Diacetyl

[JBeaber]

Hey all-
So, i don't understand the whole diacetyl thing. I don't understand how it forms in beer or what flavor/effect it has on it. Any explanations would be greatly appreciated. Thanks.

-J

[Speyedr]

Hey J,

Welcome to the Forum!

Diacetyl is a by-product of fermentation. During a normal fermentation the yeast start by eating the material that is easiest to breakdown. They break this material down into smaller substances that are harder to breakdown than the original substances. As they use up the easy stuff they move on to harder and harder stuff, diacetyl included, and basically clean up after themselves. That's why if a fermentation stops short you will end up with a lot of off-flavors, since the yeast couldn't finish up the crumbs.

Diacetyl has the smell and flavor of buttered popcorn. It can be well hidden in ales where it is usually not a concern but shows up loud and clear in a lager that hasn't finished properly. This can be avoided by doing a dacetyl rest in lagers, which is basically bringing the temp up to room temp for 24hrs after primary fermentation and before lagering. You can also ferment lagers a la Jamil, and just start them at 48f or 50f and let them go up to about 55f by the end of the primary phase, then lager. The slow rise allows the yeast to clean up better. Since ales are fermented at higher temps they tend to clean up diacetyl on their own.

Hope this helps,

Rob

[Spidey]

Excellent explanation. I will go a step further to say that even with notorious diacetyl producers such as the Ringwood ale yeast strain, as long as you are not too quick to rack your beer out of primary (I let Ringwood yeast ferment in primary for just under 1 month), then these yeast will do a fine job of cleaning up excess diacetyl and the buttery flavor mentioned will most likely not be significant.