It’s the fermented beer that sometimes escapes me.
In other words, I F-up the fermentation from time to time. Like 2 batches in a row with diacetyl.
A simple fix for the diacetyl is to do a diacetyl rest. When fermentation starts slowing down, raise the temperature a few degrees (to 62-68 degrees, depending on your fermentation temperature). The yeast will become more active and be able to clean it up. 2 or 3 days at this temperature should help.
What if you’re already fermenting at that temp? While your advice is good for one particluar situation, that’s not answer. If you ket your fermentation go longer it will have the same affect. Also, diacetyl can be cause by contamination, not just incomplete fermentation.
I am still pondering the cause. I think I will soak my fermenters and transfer hose in a strong bleach solution just in case. I use starsan normally but maybe using a different sanitizer once will help.
Both were ales (Kolsch and Blonde). Both had reasonably quick forming krausen so I don’t think I under pitched and both were fermented in the 60’s. Both were in the fermenter 10 days total (4-6 days after fermentation visibly ended).
What yeast strain? I have worked with WY 2565 kolsch before and never experienced any diacetyl production with a proper pitch and ferment temps between 57-62F.
what’s your oxygenation routine?
If you’re using StarSan as your primary, it’s definitely a good idea to switch it up every few batches. DBSA is broad-spectrum against bacteria but there are Brett strains that can tolerate ~2 pH for ~30 min, for example. I’d suggest an iodophor instead of bleach though, just because of the odor issue.
I used WLP029.
I don’t add oxygen. Just splashing from pouring into fermenter from fermenter.
I have been wanting to try Iodophor but the LHBS only sells Starsan. I wonder if Iodophor is available at other types of stores. I know my grandparents used something very similar at their dairy for years.
How about that new stuff I see in the advertisement of Zymurgy? (San Step NS) …or is it only effective against the same stuff as Star San?
Yes and no…you can get iodophor from other sources, like dairy supply places. But it isn’t the same. I don’t have the info at hand, but if you like I can get someone from BTF Iodophor to explain the differences. That doesn’t mean the dairy stuff won’t work…just that the stuff designed for brewing works a lot better!
Weird that 029 is throwing diacetyl for you. Are you picking the diacetyl up straight from the fermentor or after racking? Oxidizing after racking or packaging is a huge factor in diacetyl in finished beers.
Swing by the brewery and I’ll give you a pint of peracedic acid. In exchange for some diacetyl free homerew. 
I had issues with diacetyl that had me flummoxed. About 2-3 weeks after bottling, diacetyl showed up. All was good and tasted fine til then. Turns out after trial and error on several batches, it was oxidation when I transferred to secondary and/or bottling bucket. Thanks to a few brewers in the AHA forum who are way smarter than me, I found an article that discussed the formation of acetolactic acid due to yeast issues that carried over into the bottle. Oxidation reacts with it and forms diacetyl a couple of weeks after bottling. I began purging my transfer vessels with Co2 and diacetyl disappeared.
https://www.morebeer.com/brewingtechniques/library/backissues/issue1.2/fix.html
I don’t currently have any diacetyl free home brew 
But, I think my Kölsch should be recovered in the next few days.
The blonde picked up a hint of diacetyl very late. That keg is kicked now. I bottled 2 bottles about 1 month ago. I tasted one of the bottles a few days ago and it had diacetyl bad. The last pint of if that beer from the keg had just a hint when it warmed up.
The Kölsch had a hint when I kegged it. I thought it would go away. I often get a caramel taste at packaging which goes away after a few days of conditioning. I thought that was what I had. But after 7-10 days in the keg this beer has a much more pronounced problem. I am hoping the krausening fixed it. I will be placing the keg back in the fridge tomorrow morning. My recipe was 100% Pils with Magnum at bitter and Crystal at 5 min. Yours is far more crisp than mine. I am hoping the krausening will get me there.
Worsening over time, both within a batch and batch-to-batch, makes me suspect contamination first.
Do you use a heat exchanger?
I place a copper chiller in the wort but I don’t run the wort through anything.
Agree with Sean, sounds like a possible contamination issue. Though it could still also be an oxygen issue. Both would get progressively worse with time. Are you purging your kegs before racking?
Bring a small sealable container by the brewery and I’ll give you some peracedic. Just don’t get any full strength concentration on you.
I’ll third the contamination guess.
I get the potential for contamination and will work on that (new hoses, deep soak fermenter and kegs).
There is one area where I am confused. On the oxidation front: I purge my kegs with CO2 but then open the lid and use gravity to fill the keg through a spigot on the fermenter. There is oxygen exposure no doubt. Are you saying the oxygen is turning diacetyl precursor into diacetyl or that I am confusing oxidized beer with diacetyl? I am sure both are possible. I have always assumed a good diacetyl rest would convert all the diacetyl precursor and allow yeast to consume it. This is supposed to stop the later conversion of precursor to diacetyl because the precursor is supposed to be gone. Hence, my question. On ales fermented in the 60’s I don’t expect much precursor to be left.
I am looking at how to purge with a full keg of star san and then fill without opening the keg using gravity. I have a plan. We will see if it works.
By the way: for the krausening of the Kölsch here is what I did. I had drank it down a from 2.5G to about 2.0G. I had a Helles in the fermenter that was just hitting high krausen. I took 24oz from the Helles and put it in the warmed keg and resealed. The keg has been sitting for 4 days at 65F in the basement. I put a pressure gauge on Day 1. Pressure has slowly climbed to 27 PSI. I plan to put back in fridge tonight (if pressure is stable) and sample tomorrow night.
If oxidation caused precursor to change to diacetyl then I expect that keg to have cleaned up and be stable since all diacetyl and oxygen should have been consumed by the krausen step. If this is a contamination issue I think the diacetyl flavor will still be there and may have gotten worse.
Thoughts?
Not to imply that this is the definitive cause but just another facet to consider along with contamination… I found this in a 1993 George Fix article on Diacetyl, Which turned out to be the problem I was having. My diacetyl problems would show up in the bottle but after cold conditioning for a few weeks, would mostly disappear.
“The reaction acetolactic acid → diacetyl is of the redox type (2). Acetolactic acid is oxidized to diacetyl, and other constituents (for example, various aldehydes as well as wort-derived melanoidins and tannins) are reduced. In all of the mechanisms described so far in this article, this is done enzymatically by microbes, culture yeast, and, in adverse cases, by other guests in our worts. The reaction can occur nonenzymatically, however, in the presence of an appropriate oxidizing agent. Indeed, a widely observed but little discussed phenomenon occurs when diacetyl appears spontaneously in a beer that seemed to have normal flavors. Strong evidence indicates that this can occur when marginally dysfunctional yeast have been used in the main fermentation – they tend not to metabolize all the acetolactic acid in the wort. The acetolactic acid spills over into the finished beer and later is oxidized to diacetyl. Mechanical abuse of packaged beer can promote this; headspace air is the oxidizing agent. Elevated temperatures augment the effect. I have seen cases in which wort constituents (melanoidins and tannins), oxidized on the hot side in wort production, were passed on to the final beer, only to play the role of oxidizer there.”