites. The beer may also appear unusually turbid or murky, even if it was clear before. This ropiness is caused by polysaccharides that the bacteria produce as they grow. In sour beer styles, this is sometimes intentional and temporary. In a beer that wasn’t meant to be sour, it’s a clear sign of contamination.
Mold: Not Always Fuzzy
Many brewers assume that mold will be obvious, showing up as fuzzy green or black spots like you’d see on bread. That can happen, but mold on beer takes many forms that are harder to identify. Mold colonies can be mucoid (smooth and glistening, looking almost identical to bacterial colonies), flat and coin-shaped, or thin and crusty. Colors range from white and beige to vivid greens, reds, and blacks. A fuzzy growth is almost certainly mold, but a smooth white colony floating on the surface could also be mold rather than yeast or bacteria.
Under a microscope, mold cells are dramatically different from yeast. They grow as long filaments called hyphae, sometimes forming branching chains of interconnected cells that are much larger than the small oval shapes of yeast. Without a microscope, the practical rule is: if it’s fuzzy, it’s mold. If it’s not fuzzy but looks unusual and you didn’t pitch wild cultures, it could still be mold, and you should treat it with suspicion.
What Healthy Fermentation Looks Like
Not every strange-looking surface means infection. During active fermentation, yeast naturally clumps together and floats to the top in formations called yeast rafts. These are white or beige, irregularly shaped blobs that sit on the surface. They feel slippery if you scoop one out. They tend to break apart easily and have a clean yeast smell.
The difference between yeast rafts and an infection often comes down to texture and behavior over time. Yeast rafts look like loose clumps or islands, not a continuous film. They typically settle back into the beer as fermentation slows. A pellicle, by contrast, tends to grow and spread over days, becoming more structured. Hop residue can also create a thin, oily-looking film on the surface that mimics an early infection. If you dry-hopped your beer, a slight sheen or scattered particles on the surface are normal.
Signs in Bottled or Canned Beer
You won’t see a pellicle in a sealed bottle, but infection still leaves visible clues. The most dramatic is gushing: when you open a bottle or can and beer erupts out violently, far beyond normal carbonation. This happens because contaminating bacteria or wild yeast continue fermenting sugars inside the sealed container, producing excess carbon dioxide. If a beer gushes every time you open one from the same batch, bacterial contamination is a likely cause.
Before you even open it, check the packaging. A can that feels rock-hard and won’t give when you squeeze it is under abnormal pressure. Bottles may show a visible ring of residue around the neck or sediment that looks different from normal yeast settling. Standard bottle-conditioned yeast sediment is compact and settles to the bottom in a thin, even layer. Infected beer may have chunky, floating particles or a haze that won’t settle no matter how long you let it sit.
How Infection Changes Taste and Smell
Visual signs usually come paired with off-flavors. Beer contaminated with Pediococcus often develops a strong buttery or butterscotch taste from a compound called diacetyl, along with a sharp sourness. Acetobacter, the same bacteria that turns wine into vinegar, produces acetic acid that gives beer a harsh vinegar smell and taste. Wild yeast strains like Brettanomyces create funky, barnyard-like aromas that are unmistakable if you’re not expecting them.
If your beer looks normal but tastes sharply sour, excessively buttery, or like vinegar, an infection may be present at a level too low to produce visible changes yet. Trust your nose and palate alongside your eyes.
Is Infected Beer Dangerous to Drink?
Standard-strength beer is a remarkably hostile environment for human pathogens. The combination of low pH (typically around 3.5 to 4.5), alcohol content, hop acids, and limited oxygen makes it very difficult for dangerous bacteria like E. coli or Salmonella to survive. No one has documented a case of foodborne illness from drinking infected beer at normal alcohol levels.
The exception is low-alcohol and nonalcoholic beer. Research has shown that when alcohol drops below about 0.5% ABV, foodborne pathogens can survive and even grow, particularly if stored above refrigerator temperature. In one study, E. coli and Salmonella increased roughly 100-fold in nonalcoholic beer stored at 14°C (about 57°F) over 63 days, while cold storage at 4°C (39°F) prevented growth entirely. For standard beer above 3% ABV, infection will ruin the flavor but won’t make you sick. For nonalcoholic products, proper refrigeration matters more.