Goaty and waxy notes from caprylic acid in tired yeast

What caprylic acid is and how it tastes

Caprylic acid, also called octanoic acid or C8, is one of three medium chain fatty acids that brewing yeast makes during fermentation. Its neighbours are caproic acid (C6) and capric acid (C10). The name comes from the Latin for goat and the smell lives up to it: goaty, waxy, tallowy, sometimes soapy or like a snuffed candle.

At low levels these acids sit in the background of almost every beer. They become a fault when yeast is stressed or left in poor condition. The same acids also damage foam. A beer with a goaty edge often has a head that collapses quickly too, which is a useful second clue.

Why tired yeast makes more of it

Yeast makes medium chain fatty acids on the way to building its cell membranes. When the cell has plenty of oxygen and nutrients, it turns most of those intermediates into the long chain fatty acids it needs. When it lacks oxygen, or is old, crowded and starved, more of the medium chain acids leak out into the beer.

They also appear when yeast starts to break down. A slurry held warm for days, or beer left sitting on a thick cake of dead yeast at the bottom of a warm tank, releases fatty acids as cells die. This is closely related to the meaty and brothy flavours of autolysis described in our guide to yeast autolysis. Goaty usually arrives first.

The cellar conditions that push it up

In practice we see caprylic notes rise with a recognisable set of habits:

• Poor aeration at pitching. Wort that goes into the fermenter well short of the usual 8 to 10 mg/L of dissolved oxygen for ales leaves yeast unable to build healthy membranes.
• Old or overworked slurry. Many generations, low viability or slurry stored for a week or more.
• Warm storage of yeast. Slurry that should sit at 2 to 4 °C left in a warm room or a cold room that keeps tripping during power cuts.
• Long contact with the yeast cake. Beer held warm on settled yeast after fermentation, especially in a flat bottomed vessel with no cone to dump.
• High gravity worts. Strong beers stress yeast through alcohol and osmotic pressure.

Reading the signs in a fermentation

Caprylic acid rarely arrives alone. Look for a sluggish start, a fermentation that finishes a few points high, a soapy or waxy nose and weak foam. When those turn up together, check the yeast numbers before blaming the recipe.

Three measurements tell the story: viability from a stained count, a cell count to set the pitch and the dissolved oxygen at the fermenter inlet. Our guide to yeast viability and vitality explains the staining. If viability is under about 90%, propagate fresh rather than repitching.

Cellar habits that keep it down

The fixes are about looking after the yeast at every stage:

• Oxygenate wort properly and check it with a dissolved oxygen meter, not by guesswork.
• Crop yeast promptly once fermentation finishes and store it cold, ideally used within a few days.
• Dump settled yeast from conical tanks before warm conditioning runs long.
• Set a generation limit for each strain and propagate fresh when you reach it or when attenuation and flavour drift.
• Keep cold rooms on backup power where you can, since a warm night in an Indian summer can damage a whole slurry.

The floor detail that matters most: dump the cone before the diacetyl rest finishes in a warm tank. Beer sitting at 18 to 20 °C on a deep layer of dead yeast is where goaty notes grow fastest.

Training the panel to name it

Goaty, soapy and waxy get confused with each other and with isovaleric and butyric acids, which smell of cheese and sweat. Our guide to cheesy and sweaty notes separates those. A panel that can tell them apart can tell a yeast handling problem from an old hop problem or a bacterial one, which saves a great deal of guessing.

Your first step: on the next batch, measure dissolved oxygen at the fermenter inlet and note the slurry's age and viability alongside it. Two batches of data will tell you more about your goaty notes than a year of tasting.