What are Styrenes

And why are they in my beer?


Have you ever opened a beer and noticed that it has a plasticky flavour? You swear that chlorine didn't come anywhere near it. You scour your water reports, boil and metabisulphate it overnight, but sure enough, your next batch has it as well? 

You check your yeast health, obsess about the dates on smack packs, make a huge starter and the next batch...

You, my friend, have discovered styrenes. 

Styrenes are a class of phenol and up until recently, they've been almost entirely absent from brewing courses and research papers. It's likely because, at least in homebrewing circles, they only show up with assumed good habits. 

So let's dive in. 

What Styrenes are not

If you end up with an inexperienced judge, you're likely going to have them conclude that the offending flavour is due to chlorophenols. The BJCP does not train its judges to recognize styrene and most testing kits don't either. When I was a young BJCP judge, I know that I frequently mistook it as such. Instead, styrenes are formed through perfectly normal and healthy fermentations, which is why it's frustrating. 

What causes Styrene?

Styrenes are formed mostly by POF+ yeast strains, which include wheat, Belgian and saison strains. However, they aren't limited to those strains. British ale strains are also guilty of styrene production. The cause is the decarboxylation of cinnamic acid which means that reducing cinnamic acid is your first step. 

LIke wheat yeast, British yeast can be coaxed into producing a range of flavours depending on temperature, pitching rate, pressure and oxygen level. Higher temperatures, lower pitching rates, lower pressure and less oxygen gives you more esters. The opposite gives you a cleaner profile. With wheat yeast, that opposite gives you a more phenolic profile - cloves, rather than bananas, to simplify. Ferrulic acid is necessary to make those clove flavours. Unlike ferrulic acid, however, cinnamic acid is more malt dependent than process dependent. 

So where does cinnamic acid come from? It's responsible for the nutty flavours in malt  - which means Maris Otter is chock full of it.

Thankfully, cinnamic acid - while it has a directly proportional relationship to styrenes, is not the sole contributer. The rest has to do with fermentation controls, so lets go through them. 

1. Pressure - Open fermentation promotes styrene formation. Closed fermentation does not, but will not stop it if the pitch rate is high. 

2. Pitch rate - Low pitch rate reduces styrene production. High pitch rate promotes it. 

3. Temperature - Higher temperature reduces styrene production, lower temperature promotes it. 

Oxygen concentration has not been studied in relation to styrene production as of yet, but if it's anything like wheat yeast, I'm going to guess that higher oxygen levels will promote it. 

The other thing about styrenes is that they are produced quite early in fermentation. If you notice that your beer has a styrene issue, all is not lost. Raise the temperature and it is likely to boil off. Also understand that If you bottle condition a beer that previously had a styrene issue, that yeast will continue to produce styrenes when it referments. Keep your beer warm (25C) for bottle conditioning to reduce the chance of this happening. 

As always, this information is conditional on new studies being produced.