As promised, more on the butter bomb! Diacetyl. It is a buttery, waxy off-flavor that is normally unwanted in beer.
Human detection of diacetyl varies. Flavor thresholds of 0.1 – 0.15 parts per million are considered average, although even some beer judges I know come right and say that they don’t perceive it well, or at all. Compare this with the flavor threshold of ethyl alcohol in beer, somewhere around 55,000 ppm, or 5.5% ABV.
In a previous article I mentioned how certain yeast strains and brewing practices could exacerbate the mal-flavor of diacetyl. But sometimes, even a tasty beer without noticeable diacetyl ends up tasting of the Big D. It happens when tap lines and faucets are not kept squeeky clean.
A brewery I used to frequent made an IPA that was always loaded with diacetyl. One day the brewer gave me a sample of the latest batch right off the serving tank. It didn’t have the Big D. I pointed out that IPA from the faucet behind the bar did have it. So he poured us a sample and it was in there… but the brewer couldn’t find anything wrong with the beer. He honestly couldn’t perceive it.Tap lines require regular cleaning with a caustic cleaner. American and German professional guidances suggest cleaning every two weeks. But tap lines and faucets that are old or neglected may have resident funk in them that can be impossible to remove. Faucets can be infected by dirty little fruit flies or they may be worn and admit some air to the faucet.
Lactobacillus and Pediococcus bacteria can infect the beer and produce lactic acid and α–acetolactate (also called acetolactic acid or AAL). AAL is a precursor to diacetyl production. When a beer has both diacetyl taint and a sharp acetic note, chances are that bacterial spoilage is occurring.
A number of factors lead to diacetyl formation, but only one reliable method can reduce diacetyl levels: enzymatical reduction by yeast. (George Fix in “Diacetyl Formation, Reduction and Control”)
If lively yeast were still present, the yeast would convert these acids into less noticeable byproducts, but since we’re talking about the finished beer here, there isn’t much that can be done to eat up late-forming diacetyl.
If this is such a well-known problem (Louis Pasteur wrote about it in the 19th century!) why do breweries and taverns allow it to occur? Simple my friends – sloth and avarice. They’re too lazy to take to the time necessary to properly clean their fixtures and they don’t like wasting the beer that is in the lines prior to cleaning. Often the staff isn’t even aware of a flavor defect.
It is up to the consumer to point out the possibility of dirty tap lines. If enough beers are returned to the bartender after just a sniff or a sip, I can only hope that they will get the message. TPJ