Pellicles, mould and when to pour a batch away

What a pellicle actually is

A pellicle is a film that wild yeast or bacteria build on the surface of beer. It forms where they meet oxygen, so it grows across the top of the liquid. Pellicles look odd: white and powdery, wrinkled like skin, bubbly or thin and oily with a shine. Some look like a sheet of wet paper.

Brettanomyces, Lactobacillus, Pediococcus and Acetobacter can all make them. In a sour beer brewed on purpose, a pellicle is expected and welcome. In a pale ale you meant to be clean, it means something has got in.

How mould is different

Mould grows as separate, raised colonies rather than a continuous film. It is fuzzy or furry, often round and often coloured: green, blue, black or white. It usually sits on something floating, such as fruit, hops or a clump of foam.

Here is the quick test. A pellicle is flat or wrinkled and covers the surface like a skin. Mould is fluffy and sits in spots. If you are unsure, take a photo in good light and look at it closely. Fuzz is the giveaway.

Mould means pour it away

We do not argue with mould. Some moulds produce toxins and you cannot tell which by looking, so the batch goes down the drain. Do not skim it off and hope; the visible colony is only part of the growth.

Mould is most common in two situations. One is a slow start, where wort sat for a day or more before fermentation began. The other is a beer with fruit or other additions floating at the top in contact with air. Both are avoidable: pitch enough healthy yeast to start fermentation within 12 to 24 hours and keep fruit submerged or bagged.

A pellicle is a decision, not a sentence

Beer's alcohol, low pH and hop compounds make it a hostile place for the bacteria that make people ill. That is why an infected batch is usually a flavour problem rather than a safety one. So with a pellicle, the question is whether the beer tastes good.

Draw a sample from below the pellicle, through the tap or with a sanitised thief, without breaking the skin. Smell it, then taste a small sip. If it smells of vinegar, nail varnish or sour milk, or tastes harsh, pour it away. If it is clean, mildly tart or pleasantly funky, it may be worth keeping as a different beer from the one you planned.

Keeping an infected batch safely

If you keep it, plan for one more problem: wild yeast and some bacteria keep fermenting sugars that normal brewing yeast leaves behind. Gravity can keep falling for weeks. If you bottle it at the usual point, pressure builds in the bottle and glass can burst.

So wait. Take gravity readings a week apart. Bottle only when the reading has held steady for at least two to three weeks. Use thick bottles and put one PET bottle in the batch so you can squeeze it to feel the pressure build. Store the bottles cold once carbonated and drink them within a couple of months. Kegging avoids most of the risk. If any bottles gush when opened, chill the whole batch and open the rest carefully over a sink. Our guide to common homebrew infections describes what each microbe tastes like.

What to do with the kit afterwards

After an infected batch, what you do with the equipment matters more than what you do with the beer. Brett in particular is persistent.

• Soft plastic: tubing, seals, gaskets and scratched buckets go in the bin. Microbes hide in scratches and in porous material where sanitiser cannot reach.
• Glass and stainless steel: clean thoroughly with a hot alkaline brewery cleaner, then sanitise. These can be kept.
• Taps and valves: take them apart completely, clean every part and replace the O-rings.

Many homebrewers keep a separate set of plastic kit for sour and wild beers. It is cheap insurance. Mark it with tape so it never mixes back in.

Find the cause before the next brew

Pouring away 20 litres of beer is painful, but brewing the next batch with the same weak point is worse. Go back over the cold side: when the wort was chilled, what it touched, how long before fermentation started, whether the lid sealed and the airlock stayed full. In hot weather an airlock can dry out in under a week. That alone can let air and a pellicle in.

Your first step: write down every piece of kit that touched the batch after the boil, then replace anything soft and scratched on that list before brewing again.

Common questions

Can I drink beer with a pellicle?

Often yes, if the beer underneath smells and tastes good. Pellicles are a flavour problem in most cases. Pour away any batch that smells of vinegar or solvent or tastes harsh.

Should I pour away beer with mould on it?

Yes. Some moulds produce toxins and you cannot tell which by looking, so pour the batch away and clean or replace everything it touched.

How long should I wait to bottle an infected beer?

Until the gravity has held steady for at least two to three weeks, because wild yeast and bacteria keep fermenting sugars that brewing yeast leaves behind.

Can I reuse a plastic fermenter after an infection?

Replace it if it is scratched, along with tubing and seals. Glass and stainless steel can be cleaned and kept.