After brewing ales for years I finally attempted a Lager. Tasted great when I racked it to secondary (after a 2-day rest at room temp), after lagering 3 weeks, the diacetyl is overpowering. Can this be fixed by pitching a yeast starter (I read that somewhere)? And how can I avoid it in the future? Help!

Curlyfat

Beer tastes better in Wyoming...

One thing that can cause diacetyl is too low of a boil or covered boil pot during boil.

You did not mention the recipe or ingredients or temperature of fermentation.

Here's what the BCJP vocabulary says about it:
di·a·ce·tyl [] noun
liquid with strong odor: a yellow-green liquid with a penetrating odor that occurs naturally in plants such as bay and also in butter. It is used to enhance the flavor and smell of other foods.
In Beer: Diacetyl is recognized as a buttery or butterscotch flavor often accompanied by a 'slickness' on the palate.
Brewers Control (how to change)
It is generated as a fermentation byproduct that may be reabsorbed, depending on process and strain, or as a bacterial infection. Extended warmer temperatures during fermentations tends to reduce diacetyl. Worts with high ratios of adjuncts tend to produce higher diacetyl levels. Additionally diacetyl removal is affected by the constituents of the wort. Early fermentation cooling may result in higher diacetyl levels by virtue of Time verse removal mechanisms in contact with beer. Agitated ferments reduces diacetyl by increasing surface area over Time (rousting), however care must be taken not to facilitate production of acetaldehyde!

There is no treatment for excessive diacetyl, save for the methods previously describe to affect reduction upon fermentation, that - and good sanitation!
Sources of...
Primary bacterial infectious causes for diacetyl eminate from Pediococcus damnosus or or subspecies of Lactobacillus; late affectors in the fermentation cycle, but also may be found during storage of the finished product. It can reside in sedimented yeast, thus affecting future beers if repitched. Both Pediococcus & Lactobacillus are common for Lambics, yet by virtue of process (active fermentation: the mechanical process of CO2-scrubbing), diacetyl is reduced to imperseptable levels
Here's the link: http://www.hordsoffun.com/bjcp/vocab/

One thing that can cause diacetyl is too low of a boil or covered boil pot during boil.

Are you sure aren't referring to DMS or rather DMS precussors? I don't believe this is a cause of diacetyle. I could be wrong but....

You has a diacetyl rest which should have allowed the yeast to resorb the diacetyl. It is possible the there is indeed some sort of contamination problem and there was simply too much diacetyl for the yeast to handle.

Having said that I believe there were postings fairly recently on Homebrew Digest dealing diacetyle problems. If I recall the only suggestion to reduce diacetly at this late stage was to add a yeast starter and high kraussen and hope the active yeast will resorb the excess diacetyl present. I believe this was just a suggestion and not a tried and true method.

It might be worth a try. If you do give this a go please let us know how it turns out.

- Scott

Thanks for the help, guys! I'll try the yeast starter and pray...I've never had an infection before, but there's bound to be a first time at some point.

Curlyfat

Beer tastes better in Wyoming...

I pitched a 22oz starter (with ale yeast since that's all I had) and let it sit at room temperature (70 degrees) for 48 hours. The Diacetyl is greatly reduced! Although I would still call the beer flawed, it is not ruined and is completely drinkable. Just letting you know there seems to be some validity to this theory. On a side note, I lost the beautiful clarity I'd gained from lagering. You win some, you lose some.