I do not experience it often but I do occasionally and it might be with a lager yeast or ale yeast. My standard process is fermentation in a fridge (about 50° for lagers, between 60° and 65° for ales) for 4-5 days and then the fermenter is taken out of the fridge and placed on the basement floor. I know that diacetyl can be removed by raising the temp so when we have cooler weather I will place a piece of wood on the ground so it’s not as cold as the cement floor and also place the fermenter between two fridges that throw a bit of heat to keep things warmer. When I had the Tilt in the fermenter I noticed that the temp was 68° during that time out of the fridge. But what else can I look at? I have an ale made with WLP001 on tap now with a slight diacetyl character. What’s the best way to prevent it?
Dicetytl can be thrown by the yeast but it can also be caused by infection. Oxidation during racking or in unpurged secondary, keg or bottle can cause diacetyl to reform if it was present. You can either let the beer sit until the diacetyl is gone (by performing a forced d-test) or krausen the beer with some freshly fermenting beer. Below is the a forced diacetyl test I lifted from Craft Brewing magazine
From your primary fermentor, draw two samples into two jars and seal the jars. Heat one of those jars (a hot-water bath works well) to about 140°F (60°C). Hold it there for about fifteen minutes, then cool to the same temperature as your unheated sample. Now, smell/taste both. If they taste the same, then you’re in the clear! There’s likely very little chance of a meaningful diacetyl flavor. However, if the heated beer tastes strongly of diacetyl, you should let the yeast continue to work on it (and maybe increase the temperature a bit to goose the yeast cells into activity).
I’m not hyper sensitive to diacetyl, but I have definitely had some beers were it was noticeable. Mostly lagers, but also some ales that were dry hopped. I think maybe adding the dry hops can introduce some oxygen and kick up some minor fermentation/hop creep. In those dry hopped ales the diacetyl usually disappears over some time. I made a Festbier this past fall that was loaded with diacetyl and I had to dump it even after warming it up for a while. That Festbier was most probably under pitched and I didn’t realize that the fermentation chamber was around 40 degrees for the first couple of days and it was slow to start. I think that a healthy amount of yeast is probably the best way to prevent it. I think I will try that diacetyl test the next time I make a lager. Thanks Majorvices.
I’ve been working towards lower-O2 and less oxidation so I would love to eliminate that although I know I can’t. I would also have no issue keeping the beer warmer for a longer time so I could burn it off. But I guess I wonder what the culprit is from a process standpoint. I know how to remove it if I have it but how do I avoid it in the first place? Healthier yeast? Give the yeast more O2? Ferment warmer from the start? When my kids were home for winter break last year I had a lager that I put on tap and my one son had a few glasses of it. I had not tried it yet so I tapped a glass and HOLY DIACETYL! I asked my son, “Do you think this beer is okay?” and he said, “I like it. Why? Is something wrong with it?”. I took it out of the fridge and placed the keg next to the furnace for about 3 days. Then I put the keg in the cold garage and eventually put it back in the draft fridge. The diacetyl was gone. That’s a good “plan B” but it would be cool to know how to avoid it in the first place. I have some 1968 beers coming up shortly and I believe that strain is a known diacetyl producer. 
Just a small point, yeast make the precursor alpha-acetolactate which is is oxidized outside of the cell and turns into 2,3 butanedione (diacetyl). Yeast can’t reabsorb the alpha-acetolactate, but can absorb the diacetyl and use it as an energy source later in fermentation.
It there is alpha-acetolactate in the beer, exposure to O2 can turn it into perceptible Diacetyl. I’ve had this happen.
Hop creep is known to bump up more Diacetyl. Some hops have enzymes that break down higher sugars into fermentables which cause the yeast to ferment more, creating more alpha-acetolactate, which then forms Diacetyl. You need enough time after dry hopping to let that clean up.
I don’t do much of a Diacetyl rest these days. 55F for my lagers at end of fermentation, then slowly take the temp down, hold at 40 F for a week or more, then slowly down to -1C. Lower temperatures, but longer times.
Re:1968. To me that strain only has diacetyl if you taste it the day or two after the yeast drops out of suspension. I have taken a taste many times of wlp002 (same yeast) beer right after flocculation and tasted butter. If you just wait a few days and taste again the diacetyl is always gone. I think that yeast just mostly flocs before that cleanup is done. I don’t think it really produces more diacetyl or has a lingering problem if you are patient.
