Musty and mouldy notes in beer and where they usually come from

Musty is a smell you already know

You have met this smell before: a monsoon cupboard, a wet newspaper, a cardboard box left on a damp floor, an old cellar. In beer it shows up as a flat, dusty, damp quality that sits on top of everything else. It also tends to mute the good aromas. A musty pale ale often smells of very little at all, with a faint wet cardboard edge behind the hops.

Tasters describe it as mouldy, earthy, dank or cellar-like. Those words overlap, but they point at a small family of compounds that the human nose picks up at astonishingly low levels. Some are detectable at a few nanograms per litre. A nanogram is a billionth of a gram. At those levels a tiny source, such as one damp carton or one mouldy sack, can spoil a whole batch.

The compounds behind the smell

Four names come up again and again:

• TCA (2,4,6-trichloroanisole). The famous cork taint compound from wine. Wet cardboard and damp cellar. Beer can pick it up without a cork in sight, which we cover in our guide to cork taint in beer.
• TBA (2,4,6-tribromoanisole). A close cousin of TCA with a similar damp, musty smell.
• Geosmin. Earthy, like beetroot or fresh soil after rain. Made by soil bacteria and some algae.
• 2-methylisoborneol (MIB). Musty and earthy, often found alongside geosmin in water supplies.

The anisoles form when moulds and bacteria act on chlorinated or brominated phenols. Those phenols come from wood preservatives, some fungicides and chlorine reacting with phenolic material. That is why so many musty problems trace back to damp wood, damp cardboard or chlorine used in the wrong place.

Where it usually gets in

Brewers who chase this fault tend to find it in the same handful of places:

• Malt and grain. Malt that took on moisture in storage can grow mould. Healthy malt leaves the maltings at about 4 to 5% moisture. A torn sack sitting through an Indian monsoon can pick up far more than that.
• Water. Surface water from reservoirs can carry geosmin and MIB, especially after algal blooms in hot weather. Municipal treatment does not always remove them.
• Wood and packaging. Wooden pallets, cardboard cartons and filter sheets stored in a damp warehouse can absorb anisoles and pass them on.
• Barrels. A wooden barrel that has been left dry, damp or poorly cleaned can grow mould and taint whatever goes in next.
• Your glass. A glass dried with a damp cloth or stored in a closed, humid cabinet can smell musty on its own.

Check the glass before you blame the brewer

The cheapest test happens at your table. Smell the empty glass. If it smells of a damp cloth, ask for a fresh one or rinse it with cold water and try again. A surprising number of musty beers in bars are musty glasses.

If the beer still smells damp in a clean glass, try a second bottle or can from a different pack. A fault that shows in every sample points back to the brewery. One that turns up in a single bottle points at that bottle, its carton or how it was stored on the way to you.

What brewers do about it

For a brewery, a musty complaint starts a search. The usual order is water first, then grain, then anything wooden or cardboard that touches the beer or its packaging. Activated carbon filters are the standard tool for removing geosmin, MIB and chlorine from brewing water. Malt is stored off the floor in a dry room and used in rotation. Chlorine is kept well away from wooden surfaces.

Brewers also train their tasting panels to spot musty notes early. Thresholds vary a lot between people. One taster may find a beer clean while the person next to them finds it unpleasant. Knowing who on a panel is sensitive to which compound is a real skill and it does not come from reading about it.

When earthy is part of the style

Not every earthy note is a fault. Some English hop varieties give a gentle earthy, woody character that brewers want in a best bitter. Some farmhouse and wild beers have a deliberate cellar-like funk from Brettanomyces, which is a different family of aromas covered in our guide to Brettanomyces. The difference is that intended earthiness sits alongside the other flavours. Musty taint flattens them.

Your first step: next time a beer seems dull and faintly damp, smell the empty glass, then the beer, then a fresh glass of the same beer. Three sniffs will tell you more than any tasting note.