Common homebrew infections and what they look like

Most scary-looking fermentations are normal

A healthy ale fermentation looks alarming the first time you see it. A thick, rocky foam called krausen rises in the first 24 to 72 hours. It can be cream, tan or streaked with brown hop resin. It often leaves islands of foam and a dark ring on the fermenter wall when it falls. None of that is infection.

Normal fermentation also smells of more than beer. Sulphur, like a struck match, is common with lager yeast and some ale strains. Green apple, from acetaldehyde, is normal early on and fades as the yeast cleans up. Judge by what happens over a few days, not by one worrying look.

The signs that something else is growing

An infection usually announces itself after fermentation has slowed, when there is less yeast activity to compete with. Look for:

• A thin, continuous skin across the surface that is white, wrinkled, bubbly or shiny.
• Fuzzy, raised spots of green, black, blue or white, which mean mould.
• A smell of vinegar, sour milk, nail varnish, sweaty horse or plasters.
• A gravity that keeps falling well below what the yeast should reach.
• Bottles that gush or overcarbonate weeks after packaging.

One sign alone is not proof. Two together usually are.

Lactic acid bacteria: sour and sometimes ropy

Lactobacillus and Pediococcus are the classic beer spoilers. Both produce lactic acid, so the beer turns sour, sometimes pleasantly and often not. Lactobacillus tends to give a clean sourness and sometimes a faint white haze or film.

Pediococcus is the troublesome one. It can produce diacetyl, a butterscotch or movie popcorn flavour. Some strains also make the beer ropy: thick, oily and stringy when poured. Strangely, the rope can disappear after a few months, though the diacetyl often stays. Both bacteria are favoured by warm conditions, which is one reason Indian summer brewing needs careful sanitation.

Acetobacter: the vinegar maker

Acetobacter turns alcohol into acetic acid, the acid in vinegar, but it needs oxygen to do it. So it shows up where air gets in: a loose lid, a dried-out airlock, a half-empty fermenter left for weeks or a bottle with a poor cap. The beer smells sharply of vinegar and may grow a thin film on top.

The fix is mostly about oxygen. Keep airlocks topped up, fill fermenters properly and do not leave finished beer sitting on a large headspace of air.

Wild yeast: Brett and diastaticus

Brettanomyces, called Brett, is a slow wild yeast. In a clean beer it brings phenolic, smoky, farmyard or sticking plaster notes and often a white pellicle, a skin on the surface. Because Brett can ferment sugars normal yeast leaves behind, gravity keeps dropping for weeks or months.

Saccharomyces cerevisiae var. diastaticus is the bigger danger for bottlers. It ferments dextrins that brewing yeast cannot, so bottles that tasted fine at packaging slowly build pressure. The result is gushers at best and exploding bottles at worst. If bottles from a batch keep gushing, chill them all and open them carefully with a cloth over the cap. Our guide to pellicles, mould and when to pour a batch away covers what to do with an affected batch.

Mould is a different problem

Mould grows as raised, fuzzy colonies, usually on the surface or on fruit or hops floating at the top. It looks like mould on bread because it is the same kind of organism. It is most likely in the first hours before fermentation gets going, or in a beer left exposed to air. Mould is the one sign where we would not taste the beer. Pour it away and clean thoroughly.

Finding the source and stopping a repeat

Not every sour or odd beer is infected, either. A beer that tastes of green apple may simply be young. Butterscotch can come from yeast that was rushed off before it reabsorbed diacetyl. A solvent edge often means fermentation ran too hot, which in an Indian summer is far more common than infection. Check your fermentation temperature record before you blame bacteria. Tasting for those faults first separates process problems from microbes.

One infection is bad luck. Two in a row is a process problem. Work through the cold side, where wort is below about 60 °C and can be infected:

• Scratched plastic fermenters, buckets and spoons. Scratches hide bacteria that sanitiser cannot reach. Replace them.
• Tubing, taps and siphons. Take taps apart. Replace tubing every few batches.
• Sanitiser made up at the wrong strength or left too long.
• Cleaning skipped before sanitising. Sanitiser does not work on dirty surfaces; see cleaning versus sanitising.
• Slow starts. Pitch enough healthy yeast so fermentation begins within 12 to 24 hours and outcompetes stray microbes.

Your first step: take apart the tap on your fermenter tonight and look inside. If you find residue, you have probably found your source.

Common questions

How do I know if my homebrew is infected?

Look for a thin skin on the surface after fermentation, fuzzy mould, smells of vinegar or sour milk, gravity that keeps falling or bottles that gush. Two signs together usually confirm it.

Is krausen a sign of infection?

No. Krausen is the normal foam of active fermentation and can be cream, tan or streaked with brown. It falls back as fermentation finishes.

What causes gushing homebrew bottles?

Often a wild yeast such as diastaticus fermenting sugars that brewing yeast left behind. Bottling before fermentation finished causes the same symptom.