Okay, I am willing to use a PLAN B to ensure that I do not taste diacetyl. Is it enough to just hold the fermented beer in the fermenter at a higher temp for a bit longer? Swirl the fermenter a bit to make sure the yeast is in suspension and able to do the work? I’ll be honest… I’m confused about why I often do not experience diacetyl but maybe 5% of the time I will. Ale yeast, lager yeast, warmer temps, lower temps, etc. While I would love to avoid it altogether, I am okay with knowing that I can remove it.
This is a good talk on diacetyl from a yeast expert. Maybe Plan A is in there.
That was a really good talk on Diacetyl. I learned a lot.
When I looked around the room, there were a lot of Pros in the audience.
Thank you tommymorris… I’m going to check that out over coffee in the A.M. Cheers.
Thank you for saying what I was trying to say only sounding smart doing it.
I’m not sure I got all of that right. Maybe the chemical name is incorrect. Read the Kara Taylor presentation linked above y’all, it is really excellent.
My experience is that you don’t even need a higher temp if you give the yeast enough time.
Okay, I just listened to that. It was excellent but I don’t really know what my specific issue would be. So I went down the list and I am going to focus on a couple beers I have on tap now that were made with WLP001 and there is some diacetyl. These were not beers with “low nutrients” as she mentioned… no table sugar, no corn or rice. These are all malt beers. I fermented these between 62 and 65°F. I warmed them up towards the end of the fermentation but… it is getting cooler so maybe my basement floor is not warming them up as I thought. I believe my pH levels are okay… I look at that closely. I have been looking closer at making sure the right amount of yeast is pitched and I think I was overpitching (both ales and lagers) for awhile thinking “the more the better”. I am pitching less yeast these days but it still may be a little more than is suggested by calculators. One of these beers is dry hopped but that beer seems to have less diacetyl than the other one which was not DH’d. I rarely DH so I’m not really worried about that. It seems like my best first step is to maybe remove the fermenter from the fridge earlier and let the last piece of fermentation take place at a higher temp. Also rouse the fermenter a few times as fermentation finishes to make sure the yeast is back in suspension. Also allow just a bit more time in the fermenter (as Denny suggested) for things to clean up. It does seem like my diacetyl issues are happening more in cooler weather so it might be that simple. All spring and summer this year… no diacetyl issues in the beers. Thanks for that link… good stuff there by Kara.
I actually bring beers upstairs in the winter when I want to warm them up in the back half of fermentation so that they are around 70. I feel that once the bulk of fermentation is done colder warming up doesn’t do any harm like unwanted esters, at least that’s my experience, so I kind of go for it with the warm temp.
I also find that after high krausen the exothermic temp increase is way less so the beer stays close to ambient temp.
Also, I’m not sure I’m understanding the diacetyl test where one sample is heated. I am using plastic fermenters with spigots and usually 12-13 days after brewing I will closed-transfer the beer from fermenter to keg. Wouldn’t I just be able to grab a small sample though the spigot and give it a smell & taste test? I can absolutely detect diacetyl. Is the heated-sample method meant for people who have a harder time picking it up?
Also, when I move my fermenter from fridge to the basement floor, I typically rouse it at that time and then let it settle for a week and then transfer. I have a pale ale that I took out of the fridge on Friday and I roused it then. I roused it again yesterday and then again this morning… gently. I would typically transfer this beer Thursday or Friday of this coming week and then brew with that harvested yeast again over the weekend. I will grab a small sample prior to transfer and check for the big D.
Yeah, that’s a thought. Except for the obligatory Um, Ken… why is there a fermenter in the guest bedroom??? 
Those heat wraps are awesome If you insulate your thermoprobe against the fermenter with a piece of cloth/papertowel/bubblewrap/stryrofoam and wrap the heat wrap around the fermenter and plug into your thermostat you can keep the temp of the beer whatever you want (above ambient).
I’ve never owned one of those and I feel like I was always working to try to keep fermenting beer cooler. But I guess I wasn’t seeing what might happen in a Chicago-area basement in the winter. This spot I have between two fridges is warm. The beer probably reaches 68-70°. But if this first light step in the right direction is not enough, I will look into a heat wrap. Thank you for the suggestion.
The forced test makes the diacetyl easier to detect, and if the precursor is there it will oxidize faster at higher temps.