Diacetyl. You know it when you taste it. Many describe itas slickness on the tongue and a buttery or butterscotch flavor. Some people have a lower threshold for it than others, but at high concentrations it is easily detectable. Diacetyl is a normal product of fermentation. Most judges recognize it as a fault. But some people don’t find it objectionable. They believe that a little Diacetyl provides a greater depth of flavor and “roundness” to the beer. In a description of mild on the CAMRA website it says: “Slight Diacetyl (toffee/butterscotch) flavors are not inappropriate.” And the BJCP style guidelines say this for
Ordinary Bitters: “Generally no Diacetyl, although very low levels are allowed.” Reading between the lines
(huh?), with English style Real Ales you can have a little detectable diacetyl. But if you want the beer to be a champion, its best to have none. Diacetyl can also be produced by unwanted bacteria, especially pediococcus. The biggest source of diacetyl is brewers’ yeast. Yeast cells create a diacetyl precursor
called α-acetolacate. When the α-acetolacate exits the cell, it undergoes a chemical reaction and forms diacetyl. The reaction is relatively slow, but higher temperatures and low pH causes it to accelerate. Luckily, the yeast the produced the diacetyl in the first place, can absorb diacetyl and convert it into flavorless compounds. Here are some things that can cause diacetyl formation:

• Certain yeast strains are known to produce diacetyl;
• Highly flocculent yeast strains may settle out before
it has had a chance to take in the diacetyl;
• Not enough oxygen during the initial fermentation
stage;
• Too much oxygen during the middle and end of
fermentation;
• Too low of a yeast pitching rate; and
• A higher than optimal fermentation temperature

So how can we help the yeast scrub out the diacetyl in time for kegging? The easiest would be to prevent the formation of α-acetolacate. However, that is not an option. Holding a beer at a temperature around 65–70 °F (18–21 °C) at the end of fermentation for one to two days will give the yeast time to take in the diacetyl. This is commonly known as a diacetyl rest. Diacetyl rests should be started when the specific gravity of a beer is between 1.002-1.005. Since Real Ales usually ferment at temperatures between
65–70 °F (18–21 °C), raising the temperature is not necessary. However, the beer needs to sit for two to three days after fermentation appears to be complete. Racking the beer or crashing the temperature too quickly after fermentation will increase the possibility of diacetyl not being absorbed by the yeast. Consider not racking the beer to a secondary fermenter. Just let the fermented beer clear in the primary fermenter, and rack into your keg. BURP News October, 2012 Page 5 If your beer is infected, there is nothing you can do about diacetyl formation. Prevention is the only solution. Luckily, in the case of Real Ale, it takes some time for the bacteria to grow to a level that produces diacetyl at detectable levels. The reaction is also slowed by lower temperatures. Real Ales are supposed to be served at cellar temperatures, drunk young and quickly. As such, there is less time for the bacteria to grow. In summary, for preventing diacetyl in you competition Real Ale, or any other beer:

• Choose low diacetyl yeast stain that is not super
flocculent;
• Sanitation, sanitation, sanitation;
• Pitch enough yeast;
• Do not under or over aerate; and
• Allow the beer to sit at 65–70 °F, 18–21 °C for a
couple of days following the end of fermentation.
Sources:

Foster, Terry. “Diacetyl: Techniques.” Brew Your Own
Jul/Aug 2011
Bible, Chris. “The Dreaded Diacetyl.” Brew Your Own
October 2012, Vol. 18, No.6