Phenolic off-flavours from wild yeast

The POF+ gene turns malt acids into phenols

Malt contains hydroxycinnamic acids, mainly ferulic acid and p-coumaric acid. Yeasts that carry the phenolic off-flavour trait, written POF+, have working copies of two genes, PAD1 and FDC1. Together they decarboxylate these acids into volatile phenols:

• Ferulic acid becomes 4-vinyl guaiacol, with a clove-like, spicy aroma.
• p-Coumaric acid becomes 4-vinyl phenol, with a more medicinal, plastic or sticking plaster note.

German wheat beer yeasts and many Belgian strains are POF+ by design. Most ale and lager production strains are POF-, meaning they lack a working version of these genes. Our guide to phenolic off-flavours covers the full family.

Wild yeasts are often POF+

Many wild Saccharomyces strains, the yeasts found on fruit, grain dust and in the environment, carry a working POF+ trait. So does Saccharomyces cerevisiae var. diastaticus, which can also break down dextrins and cause over-attenuation, gushing and exploding packages. When one of these gets into a clean beer, phenols are often the first sign.

Brettanomyces takes the chemistry one step further. It converts the vinyl phenols into ethyl phenols. 4-ethyl guaiacol is smoky and spicy. 4-ethyl phenol is barnyard and horse blanket. Those flavours are prized in some styles and a fault in nearly all others.

Telling contamination from the house yeast

Clove or medicinal notes in a beer brewed with a POF- yeast point to something else in the tank. Before blaming wild yeast, rule out the alternatives:

• Chlorophenols from chlorine in water or sanitiser. These are sharper and more antiseptic. Our guide to chlorophenols covers that route.
• Cross-contamination from a house wheat or Belgian strain. Breweries that run a POF+ strain alongside clean strains often find it turns up in the wrong tank through shared hoses, harvest vessels or brinks.
• Raw materials. Some smoked malts and certain spices add phenolic character. Check the recipe before you condemn the yeast.

Signs that point firmly to contamination: the phenolic note grows over time in the package, the beer over-attenuates, haze increases or bottles and cans gain pressure.

Confirming it in the lab

Several standard tests help:

• Lysine medium. Brewing Saccharomyces strains cannot grow on lysine as their only nitrogen source. Many non-Saccharomyces wild yeasts can. It does not catch wild Saccharomyces. Our guide to detecting wild yeast with lysine medium explains the method.
• Copper sulphate media are used to detect wild Saccharomyces, which tolerate copper better than most brewing strains.
• A POF test. Grow the isolate on a medium containing ferulic acid and smell it for clove. A positive result from a colony that should be POF- is a strong clue.
• PCR tests for the STA1 gene identify diastaticus specifically. Many outside labs offer them.

Where wild yeast gets in

Wild yeast is everywhere in a brewery: grain dust from the mill room, fruit flies, open fermenters, poorly cleaned heat exchangers and packaging lines. In a warm Indian climate, with temperatures that suit yeast for most of the year, a small contamination can grow fast in any wort left sitting.

The floor detail that catches many breweries: the yeast brink or harvest vessel. It is cleaned less often than fermenters and is the one piece of kit that touches every generation. A wild yeast that settles there rides along into every repitch.

Getting rid of it

• Dump affected yeast. Acid washing will not separate wild yeast from culture yeast, because both tolerate low pH.
• Start fresh from a clean lab culture or a new pitch from a trusted supplier.
• Strip down and deep clean anything that touched the contaminated batch: heat exchanger, hoses, valves, gaskets and the filler.
• Keep POF+ house strains separated in time and equipment from clean strains, with a full clean between them.
• Plate regularly. Monthly plating of pitching yeast and finished beer catches contamination before customers do.

Building a lab routine that catches wild yeast early and knowing which medium answers which question takes practice beyond a single guide.

Your first step: taste your last three batches of each clean beer next to each other, focusing on clove and plastic notes. If any batch shows them, plate a sample of the pitching yeast for that batch before you brew with it again.